content: /*
   * Copyright (c) 2007, 2013, Oracle and/or its affiliates. All rights reserved.
   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   *
   * This code is free software; you can redistribute it and/or modify it
   * under the terms of the GNU General Public License version 2 only, as
   * published by the Free Software Foundation.  Oracle designates this
   * particular file as subject to the "Classpath" exception as provided
   * by Oracle in the LICENSE file that accompanied this code.
   *
   * This code is distributed in the hope that it will be useful, but WITHOUT
   * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
   * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
   * version 2 for more details (a copy is included in the LICENSE file that
   * accompanied this code).
   *
   * You should have received a copy of the GNU General Public License version
   * 2 along with this work; if not, write to the Free Software Foundation,
   * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   *
   * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
   * or visit www.oracle.com if you need additional information or have any
   * questions.
   */
  
  package java.nio.file;
  
  import java.io.BufferedReader;
  import java.io.BufferedWriter;
  import java.io.Closeable;
  import java.io.File;
  import java.io.IOException;
  import java.io.InputStream;
  import java.io.InputStreamReader;
  import java.io.OutputStream;
  import java.io.OutputStreamWriter;
  import java.io.Reader;
  import java.io.UncheckedIOException;
  import java.io.Writer;
  import java.nio.channels.Channels;
  import java.nio.channels.FileChannel;
  import java.nio.channels.SeekableByteChannel;
  import java.nio.charset.Charset;
  import java.nio.charset.CharsetDecoder;
  import java.nio.charset.CharsetEncoder;
  import java.nio.charset.StandardCharsets;
  import java.nio.file.attribute.BasicFileAttributeView;
  import java.nio.file.attribute.BasicFileAttributes;
  import java.nio.file.attribute.DosFileAttributes;   // javadoc
  import java.nio.file.attribute.FileAttribute;
  import java.nio.file.attribute.FileAttributeView;
  import java.nio.file.attribute.FileOwnerAttributeView;
  import java.nio.file.attribute.FileStoreAttributeView;
  import java.nio.file.attribute.FileTime;
  import java.nio.file.attribute.PosixFileAttributeView;
  import java.nio.file.attribute.PosixFileAttributes;
  import java.nio.file.attribute.PosixFilePermission;
  import java.nio.file.attribute.UserPrincipal;
  import java.nio.file.spi.FileSystemProvider;
  import java.nio.file.spi.FileTypeDetector;
  import java.security.AccessController;
  import java.security.PrivilegedAction;
  import java.util.ArrayList;
  import java.util.Arrays;
  import java.util.Collections;
  import java.util.EnumSet;
  import java.util.HashSet;
  import java.util.Iterator;
  import java.util.List;
  import java.util.Map;
  import java.util.Objects;
  import java.util.ServiceLoader;
  import java.util.Set;
  import java.util.Spliterator;
  import java.util.Spliterators;
  import java.util.function.BiPredicate;
  import java.util.stream.Stream;
  import java.util.stream.StreamSupport;
  
  /**
   * This class consists exclusively of static methods that operate on files,
   * directories, or other types of files.
   *
   * <p> In most cases, the methods defined here will delegate to the associated
   * file system provider to perform the file operations.
   *
   * @since 1.7
   */
  
  public final class Files {
      private Files() { }
  
      /**
       * Returns the {@code FileSystemProvider} to delegate to.
       */
      private static FileSystemProvider provider(Path path) {
          return path.getFileSystem().provider();
      }
  
      /**
       * Convert a Closeable to a Runnable by converting checked IOException
       * to UncheckedIOException
       */
      private static Runnable asUncheckedRunnable(Closeable c) {
          return () -> {
              try {
                  c.close();
              } catch (IOException e) {
                  throw new UncheckedIOException(e);
              }
          };
      }
  
      // -- File contents --
  
      /**
       * Opens a file, returning an input stream to read from the file. The stream
       * will not be buffered, and is not required to support the {@link
       * InputStream#mark mark} or {@link InputStream#reset reset} methods. The
       * stream will be safe for access by multiple concurrent threads. Reading
       * commences at the beginning of the file. Whether the returned stream is
       * <i>asynchronously closeable</i> and/or <i>interruptible</i> is highly
       * file system provider specific and therefore not specified.
       *
       * <p> The {@code options} parameter determines how the file is opened.
       * If no options are present then it is equivalent to opening the file with
       * the {@link StandardOpenOption#READ READ} option. In addition to the {@code
       * READ} option, an implementation may also support additional implementation
       * specific options.
       *
       * @param   path
       *          the path to the file to open
       * @param   options
       *          options specifying how the file is opened
       *
       * @return  a new input stream
       *
       * @throws  IllegalArgumentException
       *          if an invalid combination of options is specified
       * @throws  UnsupportedOperationException
       *          if an unsupported option is specified
       * @throws  IOException
       *          if an I/O error occurs
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkRead(String) checkRead}
       *          method is invoked to check read access to the file.
       */
      public static InputStream newInputStream(Path path, OpenOption... options)
          throws IOException
      {
          return provider(path).newInputStream(path, options);
      }
  
      /**
       * Opens or creates a file, returning an output stream that may be used to
       * write bytes to the file. The resulting stream will not be buffered. The
       * stream will be safe for access by multiple concurrent threads. Whether
       * the returned stream is <i>asynchronously closeable</i> and/or
       * <i>interruptible</i> is highly file system provider specific and
       * therefore not specified.
       *
       * <p> This method opens or creates a file in exactly the manner specified
       * by the {@link #newByteChannel(Path,Set,FileAttribute[]) newByteChannel}
       * method with the exception that the {@link StandardOpenOption#READ READ}
       * option may not be present in the array of options. If no options are
       * present then this method works as if the {@link StandardOpenOption#CREATE
       * CREATE}, {@link StandardOpenOption#TRUNCATE_EXISTING TRUNCATE_EXISTING},
       * and {@link StandardOpenOption#WRITE WRITE} options are present. In other
       * words, it opens the file for writing, creating the file if it doesn't
       * exist, or initially truncating an existing {@link #isRegularFile
       * regular-file} to a size of {@code 0} if it exists.
       *
       * <p> <b>Usage Examples:</b>
       * <pre>
       *     Path path = ...
       *
       *     // truncate and overwrite an existing file, or create the file if
       *     // it doesn't initially exist
       *     OutputStream out = Files.newOutputStream(path);
       *
       *     // append to an existing file, fail if the file does not exist
       *     out = Files.newOutputStream(path, APPEND);
       *
       *     // append to an existing file, create file if it doesn't initially exist
       *     out = Files.newOutputStream(path, CREATE, APPEND);
       *
       *     // always create new file, failing if it already exists
       *     out = Files.newOutputStream(path, CREATE_NEW);
       * </pre>
       *
       * @param   path
       *          the path to the file to open or create
       * @param   options
       *          options specifying how the file is opened
       *
       * @return  a new output stream
       *
       * @throws  IllegalArgumentException
       *          if {@code options} contains an invalid combination of options
       * @throws  UnsupportedOperationException
       *          if an unsupported option is specified
       * @throws  IOException
       *          if an I/O error occurs
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkWrite(String) checkWrite}
       *          method is invoked to check write access to the file. The {@link
       *          SecurityManager#checkDelete(String) checkDelete} method is
       *          invoked to check delete access if the file is opened with the
       *          {@code DELETE_ON_CLOSE} option.
       */
      public static OutputStream newOutputStream(Path path, OpenOption... options)
          throws IOException
      {
          return provider(path).newOutputStream(path, options);
      }
  
      /**
       * Opens or creates a file, returning a seekable byte channel to access the
       * file.
       *
       * <p> The {@code options} parameter determines how the file is opened.
       * The {@link StandardOpenOption#READ READ} and {@link
       * StandardOpenOption#WRITE WRITE} options determine if the file should be
       * opened for reading and/or writing. If neither option (or the {@link
       * StandardOpenOption#APPEND APPEND} option) is present then the file is
       * opened for reading. By default reading or writing commence at the
       * beginning of the file.
       *
       * <p> In the addition to {@code READ} and {@code WRITE}, the following
       * options may be present:
       *
       * <table border=1 cellpadding=5 summary="Options">
       * <tr> <th>Option</th> <th>Description</th> </tr>
       * <tr>
       *   <td> {@link StandardOpenOption#APPEND APPEND} </td>
       *   <td> If this option is present then the file is opened for writing and
       *     each invocation of the channel's {@code write} method first advances
       *     the position to the end of the file and then writes the requested
       *     data. Whether the advancement of the position and the writing of the
       *     data are done in a single atomic operation is system-dependent and
       *     therefore unspecified. This option may not be used in conjunction
       *     with the {@code READ} or {@code TRUNCATE_EXISTING} options. </td>
       * </tr>
       * <tr>
       *   <td> {@link StandardOpenOption#TRUNCATE_EXISTING TRUNCATE_EXISTING} </td>
       *   <td> If this option is present then the existing file is truncated to
       *   a size of 0 bytes. This option is ignored when the file is opened only
       *   for reading. </td>
       * </tr>
       * <tr>
       *   <td> {@link StandardOpenOption#CREATE_NEW CREATE_NEW} </td>
       *   <td> If this option is present then a new file is created, failing if
       *   the file already exists or is a symbolic link. When creating a file the
       *   check for the existence of the file and the creation of the file if it
       *   does not exist is atomic with respect to other file system operations.
       *   This option is ignored when the file is opened only for reading. </td>
       * </tr>
       * <tr>
       *   <td > {@link StandardOpenOption#CREATE CREATE} </td>
       *   <td> If this option is present then an existing file is opened if it
       *   exists, otherwise a new file is created. This option is ignored if the
       *   {@code CREATE_NEW} option is also present or the file is opened only
       *   for reading. </td>
       * </tr>
       * <tr>
       *   <td > {@link StandardOpenOption#DELETE_ON_CLOSE DELETE_ON_CLOSE} </td>
       *   <td> When this option is present then the implementation makes a
       *   <em>best effort</em> attempt to delete the file when closed by the
       *   {@link SeekableByteChannel#close close} method. If the {@code close}
       *   method is not invoked then a <em>best effort</em> attempt is made to
       *   delete the file when the Java virtual machine terminates. </td>
       * </tr>
       * <tr>
       *   <td>{@link StandardOpenOption#SPARSE SPARSE} </td>
       *   <td> When creating a new file this option is a <em>hint</em> that the
       *   new file will be sparse. This option is ignored when not creating
       *   a new file. </td>
       * </tr>
       * <tr>
       *   <td> {@link StandardOpenOption#SYNC SYNC} </td>
       *   <td> Requires that every update to the file's content or metadata be
       *   written synchronously to the underlying storage device. (see <a
       *   href="package-summary.html#integrity"> Synchronized I/O file
       *   integrity</a>). </td>
       * </tr>
       * <tr>
       *   <td> {@link StandardOpenOption#DSYNC DSYNC} </td>
       *   <td> Requires that every update to the file's content be written
       *   synchronously to the underlying storage device. (see <a
       *   href="package-summary.html#integrity"> Synchronized I/O file
       *   integrity</a>). </td>
       * </tr>
       * </table>
       *
       * <p> An implementation may also support additional implementation specific
       * options.
       *
       * <p> The {@code attrs} parameter is optional {@link FileAttribute
       * file-attributes} to set atomically when a new file is created.
       *
       * <p> In the case of the default provider, the returned seekable byte channel
       * is a {@link FileChannel}.
       *
       * <p> <b>Usage Examples:</b>
       * <pre>
       *     Path path = ...
       *
       *     // open file for reading
       *     ReadableByteChannel rbc = Files.newByteChannel(path, EnumSet.of(READ)));
       *
       *     // open file for writing to the end of an existing file, creating
       *     // the file if it doesn't already exist
       *     WritableByteChannel wbc = Files.newByteChannel(path, EnumSet.of(CREATE,APPEND));
       *
       *     // create file with initial permissions, opening it for both reading and writing
       *     {@code FileAttribute<Set<PosixFilePermission>> perms = ...}
       *     SeekableByteChannel sbc = Files.newByteChannel(path, EnumSet.of(CREATE_NEW,READ,WRITE), perms);
       * </pre>
       *
       * @param   path
       *          the path to the file to open or create
       * @param   options
       *          options specifying how the file is opened
       * @param   attrs
       *          an optional list of file attributes to set atomically when
       *          creating the file
       *
       * @return  a new seekable byte channel
       *
       * @throws  IllegalArgumentException
       *          if the set contains an invalid combination of options
       * @throws  UnsupportedOperationException
       *          if an unsupported open option is specified or the array contains
       *          attributes that cannot be set atomically when creating the file
       * @throws  FileAlreadyExistsException
       *          if a file of that name already exists and the {@link
       *          StandardOpenOption#CREATE_NEW CREATE_NEW} option is specified
       *          <i>(optional specific exception)</i>
       * @throws  IOException
       *          if an I/O error occurs
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkRead(String) checkRead}
       *          method is invoked to check read access to the path if the file is
       *          opened for reading. The {@link SecurityManager#checkWrite(String)
       *          checkWrite} method is invoked to check write access to the path
       *          if the file is opened for writing. The {@link
       *          SecurityManager#checkDelete(String) checkDelete} method is
       *          invoked to check delete access if the file is opened with the
       *          {@code DELETE_ON_CLOSE} option.
       *
       * @see FileChannel#open(Path,Set,FileAttribute[])
       */
      public static SeekableByteChannel newByteChannel(Path path,
                                                       Set<? extends OpenOption> options,
                                                       FileAttribute<?>... attrs)
          throws IOException
      {
          return provider(path).newByteChannel(path, options, attrs);
      }
  
      /**
       * Opens or creates a file, returning a seekable byte channel to access the
       * file.
       *
       * <p> This method opens or creates a file in exactly the manner specified
       * by the {@link #newByteChannel(Path,Set,FileAttribute[]) newByteChannel}
       * method.
       *
       * @param   path
       *          the path to the file to open or create
       * @param   options
       *          options specifying how the file is opened
       *
       * @return  a new seekable byte channel
       *
       * @throws  IllegalArgumentException
       *          if the set contains an invalid combination of options
       * @throws  UnsupportedOperationException
       *          if an unsupported open option is specified
       * @throws  FileAlreadyExistsException
       *          if a file of that name already exists and the {@link
       *          StandardOpenOption#CREATE_NEW CREATE_NEW} option is specified
       *          <i>(optional specific exception)</i>
       * @throws  IOException
       *          if an I/O error occurs
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkRead(String) checkRead}
       *          method is invoked to check read access to the path if the file is
       *          opened for reading. The {@link SecurityManager#checkWrite(String)
       *          checkWrite} method is invoked to check write access to the path
       *          if the file is opened for writing. The {@link
       *          SecurityManager#checkDelete(String) checkDelete} method is
       *          invoked to check delete access if the file is opened with the
       *          {@code DELETE_ON_CLOSE} option.
       *
       * @see FileChannel#open(Path,OpenOption[])
       */
      public static SeekableByteChannel newByteChannel(Path path, OpenOption... options)
          throws IOException
      {
          Set<OpenOption> set = new HashSet<OpenOption>(options.length);
          Collections.addAll(set, options);
          return newByteChannel(path, set);
      }
  
      // -- Directories --
  
      private static class AcceptAllFilter
          implements DirectoryStream.Filter<Path>
      {
          private AcceptAllFilter() { }
  
          @Override
          public boolean accept(Path entry) { return true; }
  
          static final AcceptAllFilter FILTER = new AcceptAllFilter();
      }
  
      /**
       * Opens a directory, returning a {@link DirectoryStream} to iterate over
       * all entries in the directory. The elements returned by the directory
       * stream's {@link DirectoryStream#iterator iterator} are of type {@code
       * Path}, each one representing an entry in the directory. The {@code Path}
       * objects are obtained as if by {@link Path#resolve(Path) resolving} the
       * name of the directory entry against {@code dir}.
       *
       * <p> When not using the try-with-resources construct, then directory
       * stream's {@code close} method should be invoked after iteration is
       * completed so as to free any resources held for the open directory.
       *
       * <p> When an implementation supports operations on entries in the
       * directory that execute in a race-free manner then the returned directory
       * stream is a {@link SecureDirectoryStream}.
       *
       * @param   dir
       *          the path to the directory
       *
       * @return  a new and open {@code DirectoryStream} object
       *
       * @throws  NotDirectoryException
       *          if the file could not otherwise be opened because it is not
       *          a directory <i>(optional specific exception)</i>
       * @throws  IOException
       *          if an I/O error occurs
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkRead(String) checkRead}
       *          method is invoked to check read access to the directory.
       */
      public static DirectoryStream<Path> newDirectoryStream(Path dir)
          throws IOException
      {
          return provider(dir).newDirectoryStream(dir, AcceptAllFilter.FILTER);
      }
  
      /**
       * Opens a directory, returning a {@link DirectoryStream} to iterate over
       * the entries in the directory. The elements returned by the directory
       * stream's {@link DirectoryStream#iterator iterator} are of type {@code
       * Path}, each one representing an entry in the directory. The {@code Path}
       * objects are obtained as if by {@link Path#resolve(Path) resolving} the
       * name of the directory entry against {@code dir}. The entries returned by
       * the iterator are filtered by matching the {@code String} representation
       * of their file names against the given <em>globbing</em> pattern.
       *
       * <p> For example, suppose we want to iterate over the files ending with
       * ".java" in a directory:
       * <pre>
       *     Path dir = ...
       *     try (DirectoryStream&lt;Path&gt; stream = Files.newDirectoryStream(dir, "*.java")) {
       *         :
       *     }
       * </pre>
       *
       * <p> The globbing pattern is specified by the {@link
       * FileSystem#getPathMatcher getPathMatcher} method.
       *
       * <p> When not using the try-with-resources construct, then directory
       * stream's {@code close} method should be invoked after iteration is
       * completed so as to free any resources held for the open directory.
       *
       * <p> When an implementation supports operations on entries in the
       * directory that execute in a race-free manner then the returned directory
       * stream is a {@link SecureDirectoryStream}.
       *
       * @param   dir
       *          the path to the directory
       * @param   glob
       *          the glob pattern
       *
       * @return  a new and open {@code DirectoryStream} object
       *
       * @throws  java.util.regex.PatternSyntaxException
       *          if the pattern is invalid
       * @throws  NotDirectoryException
       *          if the file could not otherwise be opened because it is not
       *          a directory <i>(optional specific exception)</i>
       * @throws  IOException
       *          if an I/O error occurs
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkRead(String) checkRead}
       *          method is invoked to check read access to the directory.
       */
      public static DirectoryStream<Path> newDirectoryStream(Path dir, String glob)
          throws IOException
      {
          // avoid creating a matcher if all entries are required.
          if (glob.equals("*"))
              return newDirectoryStream(dir);
  
          // create a matcher and return a filter that uses it.
          FileSystem fs = dir.getFileSystem();
          final PathMatcher matcher = fs.getPathMatcher("glob:" + glob);
          DirectoryStream.Filter<Path> filter = new DirectoryStream.Filter<Path>() {
              @Override
              public boolean accept(Path entry)  {
                  return matcher.matches(entry.getFileName());
              }
          };
          return fs.provider().newDirectoryStream(dir, filter);
      }
  
      /**
       * Opens a directory, returning a {@link DirectoryStream} to iterate over
       * the entries in the directory. The elements returned by the directory
       * stream's {@link DirectoryStream#iterator iterator} are of type {@code
       * Path}, each one representing an entry in the directory. The {@code Path}
       * objects are obtained as if by {@link Path#resolve(Path) resolving} the
       * name of the directory entry against {@code dir}. The entries returned by
       * the iterator are filtered by the given {@link DirectoryStream.Filter
       * filter}.
       *
       * <p> When not using the try-with-resources construct, then directory
       * stream's {@code close} method should be invoked after iteration is
       * completed so as to free any resources held for the open directory.
       *
       * <p> Where the filter terminates due to an uncaught error or runtime
       * exception then it is propagated to the {@link Iterator#hasNext()
       * hasNext} or {@link Iterator#next() next} method. Where an {@code
       * IOException} is thrown, it results in the {@code hasNext} or {@code
       * next} method throwing a {@link DirectoryIteratorException} with the
       * {@code IOException} as the cause.
       *
       * <p> When an implementation supports operations on entries in the
       * directory that execute in a race-free manner then the returned directory
       * stream is a {@link SecureDirectoryStream}.
       *
       * <p> <b>Usage Example:</b>
       * Suppose we want to iterate over the files in a directory that are
       * larger than 8K.
       * <pre>
       *     DirectoryStream.Filter&lt;Path&gt; filter = new DirectoryStream.Filter&lt;Path&gt;() {
       *         public boolean accept(Path file) throws IOException {
       *             return (Files.size(file) &gt; 8192L);
       *         }
       *     };
       *     Path dir = ...
       *     try (DirectoryStream&lt;Path&gt; stream = Files.newDirectoryStream(dir, filter)) {
       *         :
       *     }
       * </pre>
       *
       * @param   dir
       *          the path to the directory
       * @param   filter
       *          the directory stream filter
       *
       * @return  a new and open {@code DirectoryStream} object
       *
       * @throws  NotDirectoryException
       *          if the file could not otherwise be opened because it is not
       *          a directory <i>(optional specific exception)</i>
       * @throws  IOException
       *          if an I/O error occurs
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkRead(String) checkRead}
       *          method is invoked to check read access to the directory.
       */
      public static DirectoryStream<Path> newDirectoryStream(Path dir,
                                                             DirectoryStream.Filter<? super Path> filter)
          throws IOException
      {
          return provider(dir).newDirectoryStream(dir, filter);
      }
  
      // -- Creation and deletion --
  
      /**
       * Creates a new and empty file, failing if the file already exists. The
       * check for the existence of the file and the creation of the new file if
       * it does not exist are a single operation that is atomic with respect to
       * all other filesystem activities that might affect the directory.
       *
       * <p> The {@code attrs} parameter is optional {@link FileAttribute
       * file-attributes} to set atomically when creating the file. Each attribute
       * is identified by its {@link FileAttribute#name name}. If more than one
       * attribute of the same name is included in the array then all but the last
       * occurrence is ignored.
       *
       * @param   path
       *          the path to the file to create
       * @param   attrs
       *          an optional list of file attributes to set atomically when
       *          creating the file
       *
       * @return  the file
       *
       * @throws  UnsupportedOperationException
       *          if the array contains an attribute that cannot be set atomically
       *          when creating the file
       * @throws  FileAlreadyExistsException
       *          if a file of that name already exists
       *          <i>(optional specific exception)</i>
       * @throws  IOException
       *          if an I/O error occurs or the parent directory does not exist
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkWrite(String) checkWrite}
       *          method is invoked to check write access to the new file.
       */
      public static Path createFile(Path path, FileAttribute<?>... attrs)
          throws IOException
      {
          EnumSet<StandardOpenOption> options =
              EnumSet.<StandardOpenOption>of(StandardOpenOption.CREATE_NEW, StandardOpenOption.WRITE);
          newByteChannel(path, options, attrs).close();
          return path;
      }
  
      /**
       * Creates a new directory. The check for the existence of the file and the
       * creation of the directory if it does not exist are a single operation
       * that is atomic with respect to all other filesystem activities that might
       * affect the directory. The {@link #createDirectories createDirectories}
       * method should be used where it is required to create all nonexistent
       * parent directories first.
       *
       * <p> The {@code attrs} parameter is optional {@link FileAttribute
       * file-attributes} to set atomically when creating the directory. Each
       * attribute is identified by its {@link FileAttribute#name name}. If more
       * than one attribute of the same name is included in the array then all but
       * the last occurrence is ignored.
       *
       * @param   dir
       *          the directory to create
       * @param   attrs
       *          an optional list of file attributes to set atomically when
       *          creating the directory
       *
       * @return  the directory
       *
       * @throws  UnsupportedOperationException
       *          if the array contains an attribute that cannot be set atomically
       *          when creating the directory
       * @throws  FileAlreadyExistsException
       *          if a directory could not otherwise be created because a file of
       *          that name already exists <i>(optional specific exception)</i>
       * @throws  IOException
       *          if an I/O error occurs or the parent directory does not exist
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkWrite(String) checkWrite}
       *          method is invoked to check write access to the new directory.
       */
      public static Path createDirectory(Path dir, FileAttribute<?>... attrs)
          throws IOException
      {
          provider(dir).createDirectory(dir, attrs);
          return dir;
      }
  
      /**
       * Creates a directory by creating all nonexistent parent directories first.
       * Unlike the {@link #createDirectory createDirectory} method, an exception
       * is not thrown if the directory could not be created because it already
       * exists.
       *
       * <p> The {@code attrs} parameter is optional {@link FileAttribute
       * file-attributes} to set atomically when creating the nonexistent
       * directories. Each file attribute is identified by its {@link
       * FileAttribute#name name}. If more than one attribute of the same name is
       * included in the array then all but the last occurrence is ignored.
       *
       * <p> If this method fails, then it may do so after creating some, but not
       * all, of the parent directories.
       *
       * @param   dir
       *          the directory to create
       *
       * @param   attrs
       *          an optional list of file attributes to set atomically when
       *          creating the directory
       *
       * @return  the directory
       *
       * @throws  UnsupportedOperationException
       *          if the array contains an attribute that cannot be set atomically
       *          when creating the directory
       * @throws  FileAlreadyExistsException
       *          if {@code dir} exists but is not a directory <i>(optional specific
       *          exception)</i>
       * @throws  IOException
       *          if an I/O error occurs
       * @throws  SecurityException
       *          in the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkWrite(String) checkWrite}
       *          method is invoked prior to attempting to create a directory and
       *          its {@link SecurityManager#checkRead(String) checkRead} is
       *          invoked for each parent directory that is checked. If {@code
       *          dir} is not an absolute path then its {@link Path#toAbsolutePath
       *          toAbsolutePath} may need to be invoked to get its absolute path.
       *          This may invoke the security manager's {@link
       *          SecurityManager#checkPropertyAccess(String) checkPropertyAccess}
       *          method to check access to the system property {@code user.dir}
       */
      public static Path createDirectories(Path dir, FileAttribute<?>... attrs)
          throws IOException
      {
          // attempt to create the directory
          try {
              createAndCheckIsDirectory(dir, attrs);
              return dir;
          } catch (FileAlreadyExistsException x) {
              // file exists and is not a directory
              throw x;
          } catch (IOException x) {
              // parent may not exist or other reason
          }
          SecurityException se = null;
          try {
              dir = dir.toAbsolutePath();
          } catch (SecurityException x) {
              // don't have permission to get absolute path
              se = x;
          }
          // find a decendent that exists
          Path parent = dir.getParent();
          while (parent != null) {
              try {
                  provider(parent).checkAccess(parent);
                  break;
              } catch (NoSuchFileException x) {
                  // does not exist
              }
              parent = parent.getParent();
          }
          if (parent == null) {
              // unable to find existing parent
              if (se == null) {
                  throw new FileSystemException(dir.toString(), null,
                      "Unable to determine if root directory exists");
              } else {
                  throw se;
              }
          }
  
          // create directories
          Path child = parent;
          for (Path name: parent.relativize(dir)) {
              child = child.resolve(name);
              createAndCheckIsDirectory(child, attrs);
          }
          return dir;
      }
  
      /**
       * Used by createDirectories to attempt to create a directory. A no-op
       * if the directory already exists.
       */
      private static void createAndCheckIsDirectory(Path dir,
                                                    FileAttribute<?>... attrs)
          throws IOException
      {
          try {
              createDirectory(dir, attrs);
          } catch (FileAlreadyExistsException x) {
              if (!isDirectory(dir, LinkOption.NOFOLLOW_LINKS))
                  throw x;
          }
      }
  
      /**
       * Creates a new empty file in the specified directory, using the given
       * prefix and suffix strings to generate its name. The resulting
       * {@code Path} is associated with the same {@code FileSystem} as the given
       * directory.
       *
       * <p> The details as to how the name of the file is constructed is
       * implementation dependent and therefore not specified. Where possible
       * the {@code prefix} and {@code suffix} are used to construct candidate
       * names in the same manner as the {@link
       * File#createTempFile(String,String,File)} method.
       *
       * <p> As with the {@code File.createTempFile} methods, this method is only
       * part of a temporary-file facility. Where used as a <em>work files</em>,
       * the resulting file may be opened using the {@link
       * StandardOpenOption#DELETE_ON_CLOSE DELETE_ON_CLOSE} option so that the
       * file is deleted when the appropriate {@code close} method is invoked.
       * Alternatively, a {@link Runtime#addShutdownHook shutdown-hook}, or the
       * {@link File#deleteOnExit} mechanism may be used to delete the
       * file automatically.
       *
       * <p> The {@code attrs} parameter is optional {@link FileAttribute
       * file-attributes} to set atomically when creating the file. Each attribute
       * is identified by its {@link FileAttribute#name name}. If more than one
       * attribute of the same name is included in the array then all but the last
       * occurrence is ignored. When no file attributes are specified, then the
       * resulting file may have more restrictive access permissions to files
       * created by the {@link File#createTempFile(String,String,File)}
       * method.
       *
       * @param   dir
       *          the path to directory in which to create the file
       * @param   prefix
       *          the prefix string to be used in generating the file's name;
       *          may be {@code null}
       * @param   suffix
       *          the suffix string to be used in generating the file's name;
       *          may be {@code null}, in which case "{@code .tmp}" is used
       * @param   attrs
       *          an optional list of file attributes to set atomically when
       *          creating the file
       *
       * @return  the path to the newly created file that did not exist before
       *          this method was invoked
       *
       * @throws  IllegalArgumentException
       *          if the prefix or suffix parameters cannot be used to generate
       *          a candidate file name
       * @throws  UnsupportedOperationException
       *          if the array contains an attribute that cannot be set atomically
       *          when creating the directory
       * @throws  IOException
       *          if an I/O error occurs or {@code dir} does not exist
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkWrite(String) checkWrite}
       *          method is invoked to check write access to the file.
       */
      public static Path createTempFile(Path dir,
                                        String prefix,
                                        String suffix,
                                        FileAttribute<?>... attrs)
          throws IOException
      {
          return TempFileHelper.createTempFile(Objects.requireNonNull(dir),
                                               prefix, suffix, attrs);
      }
  
      /**
       * Creates an empty file in the default temporary-file directory, using
       * the given prefix and suffix to generate its name. The resulting {@code
       * Path} is associated with the default {@code FileSystem}.
       *
       * <p> This method works in exactly the manner specified by the
       * {@link #createTempFile(Path,String,String,FileAttribute[])} method for
       * the case that the {@code dir} parameter is the temporary-file directory.
       *
       * @param   prefix
       *          the prefix string to be used in generating the file's name;
       *          may be {@code null}
       * @param   suffix
       *          the suffix string to be used in generating the file's name;
       *          may be {@code null}, in which case "{@code .tmp}" is used
       * @param   attrs
       *          an optional list of file attributes to set atomically when
       *          creating the file
       *
       * @return  the path to the newly created file that did not exist before
       *          this method was invoked
       *
       * @throws  IllegalArgumentException
       *          if the prefix or suffix parameters cannot be used to generate
       *          a candidate file name
       * @throws  UnsupportedOperationException
       *          if the array contains an attribute that cannot be set atomically
       *          when creating the directory
       * @throws  IOException
       *          if an I/O error occurs or the temporary-file directory does not
       *          exist
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkWrite(String) checkWrite}
       *          method is invoked to check write access to the file.
       */
      public static Path createTempFile(String prefix,
                                        String suffix,
                                        FileAttribute<?>... attrs)
          throws IOException
      {
          return TempFileHelper.createTempFile(null, prefix, suffix, attrs);
      }
  
      /**
       * Creates a new directory in the specified directory, using the given
       * prefix to generate its name.  The resulting {@code Path} is associated
       * with the same {@code FileSystem} as the given directory.
       *
       * <p> The details as to how the name of the directory is constructed is
       * implementation dependent and therefore not specified. Where possible
       * the {@code prefix} is used to construct candidate names.
       *
       * <p> As with the {@code createTempFile} methods, this method is only
       * part of a temporary-file facility. A {@link Runtime#addShutdownHook
       * shutdown-hook}, or the {@link File#deleteOnExit} mechanism may be
       * used to delete the directory automatically.
       *
       * <p> The {@code attrs} parameter is optional {@link FileAttribute
       * file-attributes} to set atomically when creating the directory. Each
       * attribute is identified by its {@link FileAttribute#name name}. If more
       * than one attribute of the same name is included in the array then all but
       * the last occurrence is ignored.
       *
       * @param   dir
       *          the path to directory in which to create the directory
       * @param   prefix
       *          the prefix string to be used in generating the directory's name;
       *          may be {@code null}
       * @param   attrs
       *          an optional list of file attributes to set atomically when
       *          creating the directory
       *
       * @return  the path to the newly created directory that did not exist before
       *          this method was invoked
       *
       * @throws  IllegalArgumentException
       *          if the prefix cannot be used to generate a candidate directory name
       * @throws  UnsupportedOperationException
       *          if the array contains an attribute that cannot be set atomically
       *          when creating the directory
       * @throws  IOException
       *          if an I/O error occurs or {@code dir} does not exist
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkWrite(String) checkWrite}
       *          method is invoked to check write access when creating the
       *          directory.
       */
      public static Path createTempDirectory(Path dir,
                                             String prefix,
                                             FileAttribute<?>... attrs)
          throws IOException
      {
          return TempFileHelper.createTempDirectory(Objects.requireNonNull(dir),
                                                    prefix, attrs);
      }
  
      /**
       * Creates a new directory in the default temporary-file directory, using
       * the given prefix to generate its name. The resulting {@code Path} is
       * associated with the default {@code FileSystem}.
       *
       * <p> This method works in exactly the manner specified by {@link
       * #createTempDirectory(Path,String,FileAttribute[])} method for the case
       * that the {@code dir} parameter is the temporary-file directory.
       *
       * @param   prefix
       *          the prefix string to be used in generating the directory's name;
       *          may be {@code null}
       * @param   attrs
       *          an optional list of file attributes to set atomically when
       *          creating the directory
       *
       * @return  the path to the newly created directory that did not exist before
       *          this method was invoked
       *
       * @throws  IllegalArgumentException
       *          if the prefix cannot be used to generate a candidate directory name
       * @throws  UnsupportedOperationException
       *          if the array contains an attribute that cannot be set atomically
       *          when creating the directory
       * @throws  IOException
       *          if an I/O error occurs or the temporary-file directory does not
       *          exist
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkWrite(String) checkWrite}
       *          method is invoked to check write access when creating the
       *          directory.
       */
      public static Path createTempDirectory(String prefix,
                                             FileAttribute<?>... attrs)
          throws IOException
      {
          return TempFileHelper.createTempDirectory(null, prefix, attrs);
      }
  
      /**
       * Creates a symbolic link to a target <i>(optional operation)</i>.
       *
       * <p> The {@code target} parameter is the target of the link. It may be an
       * {@link Path#isAbsolute absolute} or relative path and may not exist. When
       * the target is a relative path then file system operations on the resulting
       * link are relative to the path of the link.
       *
       * <p> The {@code attrs} parameter is optional {@link FileAttribute
       * attributes} to set atomically when creating the link. Each attribute is
       * identified by its {@link FileAttribute#name name}. If more than one attribute
       * of the same name is included in the array then all but the last occurrence
       * is ignored.
       *
       * <p> Where symbolic links are supported, but the underlying {@link FileStore}
       * does not support symbolic links, then this may fail with an {@link
       * IOException}. Additionally, some operating systems may require that the
       * Java virtual machine be started with implementation specific privileges to
       * create symbolic links, in which case this method may throw {@code IOException}.
       *
       * @param   link
       *          the path of the symbolic link to create
       * @param   target
       *          the target of the symbolic link
       * @param   attrs
       *          the array of attributes to set atomically when creating the
       *          symbolic link
       *
       * @return  the path to the symbolic link
       *
       * @throws  UnsupportedOperationException
       *          if the implementation does not support symbolic links or the
       *          array contains an attribute that cannot be set atomically when
       *          creating the symbolic link
       * @throws  FileAlreadyExistsException
       *          if a file with the name already exists <i>(optional specific
       *          exception)</i>
       * @throws  IOException
       *          if an I/O error occurs
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager
       *          is installed, it denies {@link LinkPermission}<tt>("symbolic")</tt>
       *          or its {@link SecurityManager#checkWrite(String) checkWrite}
       *          method denies write access to the path of the symbolic link.
       */
      public static Path createSymbolicLink(Path link, Path target,
                                            FileAttribute<?>... attrs)
          throws IOException
      {
          provider(link).createSymbolicLink(link, target, attrs);
          return link;
      }
  
      /**
       * Creates a new link (directory entry) for an existing file <i>(optional
       * operation)</i>.
       *
       * <p> The {@code link} parameter locates the directory entry to create.
       * The {@code existing} parameter is the path to an existing file. This
       * method creates a new directory entry for the file so that it can be
       * accessed using {@code link} as the path. On some file systems this is
       * known as creating a "hard link". Whether the file attributes are
       * maintained for the file or for each directory entry is file system
       * specific and therefore not specified. Typically, a file system requires
       * that all links (directory entries) for a file be on the same file system.
       * Furthermore, on some platforms, the Java virtual machine may require to
       * be started with implementation specific privileges to create hard links
       * or to create links to directories.
       *
       * @param   link
       *          the link (directory entry) to create
       * @param   existing
       *          a path to an existing file
       *
       * @return  the path to the link (directory entry)
       *
       * @throws  UnsupportedOperationException
       *          if the implementation does not support adding an existing file
       *          to a directory
       * @throws  FileAlreadyExistsException
       *          if the entry could not otherwise be created because a file of
       *          that name already exists <i>(optional specific exception)</i>
       * @throws  IOException
       *          if an I/O error occurs
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager
       *          is installed, it denies {@link LinkPermission}<tt>("hard")</tt>
       *          or its {@link SecurityManager#checkWrite(String) checkWrite}
       *          method denies write access to either the link or the
       *          existing file.
       */
      public static Path createLink(Path link, Path existing) throws IOException {
          provider(link).createLink(link, existing);
          return link;
      }
  
      /**
       * Deletes a file.
       *
       * <p> An implementation may require to examine the file to determine if the
       * file is a directory. Consequently this method may not be atomic with respect
       * to other file system operations.  If the file is a symbolic link then the
       * symbolic link itself, not the final target of the link, is deleted.
       *
       * <p> If the file is a directory then the directory must be empty. In some
       * implementations a directory has entries for special files or links that
       * are created when the directory is created. In such implementations a
       * directory is considered empty when only the special entries exist.
       * This method can be used with the {@link #walkFileTree walkFileTree}
       * method to delete a directory and all entries in the directory, or an
       * entire <i>file-tree</i> where required.
       *
       * <p> On some operating systems it may not be possible to remove a file when
       * it is open and in use by this Java virtual machine or other programs.
       *
       * @param   path
       *          the path to the file to delete
       *
       * @throws  NoSuchFileException
       *          if the file does not exist <i>(optional specific exception)</i>
       * @throws  DirectoryNotEmptyException
       *          if the file is a directory and could not otherwise be deleted
       *          because the directory is not empty <i>(optional specific
       *          exception)</i>
       * @throws  IOException
       *          if an I/O error occurs
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkDelete(String)} method
       *          is invoked to check delete access to the file
       */
      public static void delete(Path path) throws IOException {
          provider(path).delete(path);
      }
  
      /**
       * Deletes a file if it exists.
       *
       * <p> As with the {@link #delete(Path) delete(Path)} method, an
       * implementation may need to examine the file to determine if the file is a
       * directory. Consequently this method may not be atomic with respect to
       * other file system operations.  If the file is a symbolic link, then the
       * symbolic link itself, not the final target of the link, is deleted.
       *
       * <p> If the file is a directory then the directory must be empty. In some
       * implementations a directory has entries for special files or links that
       * are created when the directory is created. In such implementations a
       * directory is considered empty when only the special entries exist.
       *
       * <p> On some operating systems it may not be possible to remove a file when
       * it is open and in use by this Java virtual machine or other programs.
       *
       * @param   path
       *          the path to the file to delete
       *
       * @return  {@code true} if the file was deleted by this method; {@code
       *          false} if the file could not be deleted because it did not
       *          exist
       *
       * @throws  DirectoryNotEmptyException
       *          if the file is a directory and could not otherwise be deleted
       *          because the directory is not empty <i>(optional specific
       *          exception)</i>
       * @throws  IOException
       *          if an I/O error occurs
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkDelete(String)} method
       *          is invoked to check delete access to the file.
       */
      public static boolean deleteIfExists(Path path) throws IOException {
          return provider(path).deleteIfExists(path);
      }
  
      // -- Copying and moving files --
  
      /**
       * Copy a file to a target file.
       *
       * <p> This method copies a file to the target file with the {@code
       * options} parameter specifying how the copy is performed. By default, the
       * copy fails if the target file already exists or is a symbolic link,
       * except if the source and target are the {@link #isSameFile same} file, in
       * which case the method completes without copying the file. File attributes
       * are not required to be copied to the target file. If symbolic links are
       * supported, and the file is a symbolic link, then the final target of the
       * link is copied. If the file is a directory then it creates an empty
       * directory in the target location (entries in the directory are not
       * copied). This method can be used with the {@link #walkFileTree
       * walkFileTree} method to copy a directory and all entries in the directory,
       * or an entire <i>file-tree</i> where required.
       *
       * <p> The {@code options} parameter may include any of the following:
       *
       * <table border=1 cellpadding=5 summary="">
       * <tr> <th>Option</th> <th>Description</th> </tr>
       * <tr>
       *   <td> {@link StandardCopyOption#REPLACE_EXISTING REPLACE_EXISTING} </td>
       *   <td> If the target file exists, then the target file is replaced if it
       *     is not a non-empty directory. If the target file exists and is a
       *     symbolic link, then the symbolic link itself, not the target of
       *     the link, is replaced. </td>
       * </tr>
       * <tr>
       *   <td> {@link StandardCopyOption#COPY_ATTRIBUTES COPY_ATTRIBUTES} </td>
       *   <td> Attempts to copy the file attributes associated with this file to
       *     the target file. The exact file attributes that are copied is platform
       *     and file system dependent and therefore unspecified. Minimally, the
       *     {@link BasicFileAttributes#lastModifiedTime last-modified-time} is
       *     copied to the target file if supported by both the source and target
       *     file stores. Copying of file timestamps may result in precision
       *     loss. </td>
       * </tr>
       * <tr>
       *   <td> {@link LinkOption#NOFOLLOW_LINKS NOFOLLOW_LINKS} </td>
       *   <td> Symbolic links are not followed. If the file is a symbolic link,
       *     then the symbolic link itself, not the target of the link, is copied.
       *     It is implementation specific if file attributes can be copied to the
       *     new link. In other words, the {@code COPY_ATTRIBUTES} option may be
       *     ignored when copying a symbolic link. </td>
       * </tr>
       * </table>
       *
       * <p> An implementation of this interface may support additional
       * implementation specific options.
       *
       * <p> Copying a file is not an atomic operation. If an {@link IOException}
       * is thrown, then it is possible that the target file is incomplete or some
       * of its file attributes have not been copied from the source file. When
       * the {@code REPLACE_EXISTING} option is specified and the target file
       * exists, then the target file is replaced. The check for the existence of
       * the file and the creation of the new file may not be atomic with respect
       * to other file system activities.
       *
       * <p> <b>Usage Example:</b>
       * Suppose we want to copy a file into a directory, giving it the same file
       * name as the source file:
       * <pre>
       *     Path source = ...
       *     Path newdir = ...
       *     Files.copy(source, newdir.resolve(source.getFileName());
       * </pre>
       *
       * @param   source
       *          the path to the file to copy
       * @param   target
       *          the path to the target file (may be associated with a different
       *          provider to the source path)
       * @param   options
       *          options specifying how the copy should be done
       *
       * @return  the path to the target file
       *
       * @throws  UnsupportedOperationException
       *          if the array contains a copy option that is not supported
       * @throws  FileAlreadyExistsException
       *          if the target file exists but cannot be replaced because the
       *          {@code REPLACE_EXISTING} option is not specified <i>(optional
       *          specific exception)</i>
       * @throws  DirectoryNotEmptyException
       *          the {@code REPLACE_EXISTING} option is specified but the file
       *          cannot be replaced because it is a non-empty directory
       *          <i>(optional specific exception)</i>
       * @throws  IOException
       *          if an I/O error occurs
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkRead(String) checkRead}
       *          method is invoked to check read access to the source file, the
       *          {@link SecurityManager#checkWrite(String) checkWrite} is invoked
       *          to check write access to the target file. If a symbolic link is
       *          copied the security manager is invoked to check {@link
       *          LinkPermission}{@code ("symbolic")}.
       */
      public static Path copy(Path source, Path target, CopyOption... options)
          throws IOException
      {
          FileSystemProvider provider = provider(source);
          if (provider(target) == provider) {
              // same provider
              provider.copy(source, target, options);
          } else {
              // different providers
              CopyMoveHelper.copyToForeignTarget(source, target, options);
          }
          return target;
      }
  
      /**
       * Move or rename a file to a target file.
       *
       * <p> By default, this method attempts to move the file to the target
       * file, failing if the target file exists except if the source and
       * target are the {@link #isSameFile same} file, in which case this method
       * has no effect. If the file is a symbolic link then the symbolic link
       * itself, not the target of the link, is moved. This method may be
       * invoked to move an empty directory. In some implementations a directory
       * has entries for special files or links that are created when the
       * directory is created. In such implementations a directory is considered
       * empty when only the special entries exist. When invoked to move a
       * directory that is not empty then the directory is moved if it does not
       * require moving the entries in the directory.  For example, renaming a
       * directory on the same {@link FileStore} will usually not require moving
       * the entries in the directory. When moving a directory requires that its
       * entries be moved then this method fails (by throwing an {@code
       * IOException}). To move a <i>file tree</i> may involve copying rather
       * than moving directories and this can be done using the {@link
       * #copy copy} method in conjunction with the {@link
       * #walkFileTree Files.walkFileTree} utility method.
       *
       * <p> The {@code options} parameter may include any of the following:
       *
       * <table border=1 cellpadding=5 summary="">
       * <tr> <th>Option</th> <th>Description</th> </tr>
       * <tr>
       *   <td> {@link StandardCopyOption#REPLACE_EXISTING REPLACE_EXISTING} </td>
       *   <td> If the target file exists, then the target file is replaced if it
       *     is not a non-empty directory. If the target file exists and is a
       *     symbolic link, then the symbolic link itself, not the target of
       *     the link, is replaced. </td>
       * </tr>
       * <tr>
       *   <td> {@link StandardCopyOption#ATOMIC_MOVE ATOMIC_MOVE} </td>
       *   <td> The move is performed as an atomic file system operation and all
       *     other options are ignored. If the target file exists then it is
       *     implementation specific if the existing file is replaced or this method
       *     fails by throwing an {@link IOException}. If the move cannot be
       *     performed as an atomic file system operation then {@link
       *     AtomicMoveNotSupportedException} is thrown. This can arise, for
       *     example, when the target location is on a different {@code FileStore}
       *     and would require that the file be copied, or target location is
       *     associated with a different provider to this object. </td>
       * </table>
       *
       * <p> An implementation of this interface may support additional
       * implementation specific options.
       *
       * <p> Moving a file will copy the {@link
       * BasicFileAttributes#lastModifiedTime last-modified-time} to the target
       * file if supported by both source and target file stores. Copying of file
       * timestamps may result in precision loss. An implementation may also
       * attempt to copy other file attributes but is not required to fail if the
       * file attributes cannot be copied. When the move is performed as
       * a non-atomic operation, and an {@code IOException} is thrown, then the
       * state of the files is not defined. The original file and the target file
       * may both exist, the target file may be incomplete or some of its file
       * attributes may not been copied from the original file.
       *
       * <p> <b>Usage Examples:</b>
       * Suppose we want to rename a file to "newname", keeping the file in the
       * same directory:
       * <pre>
       *     Path source = ...
       *     Files.move(source, source.resolveSibling("newname"));
       * </pre>
       * Alternatively, suppose we want to move a file to new directory, keeping
       * the same file name, and replacing any existing file of that name in the
       * directory:
       * <pre>
       *     Path source = ...
       *     Path newdir = ...
       *     Files.move(source, newdir.resolve(source.getFileName()), REPLACE_EXISTING);
       * </pre>
       *
       * @param   source
       *          the path to the file to move
       * @param   target
       *          the path to the target file (may be associated with a different
       *          provider to the source path)
       * @param   options
       *          options specifying how the move should be done
       *
       * @return  the path to the target file
       *
       * @throws  UnsupportedOperationException
       *          if the array contains a copy option that is not supported
       * @throws  FileAlreadyExistsException
       *          if the target file exists but cannot be replaced because the
       *          {@code REPLACE_EXISTING} option is not specified <i>(optional
       *          specific exception)</i>
       * @throws  DirectoryNotEmptyException
       *          the {@code REPLACE_EXISTING} option is specified but the file
       *          cannot be replaced because it is a non-empty directory
       *          <i>(optional specific exception)</i>
       * @throws  AtomicMoveNotSupportedException
       *          if the options array contains the {@code ATOMIC_MOVE} option but
       *          the file cannot be moved as an atomic file system operation.
       * @throws  IOException
       *          if an I/O error occurs
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkWrite(String) checkWrite}
       *          method is invoked to check write access to both the source and
       *          target file.
       */
      public static Path move(Path source, Path target, CopyOption... options)
          throws IOException
      {
          FileSystemProvider provider = provider(source);
          if (provider(target) == provider) {
              // same provider
              provider.move(source, target, options);
          } else {
              // different providers
              CopyMoveHelper.moveToForeignTarget(source, target, options);
          }
          return target;
      }
  
      // -- Miscellenous --
  
      /**
       * Reads the target of a symbolic link <i>(optional operation)</i>.
       *
       * <p> If the file system supports <a href="package-summary.html#links">symbolic
       * links</a> then this method is used to read the target of the link, failing
       * if the file is not a symbolic link. The target of the link need not exist.
       * The returned {@code Path} object will be associated with the same file
       * system as {@code link}.
       *
       * @param   link
       *          the path to the symbolic link
       *
       * @return  a {@code Path} object representing the target of the link
       *
       * @throws  UnsupportedOperationException
       *          if the implementation does not support symbolic links
       * @throws  NotLinkException
       *          if the target could otherwise not be read because the file
       *          is not a symbolic link <i>(optional specific exception)</i>
       * @throws  IOException
       *          if an I/O error occurs
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager
       *          is installed, it checks that {@code FilePermission} has been
       *          granted with the "{@code readlink}" action to read the link.
       */
      public static Path readSymbolicLink(Path link) throws IOException {
          return provider(link).readSymbolicLink(link);
      }
  
      /**
       * Returns the {@link FileStore} representing the file store where a file
       * is located.
       *
       * <p> Once a reference to the {@code FileStore} is obtained it is
       * implementation specific if operations on the returned {@code FileStore},
       * or {@link FileStoreAttributeView} objects obtained from it, continue
       * to depend on the existence of the file. In particular the behavior is not
       * defined for the case that the file is deleted or moved to a different
       * file store.
       *
       * @param   path
       *          the path to the file
       *
       * @return  the file store where the file is stored
       *
       * @throws  IOException
       *          if an I/O error occurs
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkRead(String) checkRead}
       *          method is invoked to check read access to the file, and in
       *          addition it checks {@link RuntimePermission}<tt>
       *          ("getFileStoreAttributes")</tt>
       */
      public static FileStore getFileStore(Path path) throws IOException {
          return provider(path).getFileStore(path);
      }
  
      /**
       * Tests if two paths locate the same file.
       *
       * <p> If both {@code Path} objects are {@link Path#equals(Object) equal}
       * then this method returns {@code true} without checking if the file exists.
       * If the two {@code Path} objects are associated with different providers
       * then this method returns {@code false}. Otherwise, this method checks if
       * both {@code Path} objects locate the same file, and depending on the
       * implementation, may require to open or access both files.
       *
       * <p> If the file system and files remain static, then this method implements
       * an equivalence relation for non-null {@code Paths}.
       * <ul>
       * <li>It is <i>reflexive</i>: for {@code Path} {@code f},
       *     {@code isSameFile(f,f)} should return {@code true}.
       * <li>It is <i>symmetric</i>: for two {@code Paths} {@code f} and {@code g},
       *     {@code isSameFile(f,g)} will equal {@code isSameFile(g,f)}.
       * <li>It is <i>transitive</i>: for three {@code Paths}
       *     {@code f}, {@code g}, and {@code h}, if {@code isSameFile(f,g)} returns
       *     {@code true} and {@code isSameFile(g,h)} returns {@code true}, then
       *     {@code isSameFile(f,h)} will return return {@code true}.
       * </ul>
       *
       * @param   path
       *          one path to the file
       * @param   path2
       *          the other path
       *
       * @return  {@code true} if, and only if, the two paths locate the same file
       *
       * @throws  IOException
       *          if an I/O error occurs
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkRead(String) checkRead}
       *          method is invoked to check read access to both files.
       *
       * @see BasicFileAttributes#fileKey
       */
      public static boolean isSameFile(Path path, Path path2) throws IOException {
          return provider(path).isSameFile(path, path2);
      }
  
      /**
       * Tells whether or not a file is considered <em>hidden</em>. The exact
       * definition of hidden is platform or provider dependent. On UNIX for
       * example a file is considered to be hidden if its name begins with a
       * period character ('.'). On Windows a file is considered hidden if it
       * isn't a directory and the DOS {@link DosFileAttributes#isHidden hidden}
       * attribute is set.
       *
       * <p> Depending on the implementation this method may require to access
       * the file system to determine if the file is considered hidden.
       *
       * @param   path
       *          the path to the file to test
       *
       * @return  {@code true} if the file is considered hidden
       *
       * @throws  IOException
       *          if an I/O error occurs
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkRead(String) checkRead}
       *          method is invoked to check read access to the file.
       */
      public static boolean isHidden(Path path) throws IOException {
          return provider(path).isHidden(path);
      }
  
      // lazy loading of default and installed file type detectors
      private static class FileTypeDetectors{
          static final FileTypeDetector defaultFileTypeDetector =
              createDefaultFileTypeDetector();
          static final List<FileTypeDetector> installeDetectors =
              loadInstalledDetectors();
  
          // creates the default file type detector
          private static FileTypeDetector createDefaultFileTypeDetector() {
              return AccessController
                  .doPrivileged(new PrivilegedAction<FileTypeDetector>() {
                      @Override public FileTypeDetector run() {
                          return sun.nio.fs.DefaultFileTypeDetector.create();
                  }});
          }
  
          // loads all installed file type detectors
          private static List<FileTypeDetector> loadInstalledDetectors() {
              return AccessController
                  .doPrivileged(new PrivilegedAction<List<FileTypeDetector>>() {
                      @Override public List<FileTypeDetector> run() {
                          List<FileTypeDetector> list = new ArrayList<>();
                          ServiceLoader<FileTypeDetector> loader = ServiceLoader
                              .load(FileTypeDetector.class, ClassLoader.getSystemClassLoader());
                          for (FileTypeDetector detector: loader) {
                              list.add(detector);
                          }
                          return list;
                  }});
          }
      }
  
      /**
       * Probes the content type of a file.
       *
       * <p> This method uses the installed {@link FileTypeDetector} implementations
       * to probe the given file to determine its content type. Each file type
       * detector's {@link FileTypeDetector#probeContentType probeContentType} is
       * invoked, in turn, to probe the file type. If the file is recognized then
       * the content type is returned. If the file is not recognized by any of the
       * installed file type detectors then a system-default file type detector is
       * invoked to guess the content type.
       *
       * <p> A given invocation of the Java virtual machine maintains a system-wide
       * list of file type detectors. Installed file type detectors are loaded
       * using the service-provider loading facility defined by the {@link ServiceLoader}
       * class. Installed file type detectors are loaded using the system class
       * loader. If the system class loader cannot be found then the extension class
       * loader is used; If the extension class loader cannot be found then the
       * bootstrap class loader is used. File type detectors are typically installed
       * by placing them in a JAR file on the application class path or in the
       * extension directory, the JAR file contains a provider-configuration file
       * named {@code java.nio.file.spi.FileTypeDetector} in the resource directory
       * {@code META-INF/services}, and the file lists one or more fully-qualified
       * names of concrete subclass of {@code FileTypeDetector } that have a zero
       * argument constructor. If the process of locating or instantiating the
       * installed file type detectors fails then an unspecified error is thrown.
       * The ordering that installed providers are located is implementation
       * specific.
       *
       * <p> The return value of this method is the string form of the value of a
       * Multipurpose Internet Mail Extension (MIME) content type as
       * defined by <a href="http://www.ietf.org/rfc/rfc2045.txt"><i>RFC&nbsp;2045:
       * Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet
       * Message Bodies</i></a>. The string is guaranteed to be parsable according
       * to the grammar in the RFC.
       *
       * @param   path
       *          the path to the file to probe
       *
       * @return  The content type of the file, or {@code null} if the content
       *          type cannot be determined
       *
       * @throws  IOException
       *          if an I/O error occurs
       * @throws  SecurityException
       *          If a security manager is installed and it denies an unspecified
       *          permission required by a file type detector implementation.
       */
      public static String probeContentType(Path path)
          throws IOException
      {
          // try installed file type detectors
          for (FileTypeDetector detector: FileTypeDetectors.installeDetectors) {
              String result = detector.probeContentType(path);
              if (result != null)
                  return result;
          }
  
          // fallback to default
          return FileTypeDetectors.defaultFileTypeDetector.probeContentType(path);
      }
  
      // -- File Attributes --
  
      /**
       * Returns a file attribute view of a given type.
       *
       * <p> A file attribute view provides a read-only or updatable view of a
       * set of file attributes. This method is intended to be used where the file
       * attribute view defines type-safe methods to read or update the file
       * attributes. The {@code type} parameter is the type of the attribute view
       * required and the method returns an instance of that type if supported.
       * The {@link BasicFileAttributeView} type supports access to the basic
       * attributes of a file. Invoking this method to select a file attribute
       * view of that type will always return an instance of that class.
       *
       * <p> The {@code options} array may be used to indicate how symbolic links
       * are handled by the resulting file attribute view for the case that the
       * file is a symbolic link. By default, symbolic links are followed. If the
       * option {@link LinkOption#NOFOLLOW_LINKS NOFOLLOW_LINKS} is present then
       * symbolic links are not followed. This option is ignored by implementations
       * that do not support symbolic links.
       *
       * <p> <b>Usage Example:</b>
       * Suppose we want read or set a file's ACL, if supported:
       * <pre>
       *     Path path = ...
       *     AclFileAttributeView view = Files.getFileAttributeView(path, AclFileAttributeView.class);
       *     if (view != null) {
       *         List&lt;AclEntry&gt; acl = view.getAcl();
       *         :
       *     }
       * </pre>
       *
       * @param   <V>
       *          The {@code FileAttributeView} type
       * @param   path
       *          the path to the file
       * @param   type
       *          the {@code Class} object corresponding to the file attribute view
       * @param   options
       *          options indicating how symbolic links are handled
       *
       * @return  a file attribute view of the specified type, or {@code null} if
       *          the attribute view type is not available
       */
      public static <V extends FileAttributeView> V getFileAttributeView(Path path,
                                                                         Class<V> type,
                                                                         LinkOption... options)
      {
          return provider(path).getFileAttributeView(path, type, options);
      }
  
      /**
       * Reads a file's attributes as a bulk operation.
       *
       * <p> The {@code type} parameter is the type of the attributes required
       * and this method returns an instance of that type if supported. All
       * implementations support a basic set of file attributes and so invoking
       * this method with a  {@code type} parameter of {@code
       * BasicFileAttributes.class} will not throw {@code
       * UnsupportedOperationException}.
       *
       * <p> The {@code options} array may be used to indicate how symbolic links
       * are handled for the case that the file is a symbolic link. By default,
       * symbolic links are followed and the file attribute of the final target
       * of the link is read. If the option {@link LinkOption#NOFOLLOW_LINKS
       * NOFOLLOW_LINKS} is present then symbolic links are not followed.
       *
       * <p> It is implementation specific if all file attributes are read as an
       * atomic operation with respect to other file system operations.
       *
       * <p> <b>Usage Example:</b>
       * Suppose we want to read a file's attributes in bulk:
       * <pre>
       *    Path path = ...
       *    BasicFileAttributes attrs = Files.readAttributes(path, BasicFileAttributes.class);
       * </pre>
       * Alternatively, suppose we want to read file's POSIX attributes without
       * following symbolic links:
       * <pre>
       *    PosixFileAttributes attrs = Files.readAttributes(path, PosixFileAttributes.class, NOFOLLOW_LINKS);
       * </pre>
       *
       * @param   <A>
       *          The {@code BasicFileAttributes} type
       * @param   path
       *          the path to the file
       * @param   type
       *          the {@code Class} of the file attributes required
       *          to read
       * @param   options
       *          options indicating how symbolic links are handled
       *
       * @return  the file attributes
       *
       * @throws  UnsupportedOperationException
       *          if an attributes of the given type are not supported
       * @throws  IOException
       *          if an I/O error occurs
       * @throws  SecurityException
       *          In the case of the default provider, a security manager is
       *          installed, its {@link SecurityManager#checkRead(String) checkRead}
       *          method is invoked to check read access to the file. If this
       *          method is invoked to read security sensitive attributes then the
       *          security manager may be invoke to check for additional permissions.
       */
      public static <A extends BasicFileAttributes> A readAttributes(Path path,
                                                                     Class<A> type,
                                                                     LinkOption... options)
          throws IOException
      {
          return provider(path).readAttributes(path, type, options);
      }
  
      /**
       * Sets the value of a file attribute.
       *
       * <p> The {@code attribute} parameter identifies the attribute to be set
       * and takes the form:
       * <blockquote>
       * [<i>view-name</i><b>:</b>]<i>attribute-name</i>
       * </blockquote>
       * where square brackets [...] delineate an optional component and the
       * character {@code ':'} stands for itself.
       *
       * <p> <i>view-name</i> is the {@link FileAttributeView#name name} of a {@link
       * FileAttributeView} that identifies a set of file attributes. If not
       * specified then it defaults to {@code "basic"}, the name of the file
       * attribute view that identifies the basic set of file attributes common to
       * many file systems. <i>attribute-name</i> is the name of the attribute
       * within the set.
       *
       * <p> The {@code options} array may be used to indicate how symbolic links
       * are handled for the case that the file is a symbolic link. By default,
       * symbolic links are followed and the file attribute of the final target
       * of the link is set. If the option {@link LinkOption#NOFOLLOW_LINKS
       * NOFOLLOW_LINKS} is present then symbolic links are not followed.
       *
       * <p> <b>Usage Example:</b>
       * Suppose we want to set the DOS "hidden" attribute:
       * <pre>
       *    Path path = ...
       *    Files.setAttribute(path, "dos:hidden", true);
       * </pre>
       *
       * @param   path
       *          the path to the file
       * @param   attribute
       *          the attribute to set
       * @param   value
       *          the attribute value
       * @param   options
       *          options indicating how symbolic links are handled
       *
       * @return  the {@code path} parameter
       *
       * @throws  UnsupportedOperationException
       *          if the attribute view is not available
       * @throws  IllegalArgumentException
       *          if the attribute name is not specified, or is not recognized, or
       *          the attribute value is of the correct type but has an
       *          inappropriate value
       * @throws  ClassCastException
       *          if the attribute value is not of the expected type or is a
       *          collection containing elements that are not of the expected
       *          type
       * @throws  IOException
       *          if an I/O error occurs
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, its {@link SecurityManager#checkWrite(String) checkWrite}
       *          method denies write access to the file. If this method is invoked
       *          to set security sensitive attributes then the security manager
       *          may be invoked to check for additional permissions.
       */
      public static Path setAttribute(Path path, String attribute, Object value,
                                      LinkOption... options)
          throws IOException
      {
          provider(path).setAttribute(path, attribute, value, options);
          return path;
      }
  
      /**
       * Reads the value of a file attribute.
       *
       * <p> The {@code attribute} parameter identifies the attribute to be read
       * and takes the form:
       * <blockquote>
       * [<i>view-name</i><b>:</b>]<i>attribute-name</i>
       * </blockquote>
       * where square brackets [...] delineate an optional component and the
       * character {@code ':'} stands for itself.
       *
       * <p> <i>view-name</i> is the {@link FileAttributeView#name name} of a {@link
       * FileAttributeView} that identifies a set of file attributes. If not
       * specified then it defaults to {@code "basic"}, the name of the file
       * attribute view that identifies the basic set of file attributes common to
       * many file systems. <i>attribute-name</i> is the name of the attribute.
       *
       * <p> The {@code options} array may be used to indicate how symbolic links
       * are handled for the case that the file is a symbolic link. By default,
       * symbolic links are followed and the file attribute of the final target
       * of the link is read. If the option {@link LinkOption#NOFOLLOW_LINKS
       * NOFOLLOW_LINKS} is present then symbolic links are not followed.
       *
       * <p> <b>Usage Example:</b>
       * Suppose we require the user ID of the file owner on a system that
       * supports a "{@code unix}" view:
       * <pre>
       *    Path path = ...
       *    int uid = (Integer)Files.getAttribute(path, "unix:uid");
       * </pre>
       *
       * @param   path
       *          the path to the file
       * @param   attribute
       *          the attribute to read
       * @param   options
       *          options indicating how symbolic links are handled
       *
       * @return  the attribute value
       *
       * @throws  UnsupportedOperationException
       *          if the attribute view is not available
       * @throws  IllegalArgumentException
       *          if the attribute name is not specified or is not recognized
       * @throws  IOException
       *          if an I/O error occurs
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, its {@link SecurityManager#checkRead(String) checkRead}
       *          method denies read access to the file. If this method is invoked
       *          to read security sensitive attributes then the security manager
       *          may be invoked to check for additional permissions.
       */
      public static Object getAttribute(Path path, String attribute,
                                        LinkOption... options)
          throws IOException
      {
          // only one attribute should be read
          if (attribute.indexOf('*') >= 0 || attribute.indexOf(',') >= 0)
              throw new IllegalArgumentException(attribute);
          Map<String,Object> map = readAttributes(path, attribute, options);
          assert map.size() == 1;
          String name;
          int pos = attribute.indexOf(':');
          if (pos == -1) {
              name = attribute;
          } else {
              name = (pos == attribute.length()) ? "" : attribute.substring(pos+1);
          }
          return map.get(name);
      }
  
      /**
       * Reads a set of file attributes as a bulk operation.
       *
       * <p> The {@code attributes} parameter identifies the attributes to be read
       * and takes the form:
       * <blockquote>
       * [<i>view-name</i><b>:</b>]<i>attribute-list</i>
       * </blockquote>
       * where square brackets [...] delineate an optional component and the
       * character {@code ':'} stands for itself.
       *
       * <p> <i>view-name</i> is the {@link FileAttributeView#name name} of a {@link
       * FileAttributeView} that identifies a set of file attributes. If not
       * specified then it defaults to {@code "basic"}, the name of the file
       * attribute view that identifies the basic set of file attributes common to
       * many file systems.
       *
       * <p> The <i>attribute-list</i> component is a comma separated list of
       * zero or more names of attributes to read. If the list contains the value
       * {@code "*"} then all attributes are read. Attributes that are not supported
       * are ignored and will not be present in the returned map. It is
       * implementation specific if all attributes are read as an atomic operation
       * with respect to other file system operations.
       *
       * <p> The following examples demonstrate possible values for the {@code
       * attributes} parameter:
       *
       * <blockquote>
       * <table border="0" summary="Possible values">
       * <tr>
       *   <td> {@code "*"} </td>
       *   <td> Read all {@link BasicFileAttributes basic-file-attributes}. </td>
       * </tr>
       * <tr>
       *   <td> {@code "size,lastModifiedTime,lastAccessTime"} </td>
       *   <td> Reads the file size, last modified time, and last access time
       *     attributes. </td>
       * </tr>
       * <tr>
       *   <td> {@code "posix:*"} </td>
       *   <td> Read all {@link PosixFileAttributes POSIX-file-attributes}. </td>
       * </tr>
       * <tr>
       *   <td> {@code "posix:permissions,owner,size"} </td>
       *   <td> Reads the POSX file permissions, owner, and file size. </td>
       * </tr>
       * </table>
       * </blockquote>
       *
       * <p> The {@code options} array may be used to indicate how symbolic links
       * are handled for the case that the file is a symbolic link. By default,
       * symbolic links are followed and the file attribute of the final target
       * of the link is read. If the option {@link LinkOption#NOFOLLOW_LINKS
       * NOFOLLOW_LINKS} is present then symbolic links are not followed.
       *
       * @param   path
       *          the path to the file
       * @param   attributes
       *          the attributes to read
       * @param   options
       *          options indicating how symbolic links are handled
       *
       * @return  a map of the attributes returned; The map's keys are the
       *          attribute names, its values are the attribute values
       *
       * @throws  UnsupportedOperationException
       *          if the attribute view is not available
       * @throws  IllegalArgumentException
       *          if no attributes are specified or an unrecognized attributes is
       *          specified
       * @throws  IOException
       *          if an I/O error occurs
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, its {@link SecurityManager#checkRead(String) checkRead}
       *          method denies read access to the file. If this method is invoked
       *          to read security sensitive attributes then the security manager
       *          may be invoke to check for additional permissions.
       */
      public static Map<String,Object> readAttributes(Path path, String attributes,
                                                      LinkOption... options)
          throws IOException
      {
          return provider(path).readAttributes(path, attributes, options);
      }
  
      /**
       * Returns a file's POSIX file permissions.
       *
       * <p> The {@code path} parameter is associated with a {@code FileSystem}
       * that supports the {@link PosixFileAttributeView}. This attribute view
       * provides access to file attributes commonly associated with files on file
       * systems used by operating systems that implement the Portable Operating
       * System Interface (POSIX) family of standards.
       *
       * <p> The {@code options} array may be used to indicate how symbolic links
       * are handled for the case that the file is a symbolic link. By default,
       * symbolic links are followed and the file attribute of the final target
       * of the link is read. If the option {@link LinkOption#NOFOLLOW_LINKS
       * NOFOLLOW_LINKS} is present then symbolic links are not followed.
       *
       * @param   path
       *          the path to the file
       * @param   options
       *          options indicating how symbolic links are handled
       *
       * @return  the file permissions
       *
       * @throws  UnsupportedOperationException
       *          if the associated file system does not support the {@code
       *          PosixFileAttributeView}
       * @throws  IOException
       *          if an I/O error occurs
       * @throws  SecurityException
       *          In the case of the default provider, a security manager is
       *          installed, and it denies {@link RuntimePermission}<tt>("accessUserInformation")</tt>
       *          or its {@link SecurityManager#checkRead(String) checkRead} method
       *          denies read access to the file.
       */
      public static Set<PosixFilePermission> getPosixFilePermissions(Path path,
                                                                     LinkOption... options)
          throws IOException
      {
          return readAttributes(path, PosixFileAttributes.class, options).permissions();
      }
  
      /**
       * Sets a file's POSIX permissions.
       *
       * <p> The {@code path} parameter is associated with a {@code FileSystem}
       * that supports the {@link PosixFileAttributeView}. This attribute view
       * provides access to file attributes commonly associated with files on file
       * systems used by operating systems that implement the Portable Operating
       * System Interface (POSIX) family of standards.
       *
       * @param   path
       *          The path to the file
       * @param   perms
       *          The new set of permissions
       *
       * @return  The path
       *
       * @throws  UnsupportedOperationException
       *          if the associated file system does not support the {@code
       *          PosixFileAttributeView}
       * @throws  ClassCastException
       *          if the sets contains elements that are not of type {@code
       *          PosixFilePermission}
       * @throws  IOException
       *          if an I/O error occurs
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, it denies {@link RuntimePermission}<tt>("accessUserInformation")</tt>
       *          or its {@link SecurityManager#checkWrite(String) checkWrite}
       *          method denies write access to the file.
       */
      public static Path setPosixFilePermissions(Path path,
                                                 Set<PosixFilePermission> perms)
          throws IOException
      {
          PosixFileAttributeView view =
              getFileAttributeView(path, PosixFileAttributeView.class);
          if (view == null)
              throw new UnsupportedOperationException();
          view.setPermissions(perms);
          return path;
      }
  
      /**
       * Returns the owner of a file.
       *
       * <p> The {@code path} parameter is associated with a file system that
       * supports {@link FileOwnerAttributeView}. This file attribute view provides
       * access to a file attribute that is the owner of the file.
       *
       * @param   path
       *          The path to the file
       * @param   options
       *          options indicating how symbolic links are handled
       *
       * @return  A user principal representing the owner of the file
       *
       * @throws  UnsupportedOperationException
       *          if the associated file system does not support the {@code
       *          FileOwnerAttributeView}
       * @throws  IOException
       *          if an I/O error occurs
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, it denies {@link RuntimePermission}<tt>("accessUserInformation")</tt>
       *          or its {@link SecurityManager#checkRead(String) checkRead} method
       *          denies read access to the file.
       */
      public static UserPrincipal getOwner(Path path, LinkOption... options) throws IOException {
          FileOwnerAttributeView view =
              getFileAttributeView(path, FileOwnerAttributeView.class, options);
          if (view == null)
              throw new UnsupportedOperationException();
          return view.getOwner();
      }
  
      /**
       * Updates the file owner.
       *
       * <p> The {@code path} parameter is associated with a file system that
       * supports {@link FileOwnerAttributeView}. This file attribute view provides
       * access to a file attribute that is the owner of the file.
       *
       * <p> <b>Usage Example:</b>
       * Suppose we want to make "joe" the owner of a file:
       * <pre>
       *     Path path = ...
       *     UserPrincipalLookupService lookupService =
       *         provider(path).getUserPrincipalLookupService();
       *     UserPrincipal joe = lookupService.lookupPrincipalByName("joe");
       *     Files.setOwner(path, joe);
       * </pre>
       *
       * @param   path
       *          The path to the file
       * @param   owner
       *          The new file owner
       *
       * @return  The path
       *
       * @throws  UnsupportedOperationException
       *          if the associated file system does not support the {@code
       *          FileOwnerAttributeView}
       * @throws  IOException
       *          if an I/O error occurs
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, it denies {@link RuntimePermission}<tt>("accessUserInformation")</tt>
       *          or its {@link SecurityManager#checkWrite(String) checkWrite}
       *          method denies write access to the file.
       *
       * @see FileSystem#getUserPrincipalLookupService
       * @see java.nio.file.attribute.UserPrincipalLookupService
       */
      public static Path setOwner(Path path, UserPrincipal owner)
          throws IOException
      {
          FileOwnerAttributeView view =
              getFileAttributeView(path, FileOwnerAttributeView.class);
          if (view == null)
              throw new UnsupportedOperationException();
          view.setOwner(owner);
          return path;
      }
  
      /**
       * Tests whether a file is a symbolic link.
       *
       * <p> Where it is required to distinguish an I/O exception from the case
       * that the file is not a symbolic link then the file attributes can be
       * read with the {@link #readAttributes(Path,Class,LinkOption[])
       * readAttributes} method and the file type tested with the {@link
       * BasicFileAttributes#isSymbolicLink} method.
       *
       * @param   path  The path to the file
       *
       * @return  {@code true} if the file is a symbolic link; {@code false} if
       *          the file does not exist, is not a symbolic link, or it cannot
       *          be determined if the file is a symbolic link or not.
       *
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, its {@link SecurityManager#checkRead(String) checkRead}
       *          method denies read access to the file.
       */
      public static boolean isSymbolicLink(Path path) {
          try {
              return readAttributes(path,
                                    BasicFileAttributes.class,
                                    LinkOption.NOFOLLOW_LINKS).isSymbolicLink();
          } catch (IOException ioe) {
              return false;
          }
      }
  
      /**
       * Tests whether a file is a directory.
       *
       * <p> The {@code options} array may be used to indicate how symbolic links
       * are handled for the case that the file is a symbolic link. By default,
       * symbolic links are followed and the file attribute of the final target
       * of the link is read. If the option {@link LinkOption#NOFOLLOW_LINKS
       * NOFOLLOW_LINKS} is present then symbolic links are not followed.
       *
       * <p> Where it is required to distinguish an I/O exception from the case
       * that the file is not a directory then the file attributes can be
       * read with the {@link #readAttributes(Path,Class,LinkOption[])
       * readAttributes} method and the file type tested with the {@link
       * BasicFileAttributes#isDirectory} method.
       *
       * @param   path
       *          the path to the file to test
       * @param   options
       *          options indicating how symbolic links are handled
       *
       * @return  {@code true} if the file is a directory; {@code false} if
       *          the file does not exist, is not a directory, or it cannot
       *          be determined if the file is a directory or not.
       *
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, its {@link SecurityManager#checkRead(String) checkRead}
       *          method denies read access to the file.
       */
      public static boolean isDirectory(Path path, LinkOption... options) {
          try {
              return readAttributes(path, BasicFileAttributes.class, options).isDirectory();
          } catch (IOException ioe) {
              return false;
          }
      }
  
      /**
       * Tests whether a file is a regular file with opaque content.
       *
       * <p> The {@code options} array may be used to indicate how symbolic links
       * are handled for the case that the file is a symbolic link. By default,
       * symbolic links are followed and the file attribute of the final target
       * of the link is read. If the option {@link LinkOption#NOFOLLOW_LINKS
       * NOFOLLOW_LINKS} is present then symbolic links are not followed.
       *
       * <p> Where it is required to distinguish an I/O exception from the case
       * that the file is not a regular file then the file attributes can be
       * read with the {@link #readAttributes(Path,Class,LinkOption[])
       * readAttributes} method and the file type tested with the {@link
       * BasicFileAttributes#isRegularFile} method.
       *
       * @param   path
       *          the path to the file
       * @param   options
       *          options indicating how symbolic links are handled
       *
       * @return  {@code true} if the file is a regular file; {@code false} if
       *          the file does not exist, is not a regular file, or it
       *          cannot be determined if the file is a regular file or not.
       *
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, its {@link SecurityManager#checkRead(String) checkRead}
       *          method denies read access to the file.
       */
      public static boolean isRegularFile(Path path, LinkOption... options) {
          try {
              return readAttributes(path, BasicFileAttributes.class, options).isRegularFile();
          } catch (IOException ioe) {
              return false;
          }
      }
  
      /**
       * Returns a file's last modified time.
       *
       * <p> The {@code options} array may be used to indicate how symbolic links
       * are handled for the case that the file is a symbolic link. By default,
       * symbolic links are followed and the file attribute of the final target
       * of the link is read. If the option {@link LinkOption#NOFOLLOW_LINKS
       * NOFOLLOW_LINKS} is present then symbolic links are not followed.
       *
       * @param   path
       *          the path to the file
       * @param   options
       *          options indicating how symbolic links are handled
       *
       * @return  a {@code FileTime} representing the time the file was last
       *          modified, or an implementation specific default when a time
       *          stamp to indicate the time of last modification is not supported
       *          by the file system
       *
       * @throws  IOException
       *          if an I/O error occurs
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, its {@link SecurityManager#checkRead(String) checkRead}
       *          method denies read access to the file.
       *
       * @see BasicFileAttributes#lastModifiedTime
       */
      public static FileTime getLastModifiedTime(Path path, LinkOption... options)
          throws IOException
      {
          return readAttributes(path, BasicFileAttributes.class, options).lastModifiedTime();
      }
  
      /**
       * Updates a file's last modified time attribute. The file time is converted
       * to the epoch and precision supported by the file system. Converting from
       * finer to coarser granularities result in precision loss. The behavior of
       * this method when attempting to set the last modified time when it is not
       * supported by the file system or is outside the range supported by the
       * underlying file store is not defined. It may or not fail by throwing an
       * {@code IOException}.
       *
       * <p> <b>Usage Example:</b>
       * Suppose we want to set the last modified time to the current time:
       * <pre>
       *    Path path = ...
       *    FileTime now = FileTime.fromMillis(System.currentTimeMillis());
       *    Files.setLastModifiedTime(path, now);
       * </pre>
       *
       * @param   path
       *          the path to the file
       * @param   time
       *          the new last modified time
       *
       * @return  the path
       *
       * @throws  IOException
       *          if an I/O error occurs
       * @throws  SecurityException
       *          In the case of the default provider, the security manager's {@link
       *          SecurityManager#checkWrite(String) checkWrite} method is invoked
       *          to check write access to file
       *
       * @see BasicFileAttributeView#setTimes
       */
      public static Path setLastModifiedTime(Path path, FileTime time)
          throws IOException
      {
          getFileAttributeView(path, BasicFileAttributeView.class)
              .setTimes(time, null, null);
          return path;
      }
  
      /**
       * Returns the size of a file (in bytes). The size may differ from the
       * actual size on the file system due to compression, support for sparse
       * files, or other reasons. The size of files that are not {@link
       * #isRegularFile regular} files is implementation specific and
       * therefore unspecified.
       *
       * @param   path
       *          the path to the file
       *
       * @return  the file size, in bytes
       *
       * @throws  IOException
       *          if an I/O error occurs
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, its {@link SecurityManager#checkRead(String) checkRead}
       *          method denies read access to the file.
       *
       * @see BasicFileAttributes#size
       */
      public static long size(Path path) throws IOException {
          return readAttributes(path, BasicFileAttributes.class).size();
      }
  
      // -- Accessibility --
  
      /**
       * Returns {@code false} if NOFOLLOW_LINKS is present.
       */
      private static boolean followLinks(LinkOption... options) {
          boolean followLinks = true;
          for (LinkOption opt: options) {
              if (opt == LinkOption.NOFOLLOW_LINKS) {
                  followLinks = false;
                  continue;
              }
              if (opt == null)
                  throw new NullPointerException();
              throw new AssertionError("Should not get here");
          }
          return followLinks;
      }
  
      /**
       * Tests whether a file exists.
       *
       * <p> The {@code options} parameter may be used to indicate how symbolic links
       * are handled for the case that the file is a symbolic link. By default,
       * symbolic links are followed. If the option {@link LinkOption#NOFOLLOW_LINKS
       * NOFOLLOW_LINKS} is present then symbolic links are not followed.
       *
       * <p> Note that the result of this method is immediately outdated. If this
       * method indicates the file exists then there is no guarantee that a
       * subsequence access will succeed. Care should be taken when using this
       * method in security sensitive applications.
       *
       * @param   path
       *          the path to the file to test
       * @param   options
       *          options indicating how symbolic links are handled
       * .
       * @return  {@code true} if the file exists; {@code false} if the file does
       *          not exist or its existence cannot be determined.
       *
       * @throws  SecurityException
       *          In the case of the default provider, the {@link
       *          SecurityManager#checkRead(String)} is invoked to check
       *          read access to the file.
       *
       * @see #notExists
       */
      public static boolean exists(Path path, LinkOption... options) {
          try {
              if (followLinks(options)) {
                  provider(path).checkAccess(path);
              } else {
                  // attempt to read attributes without following links
                  readAttributes(path, BasicFileAttributes.class,
                                 LinkOption.NOFOLLOW_LINKS);
              }
              // file exists
              return true;
          } catch (IOException x) {
              // does not exist or unable to determine if file exists
              return false;
          }
  
      }
  
      /**
       * Tests whether the file located by this path does not exist. This method
       * is intended for cases where it is required to take action when it can be
       * confirmed that a file does not exist.
       *
       * <p> The {@code options} parameter may be used to indicate how symbolic links
       * are handled for the case that the file is a symbolic link. By default,
       * symbolic links are followed. If the option {@link LinkOption#NOFOLLOW_LINKS
       * NOFOLLOW_LINKS} is present then symbolic links are not followed.
       *
       * <p> Note that this method is not the complement of the {@link #exists
       * exists} method. Where it is not possible to determine if a file exists
       * or not then both methods return {@code false}. As with the {@code exists}
       * method, the result of this method is immediately outdated. If this
       * method indicates the file does exist then there is no guarantee that a
       * subsequence attempt to create the file will succeed. Care should be taken
       * when using this method in security sensitive applications.
       *
       * @param   path
       *          the path to the file to test
       * @param   options
       *          options indicating how symbolic links are handled
       *
       * @return  {@code true} if the file does not exist; {@code false} if the
       *          file exists or its existence cannot be determined
       *
       * @throws  SecurityException
       *          In the case of the default provider, the {@link
       *          SecurityManager#checkRead(String)} is invoked to check
       *          read access to the file.
       */
      public static boolean notExists(Path path, LinkOption... options) {
          try {
              if (followLinks(options)) {
                  provider(path).checkAccess(path);
              } else {
                  // attempt to read attributes without following links
                  readAttributes(path, BasicFileAttributes.class,
                                 LinkOption.NOFOLLOW_LINKS);
              }
              // file exists
              return false;
          } catch (NoSuchFileException x) {
              // file confirmed not to exist
              return true;
          } catch (IOException x) {
              return false;
          }
      }
  
      /**
       * Used by isReadbale, isWritable, isExecutable to test access to a file.
       */
      private static boolean isAccessible(Path path, AccessMode... modes) {
          try {
              provider(path).checkAccess(path, modes);
              return true;
          } catch (IOException x) {
              return false;
          }
      }
  
      /**
       * Tests whether a file is readable. This method checks that a file exists
       * and that this Java virtual machine has appropriate privileges that would
       * allow it open the file for reading. Depending on the implementation, this
       * method may require to read file permissions, access control lists, or
       * other file attributes in order to check the effective access to the file.
       * Consequently, this method may not be atomic with respect to other file
       * system operations.
       *
       * <p> Note that the result of this method is immediately outdated, there is
       * no guarantee that a subsequent attempt to open the file for reading will
       * succeed (or even that it will access the same file). Care should be taken
       * when using this method in security sensitive applications.
       *
       * @param   path
       *          the path to the file to check
       *
       * @return  {@code true} if the file exists and is readable; {@code false}
       *          if the file does not exist, read access would be denied because
       *          the Java virtual machine has insufficient privileges, or access
       *          cannot be determined
       *
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkRead(String) checkRead}
       *          is invoked to check read access to the file.
       */
      public static boolean isReadable(Path path) {
          return isAccessible(path, AccessMode.READ);
      }
  
      /**
       * Tests whether a file is writable. This method checks that a file exists
       * and that this Java virtual machine has appropriate privileges that would
       * allow it open the file for writing. Depending on the implementation, this
       * method may require to read file permissions, access control lists, or
       * other file attributes in order to check the effective access to the file.
       * Consequently, this method may not be atomic with respect to other file
       * system operations.
       *
       * <p> Note that result of this method is immediately outdated, there is no
       * guarantee that a subsequent attempt to open the file for writing will
       * succeed (or even that it will access the same file). Care should be taken
       * when using this method in security sensitive applications.
       *
       * @param   path
       *          the path to the file to check
       *
       * @return  {@code true} if the file exists and is writable; {@code false}
       *          if the file does not exist, write access would be denied because
       *          the Java virtual machine has insufficient privileges, or access
       *          cannot be determined
       *
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkWrite(String) checkWrite}
       *          is invoked to check write access to the file.
       */
      public static boolean isWritable(Path path) {
          return isAccessible(path, AccessMode.WRITE);
      }
  
      /**
       * Tests whether a file is executable. This method checks that a file exists
       * and that this Java virtual machine has appropriate privileges to {@link
       * Runtime#exec execute} the file. The semantics may differ when checking
       * access to a directory. For example, on UNIX systems, checking for
       * execute access checks that the Java virtual machine has permission to
       * search the directory in order to access file or subdirectories.
       *
       * <p> Depending on the implementation, this method may require to read file
       * permissions, access control lists, or other file attributes in order to
       * check the effective access to the file. Consequently, this method may not
       * be atomic with respect to other file system operations.
       *
       * <p> Note that the result of this method is immediately outdated, there is
       * no guarantee that a subsequent attempt to execute the file will succeed
       * (or even that it will access the same file). Care should be taken when
       * using this method in security sensitive applications.
       *
       * @param   path
       *          the path to the file to check
       *
       * @return  {@code true} if the file exists and is executable; {@code false}
       *          if the file does not exist, execute access would be denied because
       *          the Java virtual machine has insufficient privileges, or access
       *          cannot be determined
       *
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkExec(String)
       *          checkExec} is invoked to check execute access to the file.
       */
      public static boolean isExecutable(Path path) {
          return isAccessible(path, AccessMode.EXECUTE);
      }
  
      // -- Recursive operations --
  
      /**
       * Walks a file tree.
       *
       * <p> This method walks a file tree rooted at a given starting file. The
       * file tree traversal is <em>depth-first</em> with the given {@link
       * FileVisitor} invoked for each file encountered. File tree traversal
       * completes when all accessible files in the tree have been visited, or a
       * visit method returns a result of {@link FileVisitResult#TERMINATE
       * TERMINATE}. Where a visit method terminates due an {@code IOException},
       * an uncaught error, or runtime exception, then the traversal is terminated
       * and the error or exception is propagated to the caller of this method.
       *
       * <p> For each file encountered this method attempts to read its {@link
       * BasicFileAttributes}. If the file is not a
       * directory then the {@link FileVisitor#visitFile visitFile} method is
       * invoked with the file attributes. If the file attributes cannot be read,
       * due to an I/O exception, then the {@link FileVisitor#visitFileFailed
       * visitFileFailed} method is invoked with the I/O exception.
       *
       * <p> Where the file is a directory, and the directory could not be opened,
       * then the {@code visitFileFailed} method is invoked with the I/O exception,
       * after which, the file tree walk continues, by default, at the next
       * <em>sibling</em> of the directory.
       *
       * <p> Where the directory is opened successfully, then the entries in the
       * directory, and their <em>descendants</em> are visited. When all entries
       * have been visited, or an I/O error occurs during iteration of the
       * directory, then the directory is closed and the visitor's {@link
       * FileVisitor#postVisitDirectory postVisitDirectory} method is invoked.
       * The file tree walk then continues, by default, at the next <em>sibling</em>
       * of the directory.
       *
       * <p> By default, symbolic links are not automatically followed by this
       * method. If the {@code options} parameter contains the {@link
       * FileVisitOption#FOLLOW_LINKS FOLLOW_LINKS} option then symbolic links are
       * followed. When following links, and the attributes of the target cannot
       * be read, then this method attempts to get the {@code BasicFileAttributes}
       * of the link. If they can be read then the {@code visitFile} method is
       * invoked with the attributes of the link (otherwise the {@code visitFileFailed}
       * method is invoked as specified above).
       *
       * <p> If the {@code options} parameter contains the {@link
       * FileVisitOption#FOLLOW_LINKS FOLLOW_LINKS} option then this method keeps
       * track of directories visited so that cycles can be detected. A cycle
       * arises when there is an entry in a directory that is an ancestor of the
       * directory. Cycle detection is done by recording the {@link
       * BasicFileAttributes#fileKey file-key} of directories,
       * or if file keys are not available, by invoking the {@link #isSameFile
       * isSameFile} method to test if a directory is the same file as an
       * ancestor. When a cycle is detected it is treated as an I/O error, and the
       * {@link FileVisitor#visitFileFailed visitFileFailed} method is invoked with
       * an instance of {@link FileSystemLoopException}.
       *
       * <p> The {@code maxDepth} parameter is the maximum number of levels of
       * directories to visit. A value of {@code 0} means that only the starting
       * file is visited, unless denied by the security manager. A value of
       * {@link Integer#MAX_VALUE MAX_VALUE} may be used to indicate that all
       * levels should be visited. The {@code visitFile} method is invoked for all
       * files, including directories, encountered at {@code maxDepth}, unless the
       * basic file attributes cannot be read, in which case the {@code
       * visitFileFailed} method is invoked.
       *
       * <p> If a visitor returns a result of {@code null} then {@code
       * NullPointerException} is thrown.
       *
       * <p> When a security manager is installed and it denies access to a file
       * (or directory), then it is ignored and the visitor is not invoked for
       * that file (or directory).
       *
       * @param   start
       *          the starting file
       * @param   options
       *          options to configure the traversal
       * @param   maxDepth
       *          the maximum number of directory levels to visit
       * @param   visitor
       *          the file visitor to invoke for each file
       *
       * @return  the starting file
       *
       * @throws  IllegalArgumentException
       *          if the {@code maxDepth} parameter is negative
       * @throws  SecurityException
       *          If the security manager denies access to the starting file.
       *          In the case of the default provider, the {@link
       *          SecurityManager#checkRead(String) checkRead} method is invoked
       *          to check read access to the directory.
       * @throws  IOException
       *          if an I/O error is thrown by a visitor method
       */
      public static Path walkFileTree(Path start,
                                      Set<FileVisitOption> options,
                                      int maxDepth,
                                      FileVisitor<? super Path> visitor)
          throws IOException
      {
          /**
           * Create a FileTreeWalker to walk the file tree, invoking the visitor
           * for each event.
           */
          try (FileTreeWalker walker = new FileTreeWalker(options, maxDepth)) {
              FileTreeWalker.Event ev = walker.walk(start);
              do {
                  FileVisitResult result;
                  switch (ev.type()) {
                      case ENTRY :
                          IOException ioe = ev.ioeException();
                          if (ioe == null) {
                              assert ev.attributes() != null;
                              result = visitor.visitFile(ev.file(), ev.attributes());
                          } else {
                              result = visitor.visitFileFailed(ev.file(), ioe);
                          }
                          break;
  
                      case START_DIRECTORY :
                          result = visitor.preVisitDirectory(ev.file(), ev.attributes());
  
                          // if SKIP_SIBLINGS and SKIP_SUBTREE is returned then
                          // there shouldn't be any more events for the current
                          // directory.
                          if (result == FileVisitResult.SKIP_SUBTREE ||
                              result == FileVisitResult.SKIP_SIBLINGS)
                              walker.pop();
                          break;
  
                      case END_DIRECTORY :
                          result = visitor.postVisitDirectory(ev.file(), ev.ioeException());
  
                          // SKIP_SIBLINGS is a no-op for postVisitDirectory
                          if (result == FileVisitResult.SKIP_SIBLINGS)
                              result = FileVisitResult.CONTINUE;
                          break;
  
                      default :
                          throw new AssertionError("Should not get here");
                  }
  
                  if (Objects.requireNonNull(result) != FileVisitResult.CONTINUE) {
                      if (result == FileVisitResult.TERMINATE) {
                          break;
                      } else if (result == FileVisitResult.SKIP_SIBLINGS) {
                          walker.skipRemainingSiblings();
                      }
                  }
                  ev = walker.next();
              } while (ev != null);
          }
  
          return start;
      }
  
      /**
       * Walks a file tree.
       *
       * <p> This method works as if invoking it were equivalent to evaluating the
       * expression:
       * <blockquote><pre>
       * walkFileTree(start, EnumSet.noneOf(FileVisitOption.class), Integer.MAX_VALUE, visitor)
       * </pre></blockquote>
       * In other words, it does not follow symbolic links, and visits all levels
       * of the file tree.
       *
       * @param   start
       *          the starting file
       * @param   visitor
       *          the file visitor to invoke for each file
       *
       * @return  the starting file
       *
       * @throws  SecurityException
       *          If the security manager denies access to the starting file.
       *          In the case of the default provider, the {@link
       *          SecurityManager#checkRead(String) checkRead} method is invoked
       *          to check read access to the directory.
       * @throws  IOException
       *          if an I/O error is thrown by a visitor method
       */
      public static Path walkFileTree(Path start, FileVisitor<? super Path> visitor)
          throws IOException
      {
          return walkFileTree(start,
                              EnumSet.noneOf(FileVisitOption.class),
                              Integer.MAX_VALUE,
                              visitor);
      }
  
  
      // -- Utility methods for simple usages --
  
      // buffer size used for reading and writing
      private static final int BUFFER_SIZE = 8192;
  
      /**
       * Opens a file for reading, returning a {@code BufferedReader} that may be
       * used to read text from the file in an efficient manner. Bytes from the
       * file are decoded into characters using the specified charset. Reading
       * commences at the beginning of the file.
       *
       * <p> The {@code Reader} methods that read from the file throw {@code
       * IOException} if a malformed or unmappable byte sequence is read.
       *
       * @param   path
       *          the path to the file
       * @param   cs
       *          the charset to use for decoding
       *
       * @return  a new buffered reader, with default buffer size, to read text
       *          from the file
       *
       * @throws  IOException
       *          if an I/O error occurs opening the file
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkRead(String) checkRead}
       *          method is invoked to check read access to the file.
       *
       * @see #readAllLines
       */
      public static BufferedReader newBufferedReader(Path path, Charset cs)
          throws IOException
      {
          CharsetDecoder decoder = cs.newDecoder();
          Reader reader = new InputStreamReader(newInputStream(path), decoder);
          return new BufferedReader(reader);
      }
  
      /**
       * Opens a file for reading, returning a {@code BufferedReader} to read text
       * from the file in an efficient manner. Bytes from the file are decoded into
       * characters using the {@link StandardCharsets#UTF_8 UTF-8} {@link Charset
       * charset}.
       *
       * <p> This method works as if invoking it were equivalent to evaluating the
       * expression:
       * <pre>{@code
       * Files.newBufferedReader(path, StandardCharsets.UTF_8)
       * }</pre>
       *
       * @param   path
       *          the path to the file
       *
       * @return  a new buffered reader, with default buffer size, to read text
       *          from the file
       *
       * @throws  IOException
       *          if an I/O error occurs opening the file
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkRead(String) checkRead}
       *          method is invoked to check read access to the file.
       *
       * @since 1.8
       */
      public static BufferedReader newBufferedReader(Path path) throws IOException {
          return newBufferedReader(path, StandardCharsets.UTF_8);
      }
  
      /**
       * Opens or creates a file for writing, returning a {@code BufferedWriter}
       * that may be used to write text to the file in an efficient manner.
       * The {@code options} parameter specifies how the the file is created or
       * opened. If no options are present then this method works as if the {@link
       * StandardOpenOption#CREATE CREATE}, {@link
       * StandardOpenOption#TRUNCATE_EXISTING TRUNCATE_EXISTING}, and {@link
       * StandardOpenOption#WRITE WRITE} options are present. In other words, it
       * opens the file for writing, creating the file if it doesn't exist, or
       * initially truncating an existing {@link #isRegularFile regular-file} to
       * a size of {@code 0} if it exists.
       *
       * <p> The {@code Writer} methods to write text throw {@code IOException}
       * if the text cannot be encoded using the specified charset.
       *
       * @param   path
       *          the path to the file
       * @param   cs
       *          the charset to use for encoding
       * @param   options
       *          options specifying how the file is opened
       *
       * @return  a new buffered writer, with default buffer size, to write text
       *          to the file
       *
       * @throws  IOException
       *          if an I/O error occurs opening or creating the file
       * @throws  UnsupportedOperationException
       *          if an unsupported option is specified
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkWrite(String) checkWrite}
       *          method is invoked to check write access to the file.
       *
       * @see #write(Path,Iterable,Charset,OpenOption[])
       */
      public static BufferedWriter newBufferedWriter(Path path, Charset cs,
                                                     OpenOption... options)
          throws IOException
      {
          CharsetEncoder encoder = cs.newEncoder();
          Writer writer = new OutputStreamWriter(newOutputStream(path, options), encoder);
          return new BufferedWriter(writer);
      }
  
      /**
       * Opens or creates a file for writing, returning a {@code BufferedWriter}
       * to write text to the file in an efficient manner. The text is encoded
       * into bytes for writing using the {@link StandardCharsets#UTF_8 UTF-8}
       * {@link Charset charset}.
       *
       * <p> This method works as if invoking it were equivalent to evaluating the
       * expression:
       * <pre>{@code
       * Files.newBufferedWriter(path, StandardCharsets.UTF_8, options)
       * }</pre>
       *
       * @param   path
       *          the path to the file
       * @param   options
       *          options specifying how the file is opened
       *
       * @return  a new buffered writer, with default buffer size, to write text
       *          to the file
       *
       * @throws  IOException
       *          if an I/O error occurs opening or creating the file
       * @throws  UnsupportedOperationException
       *          if an unsupported option is specified
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkWrite(String) checkWrite}
       *          method is invoked to check write access to the file.
       *
       * @since 1.8
       */
      public static BufferedWriter newBufferedWriter(Path path, OpenOption... options) throws IOException {
          return newBufferedWriter(path, StandardCharsets.UTF_8, options);
      }
  
      /**
       * Reads all bytes from an input stream and writes them to an output stream.
       */
      private static long copy(InputStream source, OutputStream sink)
          throws IOException
      {
          long nread = 0L;
          byte[] buf = new byte[BUFFER_SIZE];
          int n;
          while ((n = source.read(buf)) > 0) {
              sink.write(buf, 0, n);
              nread += n;
          }
          return nread;
      }
  
      /**
       * Copies all bytes from an input stream to a file. On return, the input
       * stream will be at end of stream.
       *
       * <p> By default, the copy fails if the target file already exists or is a
       * symbolic link. If the {@link StandardCopyOption#REPLACE_EXISTING
       * REPLACE_EXISTING} option is specified, and the target file already exists,
       * then it is replaced if it is not a non-empty directory. If the target
       * file exists and is a symbolic link, then the symbolic link is replaced.
       * In this release, the {@code REPLACE_EXISTING} option is the only option
       * required to be supported by this method. Additional options may be
       * supported in future releases.
       *
       * <p>  If an I/O error occurs reading from the input stream or writing to
       * the file, then it may do so after the target file has been created and
       * after some bytes have been read or written. Consequently the input
       * stream may not be at end of stream and may be in an inconsistent state.
       * It is strongly recommended that the input stream be promptly closed if an
       * I/O error occurs.
       *
       * <p> This method may block indefinitely reading from the input stream (or
       * writing to the file). The behavior for the case that the input stream is
       * <i>asynchronously closed</i> or the thread interrupted during the copy is
       * highly input stream and file system provider specific and therefore not
       * specified.
       *
       * <p> <b>Usage example</b>: Suppose we want to capture a web page and save
       * it to a file:
       * <pre>
       *     Path path = ...
       *     URI u = URI.create("http://java.sun.com/");
       *     try (InputStream in = u.toURL().openStream()) {
       *         Files.copy(in, path);
       *     }
       * </pre>
       *
       * @param   in
       *          the input stream to read from
       * @param   target
       *          the path to the file
       * @param   options
       *          options specifying how the copy should be done
       *
       * @return  the number of bytes read or written
       *
       * @throws  IOException
       *          if an I/O error occurs when reading or writing
       * @throws  FileAlreadyExistsException
       *          if the target file exists but cannot be replaced because the
       *          {@code REPLACE_EXISTING} option is not specified <i>(optional
       *          specific exception)</i>
       * @throws  DirectoryNotEmptyException
       *          the {@code REPLACE_EXISTING} option is specified but the file
       *          cannot be replaced because it is a non-empty directory
       *          <i>(optional specific exception)</i>     *
       * @throws  UnsupportedOperationException
       *          if {@code options} contains a copy option that is not supported
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkWrite(String) checkWrite}
       *          method is invoked to check write access to the file. Where the
       *          {@code REPLACE_EXISTING} option is specified, the security
       *          manager's {@link SecurityManager#checkDelete(String) checkDelete}
       *          method is invoked to check that an existing file can be deleted.
       */
      public static long copy(InputStream in, Path target, CopyOption... options)
          throws IOException
      {
          // ensure not null before opening file
          Objects.requireNonNull(in);
  
          // check for REPLACE_EXISTING
          boolean replaceExisting = false;
          for (CopyOption opt: options) {
              if (opt == StandardCopyOption.REPLACE_EXISTING) {
                  replaceExisting = true;
              } else {
                  if (opt == null) {
                      throw new NullPointerException("options contains 'null'");
                  }  else {
                      throw new UnsupportedOperationException(opt + " not supported");
                  }
              }
          }
  
          // attempt to delete an existing file
          SecurityException se = null;
          if (replaceExisting) {
              try {
                  deleteIfExists(target);
              } catch (SecurityException x) {
                  se = x;
              }
          }
  
          // attempt to create target file. If it fails with
          // FileAlreadyExistsException then it may be because the security
          // manager prevented us from deleting the file, in which case we just
          // throw the SecurityException.
          OutputStream ostream;
          try {
              ostream = newOutputStream(target, StandardOpenOption.CREATE_NEW,
                                                StandardOpenOption.WRITE);
          } catch (FileAlreadyExistsException x) {
              if (se != null)
                  throw se;
              // someone else won the race and created the file
              throw x;
          }
  
          // do the copy
          try (OutputStream out = ostream) {
              return copy(in, out);
          }
      }
  
      /**
       * Copies all bytes from a file to an output stream.
       *
       * <p> If an I/O error occurs reading from the file or writing to the output
       * stream, then it may do so after some bytes have been read or written.
       * Consequently the output stream may be in an inconsistent state. It is
       * strongly recommended that the output stream be promptly closed if an I/O
       * error occurs.
       *
       * <p> This method may block indefinitely writing to the output stream (or
       * reading from the file). The behavior for the case that the output stream
       * is <i>asynchronously closed</i> or the thread interrupted during the copy
       * is highly output stream and file system provider specific and therefore
       * not specified.
       *
       * <p> Note that if the given output stream is {@link java.io.Flushable}
       * then its {@link java.io.Flushable#flush flush} method may need to invoked
       * after this method completes so as to flush any buffered output.
       *
       * @param   source
       *          the  path to the file
       * @param   out
       *          the output stream to write to
       *
       * @return  the number of bytes read or written
       *
       * @throws  IOException
       *          if an I/O error occurs when reading or writing
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkRead(String) checkRead}
       *          method is invoked to check read access to the file.
       */
      public static long copy(Path source, OutputStream out) throws IOException {
          // ensure not null before opening file
          Objects.requireNonNull(out);
  
          try (InputStream in = newInputStream(source)) {
              return copy(in, out);
          }
      }
  
      /**
       * The maximum size of array to allocate.
       * Some VMs reserve some header words in an array.
       * Attempts to allocate larger arrays may result in
       * OutOfMemoryError: Requested array size exceeds VM limit
       */
      private static final int MAX_BUFFER_SIZE = Integer.MAX_VALUE - 8;
  
      /**
       * Reads all the bytes from an input stream. Uses {@code initialSize} as a hint
       * about how many bytes the stream will have.
       *
       * @param   source
       *          the input stream to read from
       * @param   initialSize
       *          the initial size of the byte array to allocate
       *
       * @return  a byte array containing the bytes read from the file
       *
       * @throws  IOException
       *          if an I/O error occurs reading from the stream
       * @throws  OutOfMemoryError
       *          if an array of the required size cannot be allocated
       */
      private static byte[] read(InputStream source, int initialSize) throws IOException {
          int capacity = initialSize;
          byte[] buf = new byte[capacity];
          int nread = 0;
          int n;
          for (;;) {
              // read to EOF which may read more or less than initialSize (eg: file
              // is truncated while we are reading)
              while ((n = source.read(buf, nread, capacity - nread)) > 0)
                  nread += n;
  
              // if last call to source.read() returned -1, we are done
              // otherwise, try to read one more byte; if that failed we're done too
              if (n < 0 || (n = source.read()) < 0)
                  break;
  
              // one more byte was read; need to allocate a larger buffer
              if (capacity <= MAX_BUFFER_SIZE - capacity) {
                  capacity = Math.max(capacity << 1, BUFFER_SIZE);
              } else {
                  if (capacity == MAX_BUFFER_SIZE)
                      throw new OutOfMemoryError("Required array size too large");
                  capacity = MAX_BUFFER_SIZE;
              }
              buf = Arrays.copyOf(buf, capacity);
              buf[nread++] = (byte)n;
          }
          return (capacity == nread) ? buf : Arrays.copyOf(buf, nread);
      }
  
      /**
       * Reads all the bytes from a file. The method ensures that the file is
       * closed when all bytes have been read or an I/O error, or other runtime
       * exception, is thrown.
       *
       * <p> Note that this method is intended for simple cases where it is
       * convenient to read all bytes into a byte array. It is not intended for
       * reading in large files.
       *
       * @param   path
       *          the path to the file
       *
       * @return  a byte array containing the bytes read from the file
       *
       * @throws  IOException
       *          if an I/O error occurs reading from the stream
       * @throws  OutOfMemoryError
       *          if an array of the required size cannot be allocated, for
       *          example the file is larger that {@code 2GB}
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkRead(String) checkRead}
       *          method is invoked to check read access to the file.
       */
      public static byte[] readAllBytes(Path path) throws IOException {
          try (SeekableByteChannel sbc = Files.newByteChannel(path);
               InputStream in = Channels.newInputStream(sbc)) {
              long size = sbc.size();
              if (size > (long)MAX_BUFFER_SIZE)
                  throw new OutOfMemoryError("Required array size too large");
  
              return read(in, (int)size);
          }
      }
  
      /**
       * Read all lines from a file. This method ensures that the file is
       * closed when all bytes have been read or an I/O error, or other runtime
       * exception, is thrown. Bytes from the file are decoded into characters
       * using the specified charset.
       *
       * <p> This method recognizes the following as line terminators:
       * <ul>
       *   <li> <code>&#92;u000D</code> followed by <code>&#92;u000A</code>,
       *     CARRIAGE RETURN followed by LINE FEED </li>
       *   <li> <code>&#92;u000A</code>, LINE FEED </li>
       *   <li> <code>&#92;u000D</code>, CARRIAGE RETURN </li>
       * </ul>
       * <p> Additional Unicode line terminators may be recognized in future
       * releases.
       *
       * <p> Note that this method is intended for simple cases where it is
       * convenient to read all lines in a single operation. It is not intended
       * for reading in large files.
       *
       * @param   path
       *          the path to the file
       * @param   cs
       *          the charset to use for decoding
       *
       * @return  the lines from the file as a {@code List}; whether the {@code
       *          List} is modifiable or not is implementation dependent and
       *          therefore not specified
       *
       * @throws  IOException
       *          if an I/O error occurs reading from the file or a malformed or
       *          unmappable byte sequence is read
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkRead(String) checkRead}
       *          method is invoked to check read access to the file.
       *
       * @see #newBufferedReader
       */
      public static List<String> readAllLines(Path path, Charset cs) throws IOException {
          try (BufferedReader reader = newBufferedReader(path, cs)) {
              List<String> result = new ArrayList<>();
              for (;;) {
                  String line = reader.readLine();
                  if (line == null)
                      break;
                  result.add(line);
              }
              return result;
          }
      }
  
      /**
       * Read all lines from a file. Bytes from the file are decoded into characters
       * using the {@link StandardCharsets#UTF_8 UTF-8} {@link Charset charset}.
       *
       * <p> This method works as if invoking it were equivalent to evaluating the
       * expression:
       * <pre>{@code
       * Files.readAllLines(path, StandardCharsets.UTF_8)
       * }</pre>
       *
       * @param   path
       *          the path to the file
       *
       * @return  the lines from the file as a {@code List}; whether the {@code
       *          List} is modifiable or not is implementation dependent and
       *          therefore not specified
       *
       * @throws  IOException
       *          if an I/O error occurs reading from the file or a malformed or
       *          unmappable byte sequence is read
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkRead(String) checkRead}
       *          method is invoked to check read access to the file.
       *
       * @since 1.8
       */
      public static List<String> readAllLines(Path path) throws IOException {
          return readAllLines(path, StandardCharsets.UTF_8);
      }
  
      /**
       * Writes bytes to a file. The {@code options} parameter specifies how the
       * the file is created or opened. If no options are present then this method
       * works as if the {@link StandardOpenOption#CREATE CREATE}, {@link
       * StandardOpenOption#TRUNCATE_EXISTING TRUNCATE_EXISTING}, and {@link
       * StandardOpenOption#WRITE WRITE} options are present. In other words, it
       * opens the file for writing, creating the file if it doesn't exist, or
       * initially truncating an existing {@link #isRegularFile regular-file} to
       * a size of {@code 0}. All bytes in the byte array are written to the file.
       * The method ensures that the file is closed when all bytes have been
       * written (or an I/O error or other runtime exception is thrown). If an I/O
       * error occurs then it may do so after the file has created or truncated,
       * or after some bytes have been written to the file.
       *
       * <p> <b>Usage example</b>: By default the method creates a new file or
       * overwrites an existing file. Suppose you instead want to append bytes
       * to an existing file:
       * <pre>
       *     Path path = ...
       *     byte[] bytes = ...
       *     Files.write(path, bytes, StandardOpenOption.APPEND);
       * </pre>
       *
       * @param   path
       *          the path to the file
       * @param   bytes
       *          the byte array with the bytes to write
       * @param   options
       *          options specifying how the file is opened
       *
       * @return  the path
       *
       * @throws  IOException
       *          if an I/O error occurs writing to or creating the file
       * @throws  UnsupportedOperationException
       *          if an unsupported option is specified
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkWrite(String) checkWrite}
       *          method is invoked to check write access to the file.
       */
      public static Path write(Path path, byte[] bytes, OpenOption... options)
          throws IOException
      {
          // ensure bytes is not null before opening file
          Objects.requireNonNull(bytes);
  
          try (OutputStream out = Files.newOutputStream(path, options)) {
              int len = bytes.length;
              int rem = len;
              while (rem > 0) {
                  int n = Math.min(rem, BUFFER_SIZE);
                  out.write(bytes, (len-rem), n);
                  rem -= n;
              }
          }
          return path;
      }
  
      /**
       * Write lines of text to a file. Each line is a char sequence and is
       * written to the file in sequence with each line terminated by the
       * platform's line separator, as defined by the system property {@code
       * line.separator}. Characters are encoded into bytes using the specified
       * charset.
       *
       * <p> The {@code options} parameter specifies how the the file is created
       * or opened. If no options are present then this method works as if the
       * {@link StandardOpenOption#CREATE CREATE}, {@link
       * StandardOpenOption#TRUNCATE_EXISTING TRUNCATE_EXISTING}, and {@link
       * StandardOpenOption#WRITE WRITE} options are present. In other words, it
       * opens the file for writing, creating the file if it doesn't exist, or
       * initially truncating an existing {@link #isRegularFile regular-file} to
       * a size of {@code 0}. The method ensures that the file is closed when all
       * lines have been written (or an I/O error or other runtime exception is
       * thrown). If an I/O error occurs then it may do so after the file has
       * created or truncated, or after some bytes have been written to the file.
       *
       * @param   path
       *          the path to the file
       * @param   lines
       *          an object to iterate over the char sequences
       * @param   cs
       *          the charset to use for encoding
       * @param   options
       *          options specifying how the file is opened
       *
       * @return  the path
       *
       * @throws  IOException
       *          if an I/O error occurs writing to or creating the file, or the
       *          text cannot be encoded using the specified charset
       * @throws  UnsupportedOperationException
       *          if an unsupported option is specified
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkWrite(String) checkWrite}
       *          method is invoked to check write access to the file.
       */
      public static Path write(Path path, Iterable<? extends CharSequence> lines,
                               Charset cs, OpenOption... options)
          throws IOException
      {
          // ensure lines is not null before opening file
          Objects.requireNonNull(lines);
          CharsetEncoder encoder = cs.newEncoder();
          OutputStream out = newOutputStream(path, options);
          try (BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(out, encoder))) {
              for (CharSequence line: lines) {
                  writer.append(line);
                  writer.newLine();
              }
          }
          return path;
      }
  
      /**
       * Write lines of text to a file. Characters are encoded into bytes using
       * the {@link StandardCharsets#UTF_8 UTF-8} {@link Charset charset}.
       *
       * <p> This method works as if invoking it were equivalent to evaluating the
       * expression:
       * <pre>{@code
       * Files.write(path, lines, StandardCharsets.UTF_8, options);
       * }</pre>
       *
       * @param   path
       *          the path to the file
       * @param   lines
       *          an object to iterate over the char sequences
       * @param   options
       *          options specifying how the file is opened
       *
       * @return  the path
       *
       * @throws  IOException
       *          if an I/O error occurs writing to or creating the file, or the
       *          text cannot be encoded as {@code UTF-8}
       * @throws  UnsupportedOperationException
       *          if an unsupported option is specified
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkWrite(String) checkWrite}
       *          method is invoked to check write access to the file.
       *
       * @since 1.8
       */
      public static Path write(Path path,
                               Iterable<? extends CharSequence> lines,
                               OpenOption... options)
          throws IOException
      {
          return write(path, lines, StandardCharsets.UTF_8, options);
      }
  
      // -- Stream APIs --
  
      /**
       * Return a lazily populated {@code Stream}, the elements of
       * which are the entries in the directory.  The listing is not recursive.
       *
       * <p> The elements of the stream are {@link Path} objects that are
       * obtained as if by {@link Path#resolve(Path) resolving} the name of the
       * directory entry against {@code dir}. Some file systems maintain special
       * links to the directory itself and the directory's parent directory.
       * Entries representing these links are not included.
       *
       * <p> The stream is <i>weakly consistent</i>. It is thread safe but does
       * not freeze the directory while iterating, so it may (or may not)
       * reflect updates to the directory that occur after returning from this
       * method.
       *
       * <p> The returned stream encapsulates a {@link DirectoryStream}.
       * If timely disposal of file system resources is required, the
       * {@code try}-with-resources construct should be used to ensure that the
       * stream's {@link Stream#close close} method is invoked after the stream
       * operations are completed.
       *
       * <p> Operating on a closed stream behaves as if the end of stream
       * has been reached. Due to read-ahead, one or more elements may be
       * returned after the stream has been closed.
       *
       * <p> If an {@link IOException} is thrown when accessing the directory
       * after this method has returned, it is wrapped in an {@link
       * UncheckedIOException} which will be thrown from the method that caused
       * the access to take place.
       *
       * @param   dir  The path to the directory
       *
       * @return  The {@code Stream} describing the content of the
       *          directory
       *
       * @throws  NotDirectoryException
       *          if the file could not otherwise be opened because it is not
       *          a directory <i>(optional specific exception)</i>
       * @throws  IOException
       *          if an I/O error occurs when opening the directory
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkRead(String) checkRead}
       *          method is invoked to check read access to the directory.
       *
       * @see     #newDirectoryStream(Path)
       * @since   1.8
       */
      public static Stream<Path> list(Path dir) throws IOException {
          DirectoryStream<Path> ds = Files.newDirectoryStream(dir);
          try {
              final Iterator<Path> delegate = ds.iterator();
  
              // Re-wrap DirectoryIteratorException to UncheckedIOException
              Iterator<Path> it = new Iterator<Path>() {
                  @Override
                  public boolean hasNext() {
                      try {
                          return delegate.hasNext();
                      } catch (DirectoryIteratorException e) {
                          throw new UncheckedIOException(e.getCause());
                      }
                  }
                  @Override
                  public Path next() {
                      try {
                          return delegate.next();
                      } catch (DirectoryIteratorException e) {
                          throw new UncheckedIOException(e.getCause());
                      }
                  }
              };
  
              return StreamSupport.stream(Spliterators.spliteratorUnknownSize(it, Spliterator.DISTINCT), false)
                                  .onClose(asUncheckedRunnable(ds));
          } catch (Error|RuntimeException e) {
              try {
                  ds.close();
              } catch (IOException ex) {
                  try {
                      e.addSuppressed(ex);
                  } catch (Throwable ignore) {}
              }
              throw e;
          }
      }
  
      /**
       * Return a {@code Stream} that is lazily populated with {@code
       * Path} by walking the file tree rooted at a given starting file.  The
       * file tree is traversed <em>depth-first</em>, the elements in the stream
       * are {@link Path} objects that are obtained as if by {@link
       * Path#resolve(Path) resolving} the relative path against {@code start}.
       *
       * <p> The {@code stream} walks the file tree as elements are consumed.
       * The {@code Stream} returned is guaranteed to have at least one
       * element, the starting file itself. For each file visited, the stream
       * attempts to read its {@link BasicFileAttributes}. If the file is a
       * directory and can be opened successfully, entries in the directory, and
       * their <em>descendants</em> will follow the directory in the stream as
       * they are encountered. When all entries have been visited, then the
       * directory is closed. The file tree walk then continues at the next
       * <em>sibling</em> of the directory.
       *
       * <p> The stream is <i>weakly consistent</i>. It does not freeze the
       * file tree while iterating, so it may (or may not) reflect updates to
       * the file tree that occur after returned from this method.
       *
       * <p> By default, symbolic links are not automatically followed by this
       * method. If the {@code options} parameter contains the {@link
       * FileVisitOption#FOLLOW_LINKS FOLLOW_LINKS} option then symbolic links are
       * followed. When following links, and the attributes of the target cannot
       * be read, then this method attempts to get the {@code BasicFileAttributes}
       * of the link.
       *
       * <p> If the {@code options} parameter contains the {@link
       * FileVisitOption#FOLLOW_LINKS FOLLOW_LINKS} option then the stream keeps
       * track of directories visited so that cycles can be detected. A cycle
       * arises when there is an entry in a directory that is an ancestor of the
       * directory. Cycle detection is done by recording the {@link
       * BasicFileAttributes#fileKey file-key} of directories,
       * or if file keys are not available, by invoking the {@link #isSameFile
       * isSameFile} method to test if a directory is the same file as an
       * ancestor. When a cycle is detected it is treated as an I/O error with
       * an instance of {@link FileSystemLoopException}.
       *
       * <p> The {@code maxDepth} parameter is the maximum number of levels of
       * directories to visit. A value of {@code 0} means that only the starting
       * file is visited, unless denied by the security manager. A value of
       * {@link Integer#MAX_VALUE MAX_VALUE} may be used to indicate that all
       * levels should be visited.
       *
       * <p> When a security manager is installed and it denies access to a file
       * (or directory), then it is ignored and not included in the stream.
       *
       * <p> The returned stream encapsulates one or more {@link DirectoryStream}s.
       * If timely disposal of file system resources is required, the
       * {@code try}-with-resources construct should be used to ensure that the
       * stream's {@link Stream#close close} method is invoked after the stream
       * operations are completed.  Operating on a closed stream will result in an
       * {@link IllegalStateException}.
       *
       * <p> If an {@link IOException} is thrown when accessing the directory
       * after this method has returned, it is wrapped in an {@link
       * UncheckedIOException} which will be thrown from the method that caused
       * the access to take place.
       *
       * @param   start
       *          the starting file
       * @param   maxDepth
       *          the maximum number of directory levels to visit
       * @param   options
       *          options to configure the traversal
       *
       * @return  the {@link Stream} of {@link Path}
       *
       * @throws  IllegalArgumentException
       *          if the {@code maxDepth} parameter is negative
       * @throws  SecurityException
       *          If the security manager denies access to the starting file.
       *          In the case of the default provider, the {@link
       *          SecurityManager#checkRead(String) checkRead} method is invoked
       *          to check read access to the directory.
       * @throws  IOException
       *          if an I/O error is thrown when accessing the starting file.
       * @since   1.8
       */
      public static Stream<Path> walk(Path start,
                                      int maxDepth,
                                      FileVisitOption... options)
          throws IOException
      {
          FileTreeIterator iterator = new FileTreeIterator(start, maxDepth, options);
          try {
              return StreamSupport.stream(Spliterators.spliteratorUnknownSize(iterator, Spliterator.DISTINCT), false)
                                  .onClose(iterator::close)
                                  .map(entry -> entry.file());
          } catch (Error|RuntimeException e) {
              iterator.close();
              throw e;
          }
      }
  
      /**
       * Return a {@code Stream} that is lazily populated with {@code
       * Path} by walking the file tree rooted at a given starting file.  The
       * file tree is traversed <em>depth-first</em>, the elements in the stream
       * are {@link Path} objects that are obtained as if by {@link
       * Path#resolve(Path) resolving} the relative path against {@code start}.
       *
       * <p> This method works as if invoking it were equivalent to evaluating the
       * expression:
       * <blockquote><pre>
       * walk(start, Integer.MAX_VALUE, options)
       * </pre></blockquote>
       * In other words, it visits all levels of the file tree.
       *
       * <p> The returned stream encapsulates one or more {@link DirectoryStream}s.
       * If timely disposal of file system resources is required, the
       * {@code try}-with-resources construct should be used to ensure that the
       * stream's {@link Stream#close close} method is invoked after the stream
       * operations are completed.  Operating on a closed stream will result in an
       * {@link IllegalStateException}.
       *
       * @param   start
       *          the starting file
       * @param   options
       *          options to configure the traversal
       *
       * @return  the {@link Stream} of {@link Path}
       *
       * @throws  SecurityException
       *          If the security manager denies access to the starting file.
       *          In the case of the default provider, the {@link
       *          SecurityManager#checkRead(String) checkRead} method is invoked
       *          to check read access to the directory.
       * @throws  IOException
       *          if an I/O error is thrown when accessing the starting file.
       *
       * @see     #walk(Path, int, FileVisitOption...)
       * @since   1.8
       */
      public static Stream<Path> walk(Path start, FileVisitOption... options) throws IOException {
          return walk(start, Integer.MAX_VALUE, options);
      }
  
      /**
       * Return a {@code Stream} that is lazily populated with {@code
       * Path} by searching for files in a file tree rooted at a given starting
       * file.
       *
       * <p> This method walks the file tree in exactly the manner specified by
       * the {@link #walk walk} method. For each file encountered, the given
       * {@link BiPredicate} is invoked with its {@link Path} and {@link
       * BasicFileAttributes}. The {@code Path} object is obtained as if by
       * {@link Path#resolve(Path) resolving} the relative path against {@code
       * start} and is only included in the returned {@link Stream} if
       * the {@code BiPredicate} returns true. Compare to calling {@link
       * Stream#filter filter} on the {@code Stream}
       * returned by {@code walk} method, this method may be more efficient by
       * avoiding redundant retrieval of the {@code BasicFileAttributes}.
       *
       * <p> The returned stream encapsulates one or more {@link DirectoryStream}s.
       * If timely disposal of file system resources is required, the
       * {@code try}-with-resources construct should be used to ensure that the
       * stream's {@link Stream#close close} method is invoked after the stream
       * operations are completed.  Operating on a closed stream will result in an
       * {@link IllegalStateException}.
       *
       * <p> If an {@link IOException} is thrown when accessing the directory
       * after returned from this method, it is wrapped in an {@link
       * UncheckedIOException} which will be thrown from the method that caused
       * the access to take place.
       *
       * @param   start
       *          the starting file
       * @param   maxDepth
       *          the maximum number of directory levels to search
       * @param   matcher
       *          the function used to decide whether a file should be included
       *          in the returned stream
       * @param   options
       *          options to configure the traversal
       *
       * @return  the {@link Stream} of {@link Path}
       *
       * @throws  IllegalArgumentException
       *          if the {@code maxDepth} parameter is negative
       * @throws  SecurityException
       *          If the security manager denies access to the starting file.
       *          In the case of the default provider, the {@link
       *          SecurityManager#checkRead(String) checkRead} method is invoked
       *          to check read access to the directory.
       * @throws  IOException
       *          if an I/O error is thrown when accessing the starting file.
       *
       * @see     #walk(Path, int, FileVisitOption...)
       * @since   1.8
       */
      public static Stream<Path> find(Path start,
                                      int maxDepth,
                                      BiPredicate<Path, BasicFileAttributes> matcher,
                                      FileVisitOption... options)
          throws IOException
      {
          FileTreeIterator iterator = new FileTreeIterator(start, maxDepth, options);
          try {
              return StreamSupport.stream(Spliterators.spliteratorUnknownSize(iterator, Spliterator.DISTINCT), false)
                                  .onClose(iterator::close)
                                  .filter(entry -> matcher.test(entry.file(), entry.attributes()))
                                  .map(entry -> entry.file());
          } catch (Error|RuntimeException e) {
              iterator.close();
              throw e;
          }
      }
  
      /**
       * Read all lines from a file as a {@code Stream}. Unlike {@link
       * #readAllLines(Path, Charset) readAllLines}, this method does not read
       * all lines into a {@code List}, but instead populates lazily as the stream
       * is consumed.
       *
       * <p> Bytes from the file are decoded into characters using the specified
       * charset and the same line terminators as specified by {@code
       * readAllLines} are supported.
       *
       * <p> After this method returns, then any subsequent I/O exception that
       * occurs while reading from the file or when a malformed or unmappable byte
       * sequence is read, is wrapped in an {@link UncheckedIOException} that will
       * be thrown from the
       * {@link Stream} method that caused the read to take
       * place. In case an {@code IOException} is thrown when closing the file,
       * it is also wrapped as an {@code UncheckedIOException}.
       *
       * <p> The returned stream encapsulates a {@link Reader}.  If timely
       * disposal of file system resources is required, the try-with-resources
       * construct should be used to ensure that the stream's
       * {@link Stream#close close} method is invoked after the stream operations
       * are completed.
       *
       *
       * @param   path
       *          the path to the file
       * @param   cs
       *          the charset to use for decoding
       *
       * @return  the lines from the file as a {@code Stream}
       *
       * @throws  IOException
       *          if an I/O error occurs opening the file
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkRead(String) checkRead}
       *          method is invoked to check read access to the file.
       *
       * @see     #readAllLines(Path, Charset)
       * @see     #newBufferedReader(Path, Charset)
       * @see     BufferedReader#lines()
       * @since   1.8
       */
      public static Stream<String> lines(Path path, Charset cs) throws IOException {
          BufferedReader br = Files.newBufferedReader(path, cs);
          try {
              return br.lines().onClose(asUncheckedRunnable(br));
          } catch (Error|RuntimeException e) {
              try {
                  br.close();
              } catch (IOException ex) {
                  try {
                      e.addSuppressed(ex);
                  } catch (Throwable ignore) {}
              }
              throw e;
          }
      }
  
      /**
       * Read all lines from a file as a {@code Stream}. Bytes from the file are
       * decoded into characters using the {@link StandardCharsets#UTF_8 UTF-8}
       * {@link Charset charset}.
       *
       * <p> This method works as if invoking it were equivalent to evaluating the
       * expression:
       * <pre>{@code
       * Files.lines(path, StandardCharsets.UTF_8)
       * }</pre>
       *
       * @param   path
       *          the path to the file
       *
       * @return  the lines from the file as a {@code Stream}
       *
       * @throws  IOException
       *          if an I/O error occurs opening the file
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, the {@link SecurityManager#checkRead(String) checkRead}
       *          method is invoked to check read access to the file.
       *
       * @since 1.8
       */
      public static Stream<String> lines(Path path) throws IOException {
          return lines(path, StandardCharsets.UTF_8);
      }
  }
  