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   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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   *
   * 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).
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   * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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  package java.nio.file;
  
  import java.nio.file.attribute.*;
  import java.nio.file.spi.FileSystemProvider;
  import java.util.Set;
  import java.io.Closeable;
  import java.io.IOException;
  
  /**
   * Provides an interface to a file system and is the factory for objects to
   * access files and other objects in the file system.
   *
   * <p> The default file system, obtained by invoking the {@link FileSystems#getDefault
   * FileSystems.getDefault} method, provides access to the file system that is
   * accessible to the Java virtual machine. The {@link FileSystems} class defines
   * methods to create file systems that provide access to other types of (custom)
   * file systems.
   *
   * <p> A file system is the factory for several types of objects:
   *
   * <ul>
   *   <li><p> The {@link #getPath getPath} method converts a system dependent
   *     <em>path string</em>, returning a {@link Path} object that may be used
   *     to locate and access a file. </p></li>
   *   <li><p> The {@link #getPathMatcher  getPathMatcher} method is used
   *     to create a {@link PathMatcher} that performs match operations on
   *     paths. </p></li>
   *   <li><p> The {@link #getFileStores getFileStores} method returns an iterator
   *     over the underlying {@link FileStore file-stores}. </p></li>
   *   <li><p> The {@link #getUserPrincipalLookupService getUserPrincipalLookupService}
   *     method returns the {@link UserPrincipalLookupService} to lookup users or
   *     groups by name. </p></li>
   *   <li><p> The {@link #newWatchService newWatchService} method creates a
   *     {@link WatchService} that may be used to watch objects for changes and
   *     events. </p></li>
   * </ul>
   *
   * <p> File systems vary greatly. In some cases the file system is a single
   * hierarchy of files with one top-level root directory. In other cases it may
   * have several distinct file hierarchies, each with its own top-level root
   * directory. The {@link #getRootDirectories getRootDirectories} method may be
   * used to iterate over the root directories in the file system. A file system
   * is typically composed of one or more underlying {@link FileStore file-stores}
   * that provide the storage for the files. Theses file stores can also vary in
   * the features they support, and the file attributes or <em>meta-data</em> that
   * they associate with files.
   *
   * <p> A file system is open upon creation and can be closed by invoking its
   * {@link #close() close} method. Once closed, any further attempt to access
   * objects in the file system cause {@link ClosedFileSystemException} to be
   * thrown. File systems created by the default {@link FileSystemProvider provider}
   * cannot be closed.
   *
   * <p> A {@code FileSystem} can provide read-only or read-write access to the
   * file system. Whether or not a file system provides read-only access is
   * established when the {@code FileSystem} is created and can be tested by invoking
   * its {@link #isReadOnly() isReadOnly} method. Attempts to write to file stores
   * by means of an object associated with a read-only file system throws {@link
   * ReadOnlyFileSystemException}.
   *
   * <p> File systems are safe for use by multiple concurrent threads. The {@link
   * #close close} method may be invoked at any time to close a file system but
   * whether a file system is <i>asynchronously closeable</i> is provider specific
   * and therefore unspecified. In other words, if a thread is accessing an
   * object in a file system, and another thread invokes the {@code close} method
   * then it may require to block until the first operation is complete. Closing
   * a file system causes all open channels, watch services, and other {@link
   * Closeable closeable} objects associated with the file system to be closed.
   *
   * @since 1.7
   */
  
  public abstract class FileSystem
      implements Closeable
  {
      /**
       * Initializes a new instance of this class.
       */
      protected FileSystem() {
      }
  
      /**
       * Returns the provider that created this file system.
       *
       * @return  The provider that created this file system.
       */
      public abstract FileSystemProvider provider();
  
      /**
       * Closes this file system.
       *
       * <p> After a file system is closed then all subsequent access to the file
       * system, either by methods defined by this class or on objects associated
       * with this file system, throw {@link ClosedFileSystemException}. If the
       * file system is already closed then invoking this method has no effect.
       *
       * <p> Closing a file system will close all open {@link
       * java.nio.channels.Channel channels}, {@link DirectoryStream directory-streams},
       * {@link WatchService watch-service}, and other closeable objects associated
       * with this file system. The {@link FileSystems#getDefault default} file
       * system cannot be closed.
       *
       * @throws  IOException
       *          If an I/O error occurs
       * @throws  UnsupportedOperationException
       *          Thrown in the case of the default file system
       */
      @Override
      public abstract void close() throws IOException;
  
      /**
       * Tells whether or not this file system is open.
       *
       * <p> File systems created by the default provider are always open.
       *
       * @return  {@code true} if, and only if, this file system is open
       */
      public abstract boolean isOpen();
  
      /**
       * Tells whether or not this file system allows only read-only access to
       * its file stores.
       *
       * @return  {@code true} if, and only if, this file system provides
       *          read-only access
       */
      public abstract boolean isReadOnly();
  
      /**
       * Returns the name separator, represented as a string.
       *
       * <p> The name separator is used to separate names in a path string. An
       * implementation may support multiple name separators in which case this
       * method returns an implementation specific <em>default</em> name separator.
       * This separator is used when creating path strings by invoking the {@link
       * Path#toString() toString()} method.
       *
       * <p> In the case of the default provider, this method returns the same
       * separator as {@link java.io.File#separator}.
       *
       * @return  The name separator
       */
      public abstract String getSeparator();
  
      /**
       * Returns an object to iterate over the paths of the root directories.
       *
       * <p> A file system provides access to a file store that may be composed
       * of a number of distinct file hierarchies, each with its own top-level
       * root directory. Unless denied by the security manager, each element in
       * the returned iterator corresponds to the root directory of a distinct
       * file hierarchy. The order of the elements is not defined. The file
       * hierarchies may change during the lifetime of the Java virtual machine.
       * For example, in some implementations, the insertion of removable media
       * may result in the creation of a new file hierarchy with its own
       * top-level directory.
       *
       * <p> When a security manager is installed, it is invoked to check access
       * to the each root directory. If denied, the root directory is not returned
       * by the iterator. In the case of the default provider, the {@link
       * SecurityManager#checkRead(String)} method is invoked to check read access
       * to each root directory. It is system dependent if the permission checks
       * are done when the iterator is obtained or during iteration.
       *
       * @return  An object to iterate over the root directories
       */
      public abstract Iterable<Path> getRootDirectories();
  
      /**
       * Returns an object to iterate over the underlying file stores.
       *
       * <p> The elements of the returned iterator are the {@link
       * FileStore FileStores} for this file system. The order of the elements is
       * not defined and the file stores may change during the lifetime of the
       * Java virtual machine. When an I/O error occurs, perhaps because a file
       * store is not accessible, then it is not returned by the iterator.
       *
       * <p> In the case of the default provider, and a security manager is
       * installed, the security manager is invoked to check {@link
       * RuntimePermission}<tt>("getFileStoreAttributes")</tt>. If denied, then
       * no file stores are returned by the iterator. In addition, the security
       * manager's {@link SecurityManager#checkRead(String)} method is invoked to
       * check read access to the file store's <em>top-most</em> directory. If
       * denied, the file store is not returned by the iterator. It is system
       * dependent if the permission checks are done when the iterator is obtained
       * or during iteration.
       *
       * <p> <b>Usage Example:</b>
       * Suppose we want to print the space usage for all file stores:
       * <pre>
       *     for (FileStore store: FileSystems.getDefault().getFileStores()) {
       *         long total = store.getTotalSpace() / 1024;
       *         long used = (store.getTotalSpace() - store.getUnallocatedSpace()) / 1024;
       *         long avail = store.getUsableSpace() / 1024;
       *         System.out.format("%-20s %12d %12d %12d%n", store, total, used, avail);
       *     }
       * </pre>
       *
       * @return  An object to iterate over the backing file stores
       */
      public abstract Iterable<FileStore> getFileStores();
  
      /**
       * Returns the set of the {@link FileAttributeView#name names} of the file
       * attribute views supported by this {@code FileSystem}.
       *
       * <p> The {@link BasicFileAttributeView} is required to be supported and
       * therefore the set contains at least one element, "basic".
       *
       * <p> The {@link FileStore#supportsFileAttributeView(String)
       * supportsFileAttributeView(String)} method may be used to test if an
       * underlying {@link FileStore} supports the file attributes identified by a
       * file attribute view.
       *
       * @return  An unmodifiable set of the names of the supported file attribute
       *          views
       */
      public abstract Set<String> supportedFileAttributeViews();
  
      /**
       * Converts a path string, or a sequence of strings that when joined form
       * a path string, to a {@code Path}. If {@code more} does not specify any
       * elements then the value of the {@code first} parameter is the path string
       * to convert. If {@code more} specifies one or more elements then each
       * non-empty string, including {@code first}, is considered to be a sequence
       * of name elements (see {@link Path}) and is joined to form a path string.
       * The details as to how the Strings are joined is provider specific but
       * typically they will be joined using the {@link #getSeparator
       * name-separator} as the separator. For example, if the name separator is
       * "{@code /}" and {@code getPath("/foo","bar","gus")} is invoked, then the
       * path string {@code "/foo/bar/gus"} is converted to a {@code Path}.
       * A {@code Path} representing an empty path is returned if {@code first}
       * is the empty string and {@code more} does not contain any non-empty
       * strings.
       *
       * <p> The parsing and conversion to a path object is inherently
       * implementation dependent. In the simplest case, the path string is rejected,
       * and {@link InvalidPathException} thrown, if the path string contains
       * characters that cannot be converted to characters that are <em>legal</em>
       * to the file store. For example, on UNIX systems, the NUL (&#92;u0000)
       * character is not allowed to be present in a path. An implementation may
       * choose to reject path strings that contain names that are longer than those
       * allowed by any file store, and where an implementation supports a complex
       * path syntax, it may choose to reject path strings that are <em>badly
       * formed</em>.
       *
       * <p> In the case of the default provider, path strings are parsed based
       * on the definition of paths at the platform or virtual file system level.
       * For example, an operating system may not allow specific characters to be
       * present in a file name, but a specific underlying file store may impose
       * different or additional restrictions on the set of legal
       * characters.
       *
       * <p> This method throws {@link InvalidPathException} when the path string
       * cannot be converted to a path. Where possible, and where applicable,
       * the exception is created with an {@link InvalidPathException#getIndex
       * index} value indicating the first position in the {@code path} parameter
       * that caused the path string to be rejected.
       *
       * @param   first
       *          the path string or initial part of the path string
       * @param   more
       *          additional strings to be joined to form the path string
       *
       * @return  the resulting {@code Path}
       *
       * @throws  InvalidPathException
       *          If the path string cannot be converted
       */
      public abstract Path getPath(String first, String... more);
  
      // Android-changed: Removed javadoc references to UNIX and Windows.
      /**
       * Returns a {@code PathMatcher} that performs match operations on the
       * {@code String} representation of {@link Path} objects by interpreting a
       * given pattern.
       *
       * The {@code syntaxAndPattern} parameter identifies the syntax and the
       * pattern and takes the form:
       * <blockquote><pre>
       * <i>syntax</i><b>:</b><i>pattern</i>
       * </pre></blockquote>
       * where {@code ':'} stands for itself.
       *
       * <p> A {@code FileSystem} implementation supports the "{@code glob}" and
       * "{@code regex}" syntaxes, and may support others. The value of the syntax
       * component is compared without regard to case.
       *
       * <p> When the syntax is "{@code glob}" then the {@code String}
       * representation of the path is matched using a limited pattern language
       * that resembles regular expressions but with a simpler syntax. For example:
       *
       * <blockquote>
       * <table border="0" summary="Pattern Language">
       * <tr>
       *   <td>{@code *.java}</td>
       *   <td>Matches a path that represents a file name ending in {@code .java}</td>
       * </tr>
       * <tr>
       *   <td>{@code *.*}</td>
       *   <td>Matches file names containing a dot</td>
       * </tr>
       * <tr>
       *   <td>{@code *.{java,class}}</td>
       *   <td>Matches file names ending with {@code .java} or {@code .class}</td>
       * </tr>
       * <tr>
       *   <td>{@code foo.?}</td>
       *   <td>Matches file names starting with {@code foo.} and a single
       *   character extension</td>
       * </tr>
       * <tr>
       *   <td><tt>&#47;home&#47;*&#47;*</tt>
       *   <td>Matches <tt>&#47;home&#47;gus&#47;data</tt></td>
       * </tr>
       * <tr>
       *   <td><tt>&#47;home&#47;**</tt>
       *   <td>Matches <tt>&#47;home&#47;gus</tt> and
       *   <tt>&#47;home&#47;gus&#47;data</tt></td>
       * </tr>
       *
       * </table>
       * </blockquote>
       *
       * <p> The following rules are used to interpret glob patterns:
       *
       * <ul>
       *   <li><p> The {@code *} character matches zero or more {@link Character
       *   characters} of a {@link Path#getName(int) name} component without
       *   crossing directory boundaries. </p></li>
       *
       *   <li><p> The {@code **} characters matches zero or more {@link Character
       *   characters} crossing directory boundaries. </p></li>
       *
       *   <li><p> The {@code ?} character matches exactly one character of a
       *   name component.</p></li>
       *
       *   <li><p> The backslash character ({@code \}) is used to escape characters
       *   that would otherwise be interpreted as special characters. The expression
       *   {@code \\} matches a single backslash and "\{" matches a left brace
       *   for example.  </p></li>
       *
       *   <li><p> The {@code [ ]} characters are a <i>bracket expression</i> that
       *   match a single character of a name component out of a set of characters.
       *   For example, {@code [abc]} matches {@code "a"}, {@code "b"}, or {@code "c"}.
       *   The hyphen ({@code -}) may be used to specify a range so {@code [a-z]}
       *   specifies a range that matches from {@code "a"} to {@code "z"} (inclusive).
       *   These forms can be mixed so [abce-g] matches {@code "a"}, {@code "b"},
       *   {@code "c"}, {@code "e"}, {@code "f"} or {@code "g"}. If the character
       *   after the {@code [} is a {@code !} then it is used for negation so {@code
       *   [!a-c]} matches any character except {@code "a"}, {@code "b"}, or {@code
       *   "c"}.
       *   <p> Within a bracket expression the {@code *}, {@code ?} and {@code \}
       *   characters match themselves. The ({@code -}) character matches itself if
       *   it is the first character within the brackets, or the first character
       *   after the {@code !} if negating.</p></li>
       *
       *   <li><p> The {@code { }} characters are a group of subpatterns, where
       *   the group matches if any subpattern in the group matches. The {@code ","}
       *   character is used to separate the subpatterns. Groups cannot be nested.
       *   </p></li>
       *
       *   <li><p> Leading period<tt>&#47;</tt>dot characters in file name are
       *   treated as regular characters in match operations. For example,
       *   the {@code "*"} glob pattern matches file name {@code ".login"}.
       *   The {@link Files#isHidden} method may be used to test whether a file
       *   is considered hidden.
       *   </p></li>
       *
       *   <li><p> All other characters match themselves in an implementation
       *   dependent manner. This includes characters representing any {@link
       *   FileSystem#getSeparator name-separators}. </p></li>
       *
       *   <li><p> The matching of {@link Path#getRoot root} components is highly
       *   implementation-dependent and is not specified. </p></li>
       *
       * </ul>
       *
       * <p> When the syntax is "{@code regex}" then the pattern component is a
       * regular expression as defined by the {@link java.util.regex.Pattern}
       * class.
       *
       * <p>  For both the glob and regex syntaxes, the matching details, such as
       * whether the matching is case sensitive, are implementation-dependent
       * and therefore not specified.
       *
       * @param   syntaxAndPattern
       *          The syntax and pattern
       *
       * @return  A path matcher that may be used to match paths against the pattern
       *
       * @throws  IllegalArgumentException
       *          If the parameter does not take the form: {@code syntax:pattern}
       * @throws  java.util.regex.PatternSyntaxException
       *          If the pattern is invalid
       * @throws  UnsupportedOperationException
       *          If the pattern syntax is not known to the implementation
       *
       * @see Files#newDirectoryStream(Path,String)
       */
      public abstract PathMatcher getPathMatcher(String syntaxAndPattern);
  
      /**
       * Returns the {@code UserPrincipalLookupService} for this file system
       * <i>(optional operation)</i>. The resulting lookup service may be used to
       * lookup user or group names.
       *
       * <p> <b>Usage Example:</b>
       * Suppose we want to make "joe" the owner of a file:
       * <pre>
       *     UserPrincipalLookupService lookupService = FileSystems.getDefault().getUserPrincipalLookupService();
       *     Files.setOwner(path, lookupService.lookupPrincipalByName("joe"));
       * </pre>
       *
       * @throws  UnsupportedOperationException
       *          If this {@code FileSystem} does not does have a lookup service
       *
       * @return  The {@code UserPrincipalLookupService} for this file system
       */
      public abstract UserPrincipalLookupService getUserPrincipalLookupService();
  
      /**
       * Constructs a new {@link WatchService} <i>(optional operation)</i>.
       *
       * <p> This method constructs a new watch service that may be used to watch
       * registered objects for changes and events.
       *
       * @return  a new watch service
       *
       * @throws  UnsupportedOperationException
       *          If this {@code FileSystem} does not support watching file system
       *          objects for changes and events. This exception is not thrown
       *          by {@code FileSystems} created by the default provider.
       * @throws  IOException
       *          If an I/O error occurs
       */
      public abstract WatchService newWatchService() throws IOException;
  }
  