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   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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   * This code is free software; you can redistribute it and/or modify it
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   * published by the Free Software Foundation.  Oracle designates this
   * particular file as subject to the "Classpath" exception as provided
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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
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  package java.security.cert;
  
  import java.io.ByteArrayInputStream;
  import java.io.NotSerializableException;
  import java.io.ObjectStreamException;
  import java.io.Serializable;
  import java.util.Iterator;
  import java.util.List;
  
  /**
   * An immutable sequence of certificates (a certification path).
   * <p>
   * This is an abstract class that defines the methods common to all
   * {@code CertPath}s. Subclasses can handle different kinds of
   * certificates (X.509, PGP, etc.).
   * <p>
   * All {@code CertPath} objects have a type, a list of
   * {@code Certificate}s, and one or more supported encodings. Because the
   * {@code CertPath} class is immutable, a {@code CertPath} cannot
   * change in any externally visible way after being constructed. This
   * stipulation applies to all public fields and methods of this class and any
   * added or overridden by subclasses.
   * <p>
   * The type is a {@code String} that identifies the type of
   * {@code Certificate}s in the certification path. For each
   * certificate {@code cert} in a certification path {@code certPath},
   * {@code cert.getType().equals(certPath.getType())} must be
   * {@code true}.
   * <p>
   * The list of {@code Certificate}s is an ordered {@code List} of
   * zero or more {@code Certificate}s. This {@code List} and all
   * of the {@code Certificate}s contained in it must be immutable.
   * <p>
   * Each {@code CertPath} object must support one or more encodings
   * so that the object can be translated into a byte array for storage or
   * transmission to other parties. Preferably, these encodings should be
   * well-documented standards (such as PKCS#7). One of the encodings supported
   * by a {@code CertPath} is considered the default encoding. This
   * encoding is used if no encoding is explicitly requested (for the
   * {@link #getEncoded() getEncoded()} method, for instance).
   * <p>
   * All {@code CertPath} objects are also {@code Serializable}.
   * {@code CertPath} objects are resolved into an alternate
   * {@link CertPathRep CertPathRep} object during serialization. This allows
   * a {@code CertPath} object to be serialized into an equivalent
   * representation regardless of its underlying implementation.
   * <p>
   * {@code CertPath} objects can be created with a
   * {@code CertificateFactory} or they can be returned by other classes,
   * such as a {@code CertPathBuilder}.
   * <p>
   * By convention, X.509 {@code CertPath}s (consisting of
   * {@code X509Certificate}s), are ordered starting with the target
   * certificate and ending with a certificate issued by the trust anchor. That
   * is, the issuer of one certificate is the subject of the following one. The
   * certificate representing the {@link TrustAnchor TrustAnchor} should not be
   * included in the certification path. Unvalidated X.509 {@code CertPath}s
   * may not follow these conventions. PKIX {@code CertPathValidator}s will
   * detect any departure from these conventions that cause the certification
   * path to be invalid and throw a {@code CertPathValidatorException}.
   *
   * <p> Every implementation of the Java platform is required to support the
   * following standard {@code CertPath} encodings:
   * <ul>
   * <li>{@code PKCS7}</li>
   * <li>{@code PkiPath}</li>
   * </ul>
   * These encodings are described in the <a href=
   * "{@docRoot}/../technotes/guides/security/StandardNames.html#CertPathEncodings">
   * CertPath Encodings section</a> of the
   * Java Cryptography Architecture Standard Algorithm Name Documentation.
   * Consult the release documentation for your implementation to see if any
   * other encodings are supported.
   * <p>
   * <b>Concurrent Access</b>
   * <p>
   * All {@code CertPath} objects must be thread-safe. That is, multiple
   * threads may concurrently invoke the methods defined in this class on a
   * single {@code CertPath} object (or more than one) with no
   * ill effects. This is also true for the {@code List} returned by
   * {@code CertPath.getCertificates}.
   * <p>
   * Requiring {@code CertPath} objects to be immutable and thread-safe
   * allows them to be passed around to various pieces of code without worrying
   * about coordinating access.  Providing this thread-safety is
   * generally not difficult, since the {@code CertPath} and
   * {@code List} objects in question are immutable.
   *
   * @see CertificateFactory
   * @see CertPathBuilder
   *
   * @author      Yassir Elley
   * @since       1.4
   */
  public abstract class CertPath implements Serializable {
  
      private static final long serialVersionUID = 6068470306649138683L;
  
      private String type;        // the type of certificates in this chain
  
      /**
       * Creates a {@code CertPath} of the specified type.
       * <p>
       * This constructor is protected because most users should use a
       * {@code CertificateFactory} to create {@code CertPath}s.
       *
       * @param type the standard name of the type of
       * {@code Certificate}s in this path
       */
      protected CertPath(String type) {
          this.type = type;
      }
  
      /**
       * Returns the type of {@code Certificate}s in this certification
       * path. This is the same string that would be returned by
       * {@link Certificate#getType() cert.getType()}
       * for all {@code Certificate}s in the certification path.
       *
       * @return the type of {@code Certificate}s in this certification
       * path (never null)
       */
      public String getType() {
          return type;
      }
  
      /**
       * Returns an iteration of the encodings supported by this certification
       * path, with the default encoding first. Attempts to modify the returned
       * {@code Iterator} via its {@code remove} method result in an
       * {@code UnsupportedOperationException}.
       *
       * @return an {@code Iterator} over the names of the supported
       *         encodings (as Strings)
       */
      public abstract Iterator<String> getEncodings();
  
      /**
       * Compares this certification path for equality with the specified
       * object. Two {@code CertPath}s are equal if and only if their
       * types are equal and their certificate {@code List}s (and by
       * implication the {@code Certificate}s in those {@code List}s)
       * are equal. A {@code CertPath} is never equal to an object that is
       * not a {@code CertPath}.
       * <p>
       * This algorithm is implemented by this method. If it is overridden,
       * the behavior specified here must be maintained.
       *
       * @param other the object to test for equality with this certification path
       * @return true if the specified object is equal to this certification path,
       * false otherwise
       */
      public boolean equals(Object other) {
          if (this == other)
              return true;
  
          if (! (other instanceof CertPath))
              return false;
  
          CertPath otherCP = (CertPath) other;
          if (! otherCP.getType().equals(type))
              return false;
  
          List<? extends Certificate> thisCertList = this.getCertificates();
          List<? extends Certificate> otherCertList = otherCP.getCertificates();
          return(thisCertList.equals(otherCertList));
      }
  
      /**
       * Returns the hashcode for this certification path. The hash code of
       * a certification path is defined to be the result of the following
       * calculation:
       * <pre>{@code
       *  hashCode = path.getType().hashCode();
       *  hashCode = 31*hashCode + path.getCertificates().hashCode();
       * }</pre>
       * This ensures that {@code path1.equals(path2)} implies that
       * {@code path1.hashCode()==path2.hashCode()} for any two certification
       * paths, {@code path1} and {@code path2}, as required by the
       * general contract of {@code Object.hashCode}.
       *
       * @return the hashcode value for this certification path
       */
      public int hashCode() {
          int hashCode = type.hashCode();
          hashCode = 31*hashCode + getCertificates().hashCode();
          return hashCode;
      }
  
      /**
       * Returns a string representation of this certification path.
       * This calls the {@code toString} method on each of the
       * {@code Certificate}s in the path.
       *
       * @return a string representation of this certification path
       */
      public String toString() {
          StringBuffer sb = new StringBuffer();
          Iterator<? extends Certificate> stringIterator =
                                          getCertificates().iterator();
  
          sb.append("\n" + type + " Cert Path: length = "
              + getCertificates().size() + ".\n");
          sb.append("[\n");
          int i = 1;
          while (stringIterator.hasNext()) {
              sb.append("=========================================="
                  + "===============Certificate " + i + " start.\n");
              Certificate stringCert = stringIterator.next();
              sb.append(stringCert.toString());
              sb.append("\n========================================"
                  + "=================Certificate " + i + " end.\n\n\n");
              i++;
          }
  
          sb.append("\n]");
          return sb.toString();
      }
  
      /**
       * Returns the encoded form of this certification path, using the default
       * encoding.
       *
       * @return the encoded bytes
       * @exception CertificateEncodingException if an encoding error occurs
       */
      public abstract byte[] getEncoded()
          throws CertificateEncodingException;
  
      /**
       * Returns the encoded form of this certification path, using the
       * specified encoding.
       *
       * @param encoding the name of the encoding to use
       * @return the encoded bytes
       * @exception CertificateEncodingException if an encoding error occurs or
       *   the encoding requested is not supported
       */
      public abstract byte[] getEncoded(String encoding)
          throws CertificateEncodingException;
  
      /**
       * Returns the list of certificates in this certification path.
       * The {@code List} returned must be immutable and thread-safe.
       *
       * @return an immutable {@code List} of {@code Certificate}s
       *         (may be empty, but not null)
       */
      public abstract List<? extends Certificate> getCertificates();
  
      /**
       * Replaces the {@code CertPath} to be serialized with a
       * {@code CertPathRep} object.
       *
       * @return the {@code CertPathRep} to be serialized
       *
       * @throws ObjectStreamException if a {@code CertPathRep} object
       * representing this certification path could not be created
       */
      protected Object writeReplace() throws ObjectStreamException {
          try {
              return new CertPathRep(type, getEncoded());
          } catch (CertificateException ce) {
              NotSerializableException nse =
                  new NotSerializableException
                      ("java.security.cert.CertPath: " + type);
              nse.initCause(ce);
              throw nse;
          }
      }
  
      /**
       * Alternate {@code CertPath} class for serialization.
       * @since 1.4
       */
      protected static class CertPathRep implements Serializable {
  
          private static final long serialVersionUID = 3015633072427920915L;
  
          /** The Certificate type */
          private String type;
          /** The encoded form of the cert path */
          private byte[] data;
  
          /**
           * Creates a {@code CertPathRep} with the specified
           * type and encoded form of a certification path.
           *
           * @param type the standard name of a {@code CertPath} type
           * @param data the encoded form of the certification path
           */
          protected CertPathRep(String type, byte[] data) {
              this.type = type;
              this.data = data;
          }
  
          /**
           * Returns a {@code CertPath} constructed from the type and data.
           *
           * @return the resolved {@code CertPath} object
           *
           * @throws ObjectStreamException if a {@code CertPath} could not
           * be constructed
           */
          protected Object readResolve() throws ObjectStreamException {
              try {
                  CertificateFactory cf = CertificateFactory.getInstance(type);
                  return cf.generateCertPath(new ByteArrayInputStream(data));
              } catch (CertificateException ce) {
                  NotSerializableException nse =
                      new NotSerializableException
                          ("java.security.cert.CertPath: " + type);
                  nse.initCause(ce);
                  throw nse;
              }
          }
      }
  }
  