source: trunk/src/gcc/libjava/java/util/LinkedHashMap.java@ 154

/* LinkedHashMap.java -- a class providing hashtable data structure,
   mapping Object --> Object, with linked list traversal
   Copyright (C) 2001 Free Software Foundation, Inc.

This file is part of GNU Classpath.

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Linking this library statically or dynamically with other modules is
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As a special exception, the copyright holders of this library give you
permission to link this library with independent modules to produce an
executable, regardless of the license terms of these independent
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independent module, the terms and conditions of the license of that
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exception statement from your version. */


package java.util;

/**
 * This class provides a hashtable-backed implementation of the
 * Map interface, with predictable traversal order.
 * <p>
 *
 * It uses a hash-bucket approach; that is, hash collisions are handled
 * by linking the new node off of the pre-existing node (or list of
 * nodes).  In this manner, techniques such as linear probing (which
 * can cause primary clustering) and rehashing (which does not fit very
 * well with Java's method of precomputing hash codes) are avoided.  In
 * addition, this maintains a doubly-linked list which tracks either
 * insertion or access order.  Note that the insertion order is not
 * modified if a <code>put</code> simply reinserts a key in the map.
 * <p>
 *
 * One of the nice features of tracking insertion order is that you can
 * copy a hashtable, and regardless of the implementation of the original,
 * produce the same results when iterating over the copy.  This is possible
 * without needing the overhead of <code>TreeMap</code>.
 * <p>
 *
 * When using this {@link #LinkedHashMap(int, float, boolean) constructor},
 * you build an access-order mapping.  This can be used to implement LRU
 * caches, for example.  In this case, every invocation of <code>put</code>,
 * <code>putAll</code>, or <code>get</code> moves the accessed entry to
 * the end of the iteration list.  By overriding
 * {@link #removeEldestEntry(Map.Entry)}, you can also control the
 * removal of the oldest entry, and thereby do things like keep the map
 * at a fixed size.
 * <p>
 *
 * Under ideal circumstances (no collisions), LinkedHashMap offers O(1) 
 * performance on most operations (<pre>containsValue()</pre> is,
 * of course, O(n)).  In the worst case (all keys map to the same 
 * hash code -- very unlikely), most operations are O(n).
 * <p>
 *
 * LinkedHashMap accepts the null key and null values.  It is not
 * synchronized, so if you need multi-threaded access, consider using:<br>
 * <code>Map m = Collections.synchronizedMap(new LinkedHashMap(...));</code>
 * <p>
 *
 * The iterators are <i>fail-fast</i>, meaning that any structural
 * modification, except for <code>remove()</code> called on the iterator
 * itself, cause the iterator to throw a
 * {@link ConcurrentModificationException} rather than exhibit
 * non-deterministic behavior.
 *
 * @author Eric Blake <[email protected]>
 * @see Object#hashCode()
 * @see Collection
 * @see Map
 * @see HashMap
 * @see TreeMap
 * @see Hashtable
 * @since 1.4
 * @status updated to 1.4
 */
public class LinkedHashMap extends HashMap
{
  /**
   * Compatible with JDK 1.4.
   */
  private static final long serialVersionUID = 3801124242820219131L;

  /**
   * The first Entry to iterate over.
   */
  transient LinkedHashEntry head;

  /**
   * The last Entry to iterate over.
   */
  transient LinkedHashEntry tail;

  /**
   * The iteration order of this linked hash map: <code>true</code> for
   * access-order, <code>false</code> for insertion-order.
   * @serial
   */
  final boolean accessOrder;

  /**
   * Class to represent an entry in the hash table. Holds a single key-value
   * pair and the doubly-linked insertion order list.
   */
  class LinkedHashEntry extends HashEntry
  {
    /** The predecessor in the iteration list, null if this is the eldest. */
    LinkedHashEntry pred;
    /** The successor in the iteration list, null if this is the newest. */
    LinkedHashEntry succ;

    /**
     * Simple constructor.
     * @param key the key
     * @param value the value
     */
    LinkedHashEntry(Object key, Object value)
    {
      super(key, value);
      if (head == null)
        head = this;
      pred = tail;
      tail = this;
      if (pred != null)
        pred.succ = this;
    }

    /**
     * Sets the value of this entry, and shuffles it to the end of
     * the list if this is in access-order.
     * @param value the new value
     * @return the prior value
     */
    public Object setValue(Object value)
    {