LinkedHashMap.java

/* LinkedHashMap.java -- a class providing hashtable data structure,
   mapping Object --> Object, with linked list traversal
   Copyright (C) 2001, 2002 Free Software Foundation, Inc.

This file is part of GNU Classpath.

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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.
 * <p>
 *
 * In insertion order, calling <code>put</code> adds the key to the end of
 * traversal, unless the key was already in the map; changing traversal order
 * requires removing and reinserting a key.  On the other hand, in access
 * order, all calls to <code>put</code> and <code>get</code> cause the
 * accessed key to move to the end of the traversal list.  Note that any
 * accesses to the map's contents via its collection views and iterators do
 * not affect the map's traversal order, since the collection views do not
 * call <code>put</code> or <code>get</code>.
 * <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 can build an access-order mapping.  This can be used to implement LRU
 * caches, for example.  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 (<code>containsValue()</code> is,
 * of course, O(n)).  In the worst case (all keys map to the same 
 * hash code -- very unlikely), most operations are O(n).  Traversal is
 * faster than in HashMap (proportional to the map size, and not the space
 * allocated for the map), but other operations may be slower because of the
 * overhead of the maintaining the traversal order list.
 * <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 (ebb9@email.byu.edu)
 * @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 oldest Entry to begin iteration at.
   */
  transient LinkedHashEntry root;

  /**
   * The iteration order of this linked hash map: <code>true</code> for
   * access-order, <code>false</code> for insertion-order.
   *
   * @serial true for access order traversal
   */
  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. If this entry is the root
     * (eldest), pred points to the newest entry.
     */
    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 (root == null)
        {
          root = this;
          pred = this;
        }
      else
        {
          pred = root.pred;
          pred.succ = this;
          root.pred = this;
        }
    }

    /**
     * Called when this entry is accessed via put or get. This version does
     * the necessary bookkeeping to keep the doubly-linked list in order,
     * after moving this element to the newest position in access order.
     */
    void access()
    {
      if (accessOrder && succ != null)
        {
          modCount++;
          if (this == root)
            {
              root = succ;
              pred.succ = this;
              succ = null;
            }
          else
            {
              pred.succ = succ;
              succ.pred = pred;
              succ = null;
              pred = root.pred;
              pred.succ = this;
            }
        }
    }

    /**
     * Called when this entry is removed from the map. This version does
     * the necessary bookkeeping to keep the doubly-linked list in order.
     *
     * @return the value of this key as it is removed
     */
    Object cleanup()
    {
      if (this == root)
        {
          root = succ;
          if (succ != null)
            succ.pred = pred;
        }
      else if (succ == null)
        {
          pred.succ = null;
          root.pred = pred;
        }
      else
        {
          pred.succ = succ;
          succ.pred = pred;
        }
      return value;
    }
  } // class LinkedHashEntry

  /**
   * Construct a new insertion-ordered LinkedHashMap with the default
   * capacity (11) and the default load factor (0.75).
   */
  public LinkedHashMap()
  {
    super();
    accessOrder = false;
  }

  /**
   * Construct a new insertion-ordered LinkedHashMap from the given Map,
   * with initial capacity the greater of the size of <code>m</code> or
   * the default of 11.
   * <p>
   *
   * Every element in Map m will be put into this new HashMap, in the
   * order of m's iterator.
   *
   * @param m a Map whose key / value pairs will be put into
   *          the new HashMap.  <b>NOTE: key / value pairs
   *          are not cloned in this constructor.</b>
   * @throws NullPointerException if m is null
   */
  public LinkedHashMap(Map m)
  {
    super(m);
    accessOrder = false;
  }

  /**
   * Construct a new insertion-ordered LinkedHashMap with a specific
   * inital capacity and default load factor of 0.75.
   *
   * @param initialCapacity the initial capacity of this HashMap (&gt;= 0)
   * @throws IllegalArgumentException if (initialCapacity &lt; 0)
   */
  public LinkedHashMap(int initialCapacity)
  {
    super(initialCapacity);
    accessOrder = false;
  }

  /**
   * Construct a new insertion-orderd LinkedHashMap with a specific
   * inital capacity and load factor.
   *
   * @param initialCapacity the initial capacity (&gt;= 0)
   * @param loadFactor the load factor (&gt; 0, not NaN)
   * @throws IllegalArgumentException if (initialCapacity &lt; 0) ||
   *                                     ! (loadFactor &gt; 0.0)
   */
  public LinkedHashMap(int initialCapacity, float loadFactor)
  {
    super(initialCapacity, loadFactor);
    accessOrder = false;
  }

  /**
   * Construct a new LinkedHashMap with a specific inital capacity, load
   * factor, and ordering mode.
   *
   * @param initialCapacity the initial capacity (&gt;=0)
   * @param loadFactor the load factor (&gt;0, not NaN)
   * @param accessOrder true for access-order, false for insertion-order
   * @throws IllegalArgumentException if (initialCapacity &lt; 0) ||
   *                                     ! (loadFactor &gt; 0.0)
   */
  public LinkedHashMap(int initialCapacity, float loadFactor,
                       boolean accessOrder)
  {
    super(initialCapacity, loadFactor);
    this.accessOrder = accessOrder;
  }

  /**
   * Clears the Map so it has no keys. This is O(1).
   */
  public void clear()
  {
    super.clear();
    root = null;
  }

  /**
   * Returns <code>true</code> if this HashMap contains a value
   * <code>o</code>, such that <code>o.equals(value)</code>.
   *
   * @param value the value to search for in this HashMap
   * @return <code>true</code> if at least one key maps to the value
   */
  public boolean containsValue(Object value)
  {
    LinkedHashEntry e = root;
    while (e != null)
      {
        if (equals(value, e.value))
          return true;
        e = e.succ;
      }
    return false;
  }

  /**
   * Return the value in this Map associated with the supplied key,
   * or <code>null</code> if the key maps to nothing.  If this is an
   * access-ordered Map and the key is found, this performs structural
   * modification, moving the key to the newest end of the list. NOTE:
   * Since the value could also be null, you must use containsKey to
   * see if this key actually maps to something.
   *
   * @param key the key for which to fetch an associated value
   * @return what the key maps to, if present
   * @see #put(Object, Object)
   * @see #containsKey(Object)
   */
  public Object get(Object key)
  {
    int idx = hash(key);
    HashEntry e = buckets[idx];
    while (e != null)
      {
        if (equals(key, e.key))
          {
            e.access();
            return e.value;
          }
        e = e.next;
      }
    return null;
  }

  /**
   * Returns <code>true</code> if this map should remove the eldest entry.
   * This method is invoked by all calls to <code>put</code> and
   * <code>putAll</code> which place a new entry in the map, providing
   * the implementer an opportunity to remove the eldest entry any time
   * a new one is added.  This can be used to save memory usage of the
   * hashtable, as well as emulating a cache, by deleting stale entries.
   * <p>
   *
   * For example, to keep the Map limited to 100 entries, override as follows:
   * <pre>
   * private static final int MAX_ENTRIES = 100;
   * protected boolean removeEldestEntry(Map.Entry eldest)
   * {
   *   return size() &gt; MAX_ENTRIES;
   * }
   * </pre><p>
   *
   * Typically, this method does not modify the map, but just uses the
   * return value as an indication to <code>put</code> whether to proceed.
   * However, if you override it to modify the map, you must return false
   * (indicating that <code>put</code> should leave the modified map alone),
   * or you face unspecified behavior.  Remember that in access-order mode,
   * even calling <code>get</code> is a structural modification, but using
   * the collections views (such as <code>keySet</code>) is not.
   * <p>
   *
   * This method is called after the eldest entry has been inserted, so
   * if <code>put</code> was called on a previously empty map, the eldest
   * entry is the one you just put in! The default implementation just
   * returns <code>false</code>, so that this map always behaves like
   * a normal one with unbounded growth.
   *
   * @param eldest the eldest element which would be removed if this
   *        returns true. For an access-order map, this is the least
   *        recently accessed; for an insertion-order map, this is the
   *        earliest element inserted.
   * @return true if <code>eldest</code> should be removed
   */
  protected boolean removeEldestEntry(Map.Entry eldest)
  {
    return false;
  }

  /**
   * Helper method called by <code>put</code>, which creates and adds a
   * new Entry, followed by performing bookkeeping (like removeEldestEntry).
   *
   * @param key the key of the new Entry
   * @param value the value
   * @param idx the index in buckets where the new Entry belongs
   * @param callRemove whether to call the removeEldestEntry method
   * @see #put(Object, Object)
   * @see #removeEldestEntry(Map.Entry)
   * @see LinkedHashEntry#LinkedHashEntry(Object, Object)
   */
  void addEntry(Object key, Object value, int idx, boolean callRemove)
  {
    LinkedHashEntry e = new LinkedHashEntry(key, value);
    e.next = buckets[idx];
    buckets[idx] = e;
    if (callRemove && removeEldestEntry(root))
      remove(root);
  }

  /**
   * Helper method, called by clone() to reset the doubly-linked list.
   *
   * @param m the map to add entries from
   * @see #clone()
   */
  void putAllInternal(Map m)
  {
    root = null;
    super.putAllInternal(m);
  }

  /**
   * Generates a parameterized iterator. This allows traversal to follow
   * the doubly-linked list instead of the random bin order of HashMap.
   *
   * @param type {@link #KEYS}, {@link #VALUES}, or {@link #ENTRIES}
   * @return the appropriate iterator
   */
  Iterator iterator(final int type)
  {
    return new Iterator()
    {
      /** The current Entry. */
      LinkedHashEntry current = root;

      /** The previous Entry returned by next(). */
      LinkedHashEntry last;

      /** The number of known modifications to the backing Map. */
      int knownMod = modCount;

      /**
       * Returns true if the Iterator has more elements.
       *
       * @return true if there are more elements
       * @throws ConcurrentModificationException if the HashMap was modified
       */
      public boolean hasNext()
      {
        if (knownMod != modCount)
          throw new ConcurrentModificationException();
        return current != null;
      }

      /**
       * Returns the next element in the Iterator's sequential view.
       *
       * @return the next element
       * @throws ConcurrentModificationException if the HashMap was modified
       * @throws NoSuchElementException if there is none
       */
      public Object next()
      {
        if (knownMod != modCount)
          throw new ConcurrentModificationException();
        if (current == null)
          throw new NoSuchElementException();
        last = current;
        current = current.succ;
        return type == VALUES ? last.value : type == KEYS ? last.key : last;
      }
      
      /**
       * Removes from the backing HashMap the last element which was fetched
       * with the <code>next()</code> method.
       *
       * @throws ConcurrentModificationException if the HashMap was modified
       * @throws IllegalStateException if called when there is no last element
       */
      public void remove()
      {
        if (knownMod != modCount)
          throw new ConcurrentModificationException();
        if (last == null)
          throw new IllegalStateException();
        LinkedHashMap.this.remove(last.key);
        last = null;
        knownMod++;
      }
    };
  }
} // class LinkedHashMap