gcc/libjava/java/util/LinkedHashMap.java

/* 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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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 <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 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)
    {
      if (accessOrder && succ != null)
        {
          succ.pred = pred;
          if (pred == null)
            head = succ;
          else
            pred.succ = succ;
          succ = null;
          pred = tail;
          pred.succ = this;
          tail = this;
        }
      return super.setValue(value);
    }

    /**
     * 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 (pred == null)
        head = succ;
      else
        pred.succ = succ;
      if (succ == null)
        tail = pred;
      else
        succ.pred = pred;
      
      return value;
    }
  }

  /**
   * 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 (>=0)
   * @param loadFactor the load factor (>0, not NaN)
   * @param accessOrder true for access-order, false for insertion-order
   * @throws IllegalArgumentException if (initialCapacity < 0) ||
   *                                     ! (loadFactor > 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();
    head = null;
    tail = null;
  }

  /**
   * Returns true if this HashMap contains a value <pre>o</pre>, such that
   * <pre>o.equals(value)</pre>.
   *
   * @param value the value to search for in this HashMap
   * @return true if at least one key maps to the value
   */
  public boolean containsValue(Object value)
  {
    LinkedHashEntry e = head;
    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 <pre>null</pre> 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))
          {
            if (accessOrder)
              {
                modCount++;
                LinkedHashEntry l = (LinkedHashEntry) e;
                if (l.succ != null)
                  {
                    l.succ.pred = l.pred;
                    if (l.pred == null)
                      head = l.succ;
                    else
                      l.pred.succ = l.succ;
                    l.succ = null;
                    l.pred = tail;
                    tail.succ = l;
                    tail = l;
                  }
              }
            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() > 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 do nothing), or face
   * unspecified behavior.
   * <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)
   */
  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(head))
      remove(head);
  }

  /**
   * 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)
  {
    head = null;
    tail = 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 = head;

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

      /** The number of known modifications to the backing HashMap */
      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 <pre>next()</pre> 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++;
      }
    };
  }
}