weakhashmap.java

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      {
        return value;
      }

      /**
       * This changes the value.  This change takes place in
       * the underlying hash map.
       * @param newVal the new value
       * @return the old value
       */
      public Object setValue(Object newVal)
      {
        Object oldVal = value;
        value = newVal;
        return oldVal;
      }

      /**
       * The hashCode as specified in the Entry interface.
       * @return the hash code
       */
      public int hashCode()
      {
        return key.hashCode() ^ WeakHashMap.hashCode(value);
      }

      /**
       * The equals method as specified in the Entry interface.
       * @param o the object to compare to
       * @return true iff o represents the same key/value pair
       */
      public boolean equals(Object o)
      {
        if (o instanceof Map.Entry)
          {
            Map.Entry e = (Map.Entry) o;
            return key.equals(e.getKey())
              && WeakHashMap.equals(value, e.getValue());
          }
        return false;
      }

      public String toString()
      {
        return key + "=" + value;
      }
    }

    /**
     * This returns the entry stored in this bucket, or null, if the
     * bucket got cleared in the mean time.
     * @return the Entry for this bucket, if it exists
     */
    WeakEntry getEntry()
    {
      final Object key = this.get();
      if (key == null)
        return null;
      return new WeakEntry(key);
    }
  }

  /**
   * The entry set returned by <code>entrySet()</code>.
   */
  private final WeakEntrySet theEntrySet;

  /**
   * The hash buckets.  These are linked lists. Package visible for use in
   * nested classes.
   */
  WeakBucket[] buckets;

  /**
   * Creates a new weak hash map with default load factor and default
   * capacity.
   */
  public WeakHashMap()
  {
    this(DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR);
  }

  /**
   * Creates a new weak hash map with default load factor and the given
   * capacity.
   * @param initialCapacity the initial capacity
   * @throws IllegalArgumentException if initialCapacity is negative
   */
  public WeakHashMap(int initialCapacity)
  {
    this(initialCapacity, DEFAULT_LOAD_FACTOR);
  }

  /**
   * Creates a new weak hash map with the given initial capacity and
   * load factor.
   * @param initialCapacity the initial capacity.
   * @param loadFactor the load factor (see class description of HashMap).
   * @throws IllegalArgumentException if initialCapacity is negative, or
   *         loadFactor is non-positive
   */
  public WeakHashMap(int initialCapacity, float loadFactor)
  {
    // Check loadFactor for NaN as well.
    if (initialCapacity < 0 || ! (loadFactor > 0))
      throw new IllegalArgumentException();
    this.loadFactor = loadFactor;
    threshold = (int) (initialCapacity * loadFactor);
    theEntrySet = new WeakEntrySet();
    queue = new ReferenceQueue();
    buckets = new WeakBucket[initialCapacity];
  }

  /**
   * Construct a new WeakHashMap with the same mappings as the given map.
   * The WeakHashMap has a default load factor of 0.75.
   *
   * @param m the map to copy
   * @throws NullPointerException if m is null
   * @since 1.3
   */
  public WeakHashMap(Map m)
  {
    this(m.size(), DEFAULT_LOAD_FACTOR);
    putAll(m);
  }

  /**
   * Simply hashes a non-null Object to its array index.
   * @param key the key to hash
   * @return its slot number
   */
  private int hash(Object key)
  {
    return Math.abs(key.hashCode() % buckets.length);
  }

  /**
   * Cleans the reference queue.  This will poll all references (which
   * are WeakBuckets) from the queue and remove them from this map.
   * This will not change modCount, even if it modifies the map.  The
   * iterators have to make sure that nothing bad happens.  <br>
   *
   * Currently the iterator maintains a strong reference to the key, so
   * that is no problem.
   */
  // Package visible for use by nested classes.
  void cleanQueue()
  {
    Object bucket = queue.poll();
    while (bucket != null)
      {
        internalRemove((WeakBucket) bucket);
        bucket = queue.poll();
      }
  }

  /**
   * Rehashes this hashtable.  This will be called by the
   * <code>add()</code> method if the size grows beyond the threshold.
   * It will grow the bucket size at least by factor two and allocates
   * new buckets.
   */
  private void rehash()
  {
    WeakBucket[] oldBuckets = buckets;
    int newsize = buckets.length * 2 + 1; // XXX should be prime.
    threshold = (int) (newsize * loadFactor);
    buckets = new WeakBucket[newsize];

    // Now we have to insert the buckets again.
    for (int i = 0; i < oldBuckets.length; i++)
      {
        WeakBucket bucket = oldBuckets[i];
        WeakBucket nextBucket;
        while (bucket != null)
          {
            nextBucket = bucket.next;

            Object key = bucket.get();
            if (key == null)
              {
                // This bucket should be removed; it is probably
                // already on the reference queue.  We don't insert it
                // at all, and mark it as cleared.
                bucket.slot = -1;
                size--;
              }
            else
              {
                // Add this bucket to its new slot.
                int slot = hash(key);
                bucket.slot = slot;
                bucket.next = buckets[slot];
                buckets[slot] = bucket;
              }
            bucket = nextBucket;
          }
      }
  }

  /**
   * Finds the entry corresponding to key.  Since it returns an Entry
   * it will also prevent the key from being removed under us.
   * @param key the key, may be null
   * @return The WeakBucket.WeakEntry or null, if the key wasn't found.
   */
  private WeakBucket.WeakEntry internalGet(Object key)
  {
    if (key == null)
      key = NULL_KEY;
    int slot = hash(key);
    WeakBucket bucket = buckets[slot];
    while (bucket != null)
      {
        WeakBucket.WeakEntry entry = bucket.getEntry();
        if (entry != null && key.equals(entry.key))
          return entry;

        bucket = bucket.next;
      }
    return null;
  }

  /**
   * Adds a new key/value pair to the hash map.
   * @param key the key. This mustn't exists in the map. It may be null.
   * @param value the value.
   */
  private void internalAdd(Object key, Object value)
  {
    if (key == null)
      key = NULL_KEY;
    int slot = hash(key);
    WeakBucket bucket = new WeakBucket(key, queue, value, slot);
    bucket.next = buckets[slot];
    buckets[slot] = bucket;
    size++;
  }

  /**
   * Removes a bucket from this hash map, if it wasn't removed before
   * (e.g. one time through rehashing and one time through reference queue).
   * Package visible for use in nested classes.
   *
   * @param bucket the bucket to remove.
   */
  void internalRemove(WeakBucket bucket)
  {
    int slot = bucket.slot;
    if (slot == -1)
      // This bucket was already removed.
      return;

    // Mark the bucket as removed.  This is necessary, since the
    // bucket may be enqueued later by the garbage collection, and
    // internalRemove will be called a second time.
    bucket.slot = -1;
    if (buckets[slot] == bucket)
      buckets[slot] = bucket.next;
    else
      {
        WeakBucket prev = buckets[slot];
        /* This may throw a NullPointerException.  It shouldn't but if
         * a race condition occurred (two threads removing the same
         * bucket at the same time) it may happen.  <br>
         * But with race condition many much worse things may happen
         * anyway.
         */
        while (prev.next != bucket)
          prev = prev.next;
        prev.next = bucket.next;
      }
    size--;
  }

  /**
   * Returns the size of this hash map.  Note that the size() may shrink
   * spontaneously, if the some of the keys were only weakly reachable.
   * @return the number of entries in this hash map.
   */
  public int size()
  {
    cleanQueue();
    return size;
  }

  /**
   * Tells if the map is empty.  Note that the result may change
   * spontanously, if all of the keys were only weakly reachable.
   * @return true, iff the map is empty.
   */
  public boolean isEmpty()
  {
    cleanQueue();
    return size == 0;
  }

  /**
   * Tells if the map contains the given key.  Note that the result
   * may change spontanously, if the key was only weakly
   * reachable.
   * @param key the key to look for
   * @return true, iff the map contains an entry for the given key.
   */
  public boolean containsKey(Object key)
  {
    cleanQueue();
    return internalGet(key) != null;
  }

  /**
   * Gets the value the key is mapped to.
   * @return the value the key was mapped to.  It returns null if
   *         the key wasn't in this map, or if the mapped value was
   *         explicitly set to null.
   */
  public Object get(Object key)
  {
    cleanQueue();
    WeakBucket.WeakEntry entry = internalGet(key);
    return entry == null ? null : entry.getValue();
  }

  /**
   * Adds a new key/value mapping to this map.
   * @param key the key, may be null
   * @param value the value, may be null
   * @return the value the key was mapped to previously.  It returns
   *         null if the key wasn't in this map, or if the mapped value
   *         was explicitly set to null.
   */
  public Object put(Object key, Object value)
  {
    cleanQueue();
    WeakBucket.WeakEntry entry = internalGet(key);
    if (entry != null)
      return entry.setValue(value);

    modCount++;
    if (size >= threshold)
      rehash();

    internalAdd(key, value);
    return null;
  }

  /**
   * Removes the key and the corresponding value from this map.
   * @param key the key. This may be null.
   * @return the value the key was mapped to previously.  It returns
   *         null if the key wasn't in this map, or if the mapped value was
   *         explicitly set to null.
   */
  public Object remove(Object key)
  {
    cleanQueue();
    WeakBucket.WeakEntry entry = internalGet(key);
    if (entry == null)
      return null;

    modCount++;
    internalRemove(entry.getBucket());
    return entry.getValue();
  }

  /**
   * Returns a set representation of the entries in this map.  This
   * set will not have strong references to the keys, so they can be
   * silently removed.  The returned set has therefore the same
   * strange behaviour (shrinking size(), disappearing entries) as
   * this weak hash map.
   * @return a set representation of the entries.
   */
  public Set entrySet()
  {
    cleanQueue();
    return theEntrySet;
  }

  /**
   * Clears all entries from this map.
   */
  public void clear()
  {
    super.clear();
  }

  /**
   * Returns true if the map contains at least one key which points to
   * the specified object as a value.  Note that the result
   * may change spontanously, if its key was only weakly reachable.
   * @param value the value to search for
   * @return true if it is found in the set.
   */
  public boolean containsValue(Object value)
  {
    cleanQueue();
    return super.containsValue(value);
  }

  /**
   * Returns a set representation of the keys in this map.  This
   * set will not have strong references to the keys, so they can be
   * silently removed.  The returned set has therefore the same
   * strange behaviour (shrinking size(), disappearing entries) as
   * this weak hash map.
   * @return a set representation of the keys.
   */
  public Set keySet()
  {
    cleanQueue();
    return super.keySet();
  }

  /**
   * Puts all of the mappings from the given map into this one. If the
   * key already exists in this map, its value is replaced.
   * @param m the map to copy in
   */
  public void putAll(Map m)
  {
    super.putAll(m);
  }

  /**
   * Returns a collection representation of the values in this map.  This
   * collection will not have strong references to the keys, so mappings
   * can be silently removed.  The returned collection has therefore the same
   * strange behaviour (shrinking size(), disappearing entries) as
   * this weak hash map.
   * @return a collection representation of the values.
   */
  public Collection values()
  {
    cleanQueue();
    return super.values();
  }
} // class WeakHashMap