File	Doc	Category	Size	Date	Package
WeakHashMap.java	API Doc	Java SE 6 API	33088	Tue Jun 10 00:25:56 BST 2008	java.util

WeakHashMap

• java.lang.Object
  • java.util.AbstractMap

Fields Summary
private static final int	DEFAULT_INITIAL_CAPACITY The default initial capacity -- MUST be a power of two.
private static final int	MAXIMUM_CAPACITY The maximum capacity, used if a higher value is implicitly specified by either of the constructors with arguments. MUST be a power of two <= 1<<30.
private static final float	DEFAULT_LOAD_FACTOR The load fast used when none specified in constructor.
private Entry[]	table The table, resized as necessary. Length MUST Always be a power of two.
private int	size The number of key-value mappings contained in this weak hash map.
private int	threshold The next size value at which to resize (capacity * load factor).
private final float	loadFactor The load factor for the hash table.
private final ReferenceQueue	queue Reference queue for cleared WeakEntries
private volatile int	modCount The number of times this WeakHashMap has been structurally modified. Structural modifications are those that change the number of mappings in the map or otherwise modify its internal structure (e.g., rehash). This field is used to make iterators on Collection-views of the map fail-fast.
private static final Object	NULL_KEY Value representing null keys inside tables.
private transient Set	entrySet

Constructors Summary
public WeakHashMap(int initialCapacity, float loadFactor) Constructs a new, empty WeakHashMap with the given initial capacity and the given load factor. param initialCapacity The initial capacity of the WeakHashMap param loadFactor The load factor of the WeakHashMap throws IllegalArgumentException if the initial capacity is negative, or if the load factor is nonpositive. if (initialCapacity < 0) throw new IllegalArgumentException("Illegal Initial Capacity: "+ initialCapacity); if (initialCapacity > MAXIMUM_CAPACITY) initialCapacity = MAXIMUM_CAPACITY; if (loadFactor <= 0 || Float.isNaN(loadFactor)) throw new IllegalArgumentException("Illegal Load factor: "+ loadFactor); int capacity = 1; while (capacity < initialCapacity) capacity <<= 1; table = new Entry[capacity]; this.loadFactor = loadFactor; threshold = (int)(capacity * loadFactor);
public WeakHashMap(int initialCapacity) Constructs a new, empty WeakHashMap with the given initial capacity and the default load factor (0.75). param initialCapacity The initial capacity of the WeakHashMap throws IllegalArgumentException if the initial capacity is negative this(initialCapacity, DEFAULT_LOAD_FACTOR);
public WeakHashMap() Constructs a new, empty WeakHashMap with the default initial capacity (16) and load factor (0.75). this.loadFactor = DEFAULT_LOAD_FACTOR; threshold = (int)(DEFAULT_INITIAL_CAPACITY); table = new Entry[DEFAULT_INITIAL_CAPACITY];
public WeakHashMap(Map m) Constructs a new WeakHashMap with the same mappings as the specified map. The WeakHashMap is created with the default load factor (0.75) and an initial capacity sufficient to hold the mappings in the specified map. param m the map whose mappings are to be placed in this map throws NullPointerException if the specified map is null since 1.3 this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1, 16), DEFAULT_LOAD_FACTOR); putAll(m);

Methods Summary
public void	clear() Removes all of the mappings from this map. The map will be empty after this call returns. // clear out ref queue. We don't need to expunge entries // since table is getting cleared. while (queue.poll() != null) ; modCount++; Entry[] tab = table; for (int i = 0; i < tab.length; ++i) tab[i] = null; size = 0; // Allocation of array may have caused GC, which may have caused // additional entries to go stale. Removing these entries from the // reference queue will make them eligible for reclamation. while (queue.poll() != null) ;
public boolean	containsKey(java.lang.Object key) Returns true if this map contains a mapping for the specified key. param key The key whose presence in this map is to be tested return true if there is a mapping for key; false otherwise return getEntry(key) != null;
private boolean	containsNullValue() Special-case code for containsValue with null argument Entry[] tab = getTable(); for (int i = tab.length ; i-- > 0 ;) for (Entry e = tab[i] ; e != null ; e = e.next) if (e.value==null) return true; return false;
public boolean	containsValue(java.lang.Object value) Returns true if this map maps one or more keys to the specified value. param value value whose presence in this map is to be tested return true if this map maps one or more keys to the specified value if (value==null) return containsNullValue(); Entry[] tab = getTable(); for (int i = tab.length ; i-- > 0 ;) for (Entry e = tab[i] ; e != null ; e = e.next) if (value.equals(e.value)) return true; return false;
public java.util.Set	entrySet() Returns a {@link Set} view of the mappings contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress (except through the iterator's own remove operation, or through the setValue operation on a map entry returned by the iterator) the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Set.remove, removeAll, retainAll and clear operations. It does not support the add or addAll operations. Set<Map.Entry<K,V>> es = entrySet; return es != null ? es : (entrySet = new EntrySet());
static boolean	eq(java.lang.Object x, java.lang.Object y) Checks for equality of non-null reference x and possibly-null y. By default uses Object.equals. return x == y || x.equals(y);
private void	expungeStaleEntries() Expunges stale entries from the table. Entry<K,V> e; while ( (e = (Entry<K,V>) queue.poll()) != null) { int h = e.hash; int i = indexFor(h, table.length); Entry<K,V> prev = table[i]; Entry<K,V> p = prev; while (p != null) { Entry<K,V> next = p.next; if (p == e) { if (prev == e) table[i] = next; else prev.next = next; e.next = null; // Help GC e.value = null; // " " size--; break; } prev = p; p = next; } }
public V	get(java.lang.Object key) Returns the value to which the specified key is mapped, or {@code null} if this map contains no mapping for the key. More formally, if this map contains a mapping from a key {@code k} to a value {@code v} such that {@code (key==null ? k==null : key.equals(k))}, then this method returns {@code v}; otherwise it returns {@code null}. (There can be at most one such mapping.) A return value of {@code null} does not necessarily indicate that the map contains no mapping for the key; it's also possible that the map explicitly maps the key to {@code null}. The {@link #containsKey containsKey} operation may be used to distinguish these two cases. see #put(Object, Object) Object k = maskNull(key); int h = HashMap.hash(k.hashCode()); Entry[] tab = getTable(); int index = indexFor(h, tab.length); Entry<K,V> e = tab[index]; while (e != null) { if (e.hash == h && eq(k, e.get())) return e.value; e = e.next; } return null;
java.util.WeakHashMap$Entry	getEntry(java.lang.Object key) Returns the entry associated with the specified key in this map. Returns null if the map contains no mapping for this key. Object k = maskNull(key); int h = HashMap.hash(k.hashCode()); Entry[] tab = getTable(); int index = indexFor(h, tab.length); Entry<K,V> e = tab[index]; while (e != null && !(e.hash == h && eq(k, e.get()))) e = e.next; return e;
private java.util.WeakHashMap$Entry[]	getTable() Returns the table after first expunging stale entries. expungeStaleEntries(); return table;
static int	indexFor(int h, int length) Returns index for hash code h. return h & (length-1);
public boolean	isEmpty() Returns true if this map contains no key-value mappings. This result is a snapshot, and may not reflect unprocessed entries that will be removed before next attempted access because they are no longer referenced. return size() == 0;
public java.util.Set	keySet() Returns a {@link Set} view of the keys contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress (except through the iterator's own remove operation), the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Set.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations. Set<K> ks = keySet; return (ks != null ? ks : (keySet = new KeySet()));
private static java.lang.Object	maskNull(java.lang.Object key) Use NULL_KEY for key if it is null. return (key == null ? NULL_KEY : key);
public V	put(K key, V value) Associates the specified value with the specified key in this map. If the map previously contained a mapping for this key, the old value is replaced. param key key with which the specified value is to be associated. param value value to be associated with the specified key. return the previous value associated with key, or null if there was no mapping for key. (A null return can also indicate that the map previously associated null with key.) K k = (K) maskNull(key); int h = HashMap.hash(k.hashCode()); Entry[] tab = getTable(); int i = indexFor(h, tab.length); for (Entry<K,V> e = tab[i]; e != null; e = e.next) { if (h == e.hash && eq(k, e.get())) { V oldValue = e.value; if (value != oldValue) e.value = value; return oldValue; } } modCount++; Entry<K,V> e = tab[i]; tab[i] = new Entry<K,V>(k, value, queue, h, e); if (++size >= threshold) resize(tab.length * 2); return null;
public void	putAll(java.util.Map m) Copies all of the mappings from the specified map to this map. These mappings will replace any mappings that this map had for any of the keys currently in the specified map. param m mappings to be stored in this map. throws NullPointerException if the specified map is null. int numKeysToBeAdded = m.size(); if (numKeysToBeAdded == 0) return; /* * Expand the map if the map if the number of mappings to be added * is greater than or equal to threshold. This is conservative; the * obvious condition is (m.size() + size) >= threshold, but this * condition could result in a map with twice the appropriate capacity, * if the keys to be added overlap with the keys already in this map. * By using the conservative calculation, we subject ourself * to at most one extra resize. */ if (numKeysToBeAdded > threshold) { int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1); if (targetCapacity > MAXIMUM_CAPACITY) targetCapacity = MAXIMUM_CAPACITY; int newCapacity = table.length; while (newCapacity < targetCapacity) newCapacity <<= 1; if (newCapacity > table.length) resize(newCapacity); } for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) put(e.getKey(), e.getValue());
public V	remove(java.lang.Object key) Removes the mapping for a key from this weak hash map if it is present. More formally, if this map contains a mapping from key k to value v such that (key==null ? k==null : key.equals(k)), that mapping is removed. (The map can contain at most one such mapping.) Returns the value to which this map previously associated the key, or null if the map contained no mapping for the key. A return value of null does not necessarily indicate that the map contained no mapping for the key; it's also possible that the map explicitly mapped the key to null. The map will not contain a mapping for the specified key once the call returns. param key key whose mapping is to be removed from the map return the previous value associated with key, or null if there was no mapping for key Object k = maskNull(key); int h = HashMap.hash(k.hashCode()); Entry[] tab = getTable(); int i = indexFor(h, tab.length); Entry<K,V> prev = tab[i]; Entry<K,V> e = prev; while (e != null) { Entry<K,V> next = e.next; if (h == e.hash && eq(k, e.get())) { modCount++; size--; if (prev == e) tab[i] = next; else prev.next = next; return e.value; } prev = e; e = next; } return null;
java.util.WeakHashMap$Entry	removeMapping(java.lang.Object o) Special version of remove needed by Entry set if (!(o instanceof Map.Entry)) return null; Entry[] tab = getTable(); Map.Entry entry = (Map.Entry)o; Object k = maskNull(entry.getKey()); int h = HashMap.hash(k.hashCode()); int i = indexFor(h, tab.length); Entry<K,V> prev = tab[i]; Entry<K,V> e = prev; while (e != null) { Entry<K,V> next = e.next; if (h == e.hash && e.equals(entry)) { modCount++; size--; if (prev == e) tab[i] = next; else prev.next = next; return e; } prev = e; e = next; } return null;
void	resize(int newCapacity) Rehashes the contents of this map into a new array with a larger capacity. This method is called automatically when the number of keys in this map reaches its threshold. If current capacity is MAXIMUM_CAPACITY, this method does not resize the map, but sets threshold to Integer.MAX_VALUE. This has the effect of preventing future calls. param newCapacity the new capacity, MUST be a power of two; must be greater than current capacity unless current capacity is MAXIMUM_CAPACITY (in which case value is irrelevant). Entry[] oldTable = getTable(); int oldCapacity = oldTable.length; if (oldCapacity == MAXIMUM_CAPACITY) { threshold = Integer.MAX_VALUE; return; } Entry[] newTable = new Entry[newCapacity]; transfer(oldTable, newTable); table = newTable; /* * If ignoring null elements and processing ref queue caused massive * shrinkage, then restore old table. This should be rare, but avoids * unbounded expansion of garbage-filled tables. */ if (size >= threshold / 2) { threshold = (int)(newCapacity * loadFactor); } else { expungeStaleEntries(); transfer(newTable, oldTable); table = oldTable; }
public int	size() Returns the number of key-value mappings in this map. This result is a snapshot, and may not reflect unprocessed entries that will be removed before next attempted access because they are no longer referenced. if (size == 0) return 0; expungeStaleEntries(); return size;
private void	transfer(java.util.WeakHashMap$Entry[] src, java.util.WeakHashMap$Entry[] dest) Transfers all entries from src to dest tables for (int j = 0; j < src.length; ++j) { Entry<K,V> e = src[j]; src[j] = null; while (e != null) { Entry<K,V> next = e.next; Object key = e.get(); if (key == null) { e.next = null; // Help GC e.value = null; // " " size--; } else { int i = indexFor(e.hash, dest.length); e.next = dest[i]; dest[i] = e; } e = next; } }
private static K	unmaskNull(java.lang.Object key) Returns internal representation of null key back to caller as null. return (K) (key == NULL_KEY ? null : key);
public java.util.Collection	values() Returns a {@link Collection} view of the values contained in this map. The collection is backed by the map, so changes to the map are reflected in the collection, and vice-versa. If the map is modified while an iteration over the collection is in progress (except through the iterator's own remove operation), the results of the iteration are undefined. The collection supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Collection.remove, removeAll, retainAll and clear operations. It does not support the add or addAll operations. Collection<V> vs = values; return (vs != null ? vs : (values = new Values()));