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WeakHashMap.javaAPI DocJava SE 5 API33028Fri Aug 26 14:57:24 BST 2005java.util

WeakHashMap

public class WeakHashMap extends AbstractMap implements Map
A hashtable-based Map implementation with weak keys. An entry in a WeakHashMap will automatically be removed when its key is no longer in ordinary use. More precisely, the presence of a mapping for a given key will not prevent the key from being discarded by the garbage collector, that is, made finalizable, finalized, and then reclaimed. When a key has been discarded its entry is effectively removed from the map, so this class behaves somewhat differently than other Map implementations.

Both null values and the null key are supported. This class has performance characteristics similar to those of the HashMap class, and has the same efficiency parameters of initial capacity and load factor.

Like most collection classes, this class is not synchronized. A synchronized WeakHashMap may be constructed using the Collections.synchronizedMap method.

This class is intended primarily for use with key objects whose equals methods test for object identity using the == operator. Once such a key is discarded it can never be recreated, so it is impossible to do a lookup of that key in a WeakHashMap at some later time and be surprised that its entry has been removed. This class will work perfectly well with key objects whose equals methods are not based upon object identity, such as String instances. With such recreatable key objects, however, the automatic removal of WeakHashMap entries whose keys have been discarded may prove to be confusing.

The behavior of the WeakHashMap class depends in part upon the actions of the garbage collector, so several familiar (though not required) Map invariants do not hold for this class. Because the garbage collector may discard keys at any time, a WeakHashMap may behave as though an unknown thread is silently removing entries. In particular, even if you synchronize on a WeakHashMap instance and invoke none of its mutator methods, it is possible for the size method to return smaller values over time, for the isEmpty method to return false and then true, for the containsKey method to return true and later false for a given key, for the get method to return a value for a given key but later return null, for the put method to return null and the remove method to return false for a key that previously appeared to be in the map, and for successive examinations of the key set, the value set, and the entry set to yield successively smaller numbers of elements.

Each key object in a WeakHashMap is stored indirectly as the referent of a weak reference. Therefore a key will automatically be removed only after the weak references to it, both inside and outside of the map, have been cleared by the garbage collector.

Implementation note: The value objects in a WeakHashMap are held by ordinary strong references. Thus care should be taken to ensure that value objects do not strongly refer to their own keys, either directly or indirectly, since that will prevent the keys from being discarded. Note that a value object may refer indirectly to its key via the WeakHashMap itself; that is, a value object may strongly refer to some other key object whose associated value object, in turn, strongly refers to the key of the first value object. One way to deal with this is to wrap values themselves within WeakReferences before inserting, as in: m.put(key, new WeakReference(value)), and then unwrapping upon each get.

The iterators returned by all of this class's "collection view methods" are fail-fast: if the map is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove or add methods, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.

This class is a member of the Java Collections Framework.

version
1.30, 02/19/04
author
Doug Lea
author
Josh Bloch
author
Mark Reinhold
since
1.2
see
java.util.HashMap
see
java.lang.ref.WeakReference

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 HashMap has been structurally modified Structural modifications are those that change the number of mappings in the HashMap or otherwise modify its internal structure (e.g., rehash). This field is used to make iterators on Collection-views of the HashMap fail-fast. (See ConcurrentModificationException).
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, which is 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 the default load factor (0.75).

        this.loadFactor = DEFAULT_LOAD_FACTOR;
        threshold = (int)(DEFAULT_INITIAL_CAPACITY);
        table = new Entry[DEFAULT_INITIAL_CAPACITY];
    
public WeakHashMap(Map t)
Constructs a new WeakHashMap with the same mappings as the specified Map. The WeakHashMap is created with default load factor, which is 0.75 and an initial capacity sufficient to hold the mappings in the specified Map.

param
t 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) (t.size() / DEFAULT_LOAD_FACTOR) + 1, 16),
             DEFAULT_LOAD_FACTOR);
        putAll(t);
    
Methods Summary
public voidclear()
Removes all mappings from this map.

        // 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 booleancontainsKey(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 booleancontainsNullValue()
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 booleancontainsValue(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.SetentrySet()
Returns a collection view of the mappings contained in this map. Each element in the returned collection is a Map.Entry. The collection is backed by the map, so changes to the map are reflected in the collection, and vice-versa. 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.

return
a collection view of the mappings contained in this map.
see
Map.Entry

        Set<Map.Entry<K,V>> es = entrySet;
        return (es != null ? es : (entrySet = new EntrySet()));
    
static booleaneq(java.lang.Object x, java.lang.Object y)
Check for equality of non-null reference x and possibly-null y. By default uses Object.equals.

        return x == y || x.equals(y);
    
private voidexpungeStaleEntries()
Expunge 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 Vget(java.lang.Object key)
Returns the value to which the specified key is mapped in this weak hash map, or null if the map contains no mapping for this key. A return value of null does not necessarily indicate that the map contains no mapping for the key; it is also possible that the map explicitly maps the key to null. The containsKey method may be used to distinguish these two cases.

param
key the key whose associated value is to be returned.
return
the value to which this map maps the specified key, or null if the map contains no mapping for this key.
see
#put(Object, Object)

        Object k = maskNull(key);
        int h = HashMap.hash(k);
        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$EntrygetEntry(java.lang.Object key)
Returns the entry associated with the specified key in the HashMap. Returns null if the HashMap contains no mapping for this key.

        Object k = maskNull(key);
        int h = HashMap.hash(k);
        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()
Return the table after first expunging stale entries

        expungeStaleEntries();
        return table;
    
static intindexFor(int h, int length)
Return index for hash code h.

        return h & (length-1);
    
public booleanisEmpty()
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.SetkeySet()
Returns a 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. The set supports element removal, which removes the corresponding mapping from this map, via the Iterator.remove, Set.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations.

return
a set view of the keys contained in this map.


                                                                                    
       
        Set<K> ks = keySet;
        return (ks != null ? ks : (keySet = new KeySet()));
    
private static java.lang.ObjectmaskNull(java.lang.Object key)
Use NULL_KEY for key if it is null.


                 
         
        return (key == null ? NULL_KEY : key);
    
public Vput(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
previous value associated with specified key, or null if there was no mapping for key. A null return can also indicate that the HashMap previously associated null with the specified key.

        K k = (K) maskNull(key);
        int h = HashMap.hash(k);
        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 voidputAll(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 (Iterator<? extends Map.Entry<? extends K, ? extends V>> i = m.entrySet().iterator(); i.hasNext(); ) {
            Map.Entry<? extends K, ? extends V> e = i.next();
            put(e.getKey(), e.getValue());
        }
    
public Vremove(java.lang.Object key)
Removes the mapping for this key from this map if present.

param
key key whose mapping is to be removed from the map.
return
previous value associated with specified key, or null if there was no mapping for key. A null return can also indicate that the map previously associated null with the specified key.

        Object k = maskNull(key);
        int h = HashMap.hash(k);
        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$EntryremoveMapping(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);
        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;
    
voidresize(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 intsize()
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 voidtransfer(java.util.WeakHashMap$Entry[] src, java.util.WeakHashMap$Entry[] dest)
Transfer 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 KunmaskNull(java.lang.Object key)
Return internal representation of null key back to caller as null

        return (K) (key == NULL_KEY ? null : key);
    
public java.util.Collectionvalues()
Returns a 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. The collection supports element removal, which removes the corresponding mapping from this map, via the Iterator.remove, Collection.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations.

return
a collection view of the values contained in this map.

        Collection<V> vs = values;
        return (vs != null ?  vs : (values = new Values()));