java.util

Class WeakHashMap<K,V>

Implemented Interfaces:
Map<K,V>
Known Direct Subclasses:
CacheMap

public class WeakHashMap<K,V>
extends AbstractMap<K,V>
implements Map<K,V>

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 ../../../guide/collections/index.html"> Java Collections Framework.

Since:
1.2
See Also:
HashMap, WeakReference

Constructor Summary

WeakHashMap()
Constructs a new, empty WeakHashMap with the default initial capacity (16) and the default load factor (0.75).
WeakHashMap(extends K, V> t)
Constructs a new WeakHashMap with the same mappings as the specified Map.
WeakHashMap(int initialCapacity)
Constructs a new, empty WeakHashMap with the given initial capacity and the default load factor, which is 0.75.
WeakHashMap(int initialCapacity, float loadFactor)
Constructs a new, empty WeakHashMap with the given initial capacity and the given load factor.

Method Summary

void
clear()
Removes all mappings from this map.
boolean
containsKey(Object key)
Returns true if this map contains a mapping for the specified key.
boolean
containsValue(Object value)
Returns true if this map maps one or more keys to the specified value.
Set>
entrySet()
Returns a collection view of the mappings contained in this map.
V
get(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.
boolean
isEmpty()
Returns true if this map contains no key-value mappings.
Set
keySet()
Returns a set view of the keys contained in this map.
V
put(K key, V value)
Associates the specified value with the specified key in this map.
void
putAll(extends K, V> 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.
V
remove(Object key)
Removes the mapping for this key from this map if present.
int
size()
Returns the number of key-value mappings in this map.
Collection
values()
Returns a collection view of the values contained in this map.

Methods inherited from class java.util.AbstractMap<K,V>

clear, clone, containsKey, containsValue, entrySet, equals, get, hashCode, isEmpty, keySet, put, putAll, remove, size, toString, values

Methods inherited from class java.lang.Object

clone, equals, extends Object> getClass, finalize, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Details

WeakHashMap

public WeakHashMap()
Constructs a new, empty WeakHashMap with the default initial capacity (16) and the default load factor (0.75).
Usages and Demos :

View More Examples of WeakHashMap()
   1:   public static void main (String args[]) {
   2:     map = new WeakHashMap();
   3:     map.put(new String("Maine"), "Augusta");
   4:     Runnable runner = new Runnable() {
   5:       public void run() {

View Full Code Here
   1: 
   2:     private WeakHashMap listeners = new WeakHashMap();
   3: 
   4:     public void addListener(Listener listener) {
   5:         listeners.put(listener, null);

View Full Code Here
   1: 
   2:     private WeakHashMap map = new WeakHashMap();
   3: 
   4:         ...
   5:     public void reset() {
   6:         this.map = new WeakHashMap();
   7:     }
   8: }

View Full Code Here
   1:     public EqualityEncounterCounter()
   2:     { super( new WeakHashMap() ); }
   3: }

View Full Code Here

WeakHashMap

public WeakHashMap(extends K,
                   V> 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.
Parameters:
t - the map whose mappings are to be placed in this map.
Throws:
NullPointerException - if the specified map is null.
Since:
1.3
Usages and Demos :

View More Examples of WeakHashMap(extends K,V> t)
   1:     public WeakHashSet(final int initialCapacity, final float loadFactor) {
   2:         backingStore = new WeakHashMap(initialCapacity, loadFactor);
   3:     }
   4: 
   5:     public WeakHashSet(final int initialCapacity) {

View Full Code Here

WeakHashMap

public WeakHashMap(int initialCapacity)
Constructs a new, empty WeakHashMap with the given initial capacity and the default load factor, which is 0.75.
Parameters:
initialCapacity - The initial capacity of the WeakHashMap
Throws:
IllegalArgumentException - If the initial capacity is negative.
Usages and Demos :

View More Examples of WeakHashMap(int initialCapacity)
   1:   private long cacheDuration;
   2:   private WeakHashMap subjects = new WeakHashMap(5);
   3: 
   4:   private class CacheEntry {
   5:     private long validity;

View Full Code Here
   1: 
   2:     private Map slots = new WeakHashMap(100);
   3:     private int lastFutureResultID=0;
   4:     
   5:     synchronized private int getNextFutureResultID() {

View Full Code Here
   1: 
   2:   private static Map loggerCache = new WeakHashMap(11);
   3: 
   4:   public static Logger getLogger(Object name) {
   5:     synchronized (loggerCache) {

View Full Code Here
   1: 
   2:     private static final WeakHashMap entryMap = new WeakHashMap(17);
   3:     private static final FieldComparator comparator = new FieldComparator();
   4:     private static final ServiceType[] empty = {};
   5:     private static final Class[] noArg = new Class[0];

View Full Code Here
   1: 
   2:     private static final WeakHashMap entryMap = new WeakHashMap(17);
   3:     private static final FieldComparator comparator = new FieldComparator();
   4:     private static final ServiceType[] empty = {};
   5:     private static final Class[] noArg = new Class[0];

View Full Code Here

WeakHashMap

public WeakHashMap(int initialCapacity,
                   float loadFactor)
Constructs a new, empty WeakHashMap with the given initial capacity and the given load factor.
Parameters:
initialCapacity - The initial capacity of the WeakHashMap
loadFactor - The load factor of the WeakHashMap
Throws:
IllegalArgumentException - If the initial capacity is negative, or if the load factor is nonpositive.
Usages and Demos :

View More Examples of WeakHashMap(int initialCapacity,float loadFactor)
   1:     public WeakHashSet(final int initialCapacity, final float loadFactor) {
   2:         backingStore = new WeakHashMap(initialCapacity, loadFactor);
   3:     }
   4: 
   5:     public WeakHashSet(final int initialCapacity) {

View Full Code Here

Method Details

clear

public void clear()
Removes all mappings from this map.
Specified by:
clear in interface Map<K,V>
Overrides:
clear in interface AbstractMap<K,V>
Usages and Demos :

View More Examples of clear()
   1: import java.util.Iterator;
   2: import java.util.WeakHashMap;
   3: 
   4:         ...
   5: 
   6:     private WeakHashMap listeners = new WeakHashMap();
   7: 
   8:         ...
   9:     public void clear() {
  10:         listeners.clear();
  11:     }

View Full Code Here
   1: 
   2:     WeakHashMap resources = new WeakHashMap();
   3: 
   4:         ...
   5:     public synchronized void reset(){
   6:         resources.clear();
   7:     }
   8: }

View Full Code Here
   1: 
   2: import java.util.WeakHashMap;
   3: 
   4:         ...
   5: {
   6:     private static WeakHashMap registry = new WeakHashMap();
   7: 
   8:         ...
   9:     {
  10:         registry.clear();
  11:     }

View Full Code Here
   1: 
   2: import java.util.WeakHashMap;
   3: 
   4:         ...
   5: {
   6:     private static WeakHashMap registry = new WeakHashMap();
   7:     
   8:         ...
   9:     {
  10:         registry.clear();
  11:     }

View Full Code Here
   1:   private boolean     init_,loading_;
   2:   private WeakHashMap waiters_;
   3: 
   4:         ...
   5:     loading_=false;
   6:     waiters_=new WeakHashMap(7);
   7:     setDescription("lazy icon "+_url);
   8:         ...
   9:     }
  10:     waiters_.clear();
  11:     waiters_=null;

View Full Code Here

containsKey

public boolean containsKey(Object key)
Returns true if this map contains a mapping for the specified key.
Specified by:
containsKey in interface Map<K,V>
Overrides:
containsKey in interface AbstractMap<K,V>
Parameters:
key - The key whose presence in this map is to be tested
Returns:
true if there is a mapping for key; false otherwise
Usages and Demos :

View More Examples of containsKey(Object key)
   1: import java.util.Set;
   2: import java.util.WeakHashMap;
   3: 
   4:         ...
   5: public class WeakHashSet implements Set {
   6:     private WeakHashMap map = new WeakHashMap();
   7: 
   8:         ...
   9:     public boolean contains(Object o) {
  10:         return map.containsKey(o);
  11:     }

View Full Code Here
   1: import java.util.Set;
   2: import java.util.WeakHashMap;
   3: 
   4:         ...
   5: public class WeakHashSet extends AbstractSet implements Set {
   6:     WeakHashMap backingStore = new WeakHashMap();
   7: 
   8:         ...
   9:     public boolean contains(final Object o) {
  10:         return backingStore.containsKey(o);
  11:     }

View Full Code Here
   1: import java.util.HashSet;
   2: import java.util.WeakHashMap;
   3: import javax.transaction.TransactionRolledbackException;
   4:         ...
   5:    public static final HashSet colocation = new HashSet();
   6:    public static final WeakHashMap txFailoverAuthorizations = new WeakHashMap();
   7: 
   8:         ...
   9:          {
  10:             return ! txFailoverAuthorizations.containsKey(tx);               
  11:          }

View Full Code Here
   1: import java.util.Set;
   2: import java.util.WeakHashMap;
   3: 
   4:         ...
   5:     {
   6:         if (cache.containsKey(f))
   7:         {
   8:         ...
   9:     {
  10:         private WeakHashMap map = new WeakHashMap();
  11: 
  12:         ...
  13:         {
  14:             return map.containsKey(key);

View Full Code Here

containsValue

public boolean containsValue(Object value)
Returns true if this map maps one or more keys to the specified value.
Specified by:
containsValue in interface Map<K,V>
Overrides:
containsValue in interface AbstractMap<K,V>
Parameters:
value - value whose presence in this map is to be tested.
Returns:
true if this map maps one or more keys to the specified value.

entrySet

public Set> entrySet()
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.
Overrides:
entrySet in interface AbstractMap<K,V>
Returns:
a collection view of the mappings contained in this map.
See Also:
Map.Entry

get

public V get(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.
Specified by:
get in interface Map<K,V>
Overrides:
get in interface AbstractMap<K,V>
Parameters:
key - the key whose associated value is to be returned.
Returns:
the value to which this map maps the specified key, or null if the map contains no mapping for this key.
See Also:
put(Object, Object)
Usages and Demos :

View More Examples of get(Object key)
   1: import java.awt.CheckboxGroup;
   2: import java.util.WeakHashMap;
   3: 
   4:         ...
   5: {
   6:   private static WeakHashMap map = new WeakHashMap ();
   7: 
   8:         ...
   9:       return null;
  10:     GtkCheckboxGroupPeer nat = (GtkCheckboxGroupPeer) map.get(group);
  11:     if (nat == null)

View Full Code Here
   1: import java.util.Map;
   2: import java.util.WeakHashMap;
   3: 
   4:         ...
   5:     private volatile int _managementPort;
   6:     private final WeakHashMap _beans = new WeakHashMap();
   7:     private final HashMap _unique = new HashMap();
   8:         ...
   9:     {
  10:         ObjectName bean = (ObjectName)_beans.get(object);
  11:         return bean==null?null:bean; 
  12:         ...
  13:     {
  14:         ObjectName parent=(ObjectName)_beans.get(relationship.getParent());

View Full Code Here
   1: import java.util.StringTokenizer;
   2: import java.util.WeakHashMap;
   3: 
   4:         ...
   5:     private static final String JDBC_TYPES_FILE = "jdbc-types.csv";
   6:     private static final WeakHashMap MAPPING_STRATEGIES = new WeakHashMap();
   7:     private static final Class[] PARM_CLASS_INT = new Class[] { int.class };
   8:         ...
   9:                     JdbcType jdbcType = 
  10:                         (JdbcType) JDBC_TYPES_BY_INT.get(typeSeq[seqIdx]);
  11:                     jdbcType.addGetter(methodName, token.equals("P"));
  12:         ...
  13:         }
  14:         ExpandingArrayList strategyList = (ExpandingArrayList) MAPPING_STRATEGIES.get(row.getMappingStrategyKey());

View Full Code Here
   1: import java.util.Set;
   2: import java.util.WeakHashMap;
   3: 
   4:         ...
   5:             BeadFeatureRenderer cachedRenderer =
   6:                 (BeadFeatureRenderer) cache.get(f);
   7: 
   8:         ...
   9:                 FeatureRenderer delegate =
  10:                     (FeatureRenderer) delegates.get(filter);
  11: 
  12:         ...
  13:     {
  14:         private WeakHashMap map = new WeakHashMap();

View Full Code Here

isEmpty

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.
Specified by:
isEmpty in interface Map<K,V>
Overrides:
isEmpty in interface AbstractMap<K,V>

keySet

public Set keySet()
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.
Specified by:
keySet in interface Map<K,V>
Overrides:
keySet in interface AbstractMap<K,V>
Returns:
a set view of the keys contained in this map.
Usages and Demos :

View More Examples of keySet()
   1: import java.util.Set;
   2: import java.util.WeakHashMap;
   3: 
   4:         ...
   5: public class WeakHashSet extends AbstractSet implements Set {
   6:     WeakHashMap backingStore = new WeakHashMap();
   7: 
   8:         ...
   9:     public boolean isEmpty() {
  10:         return backingStore.keySet()
  11:                            .isEmpty();
  12:         ...
  13:     public boolean containsAll(final Collection c) {
  14:         return backingStore.keySet()

View Full Code Here
   1: import java.util.Map;
   2: import java.util.WeakHashMap;
   3: 
   4:         ...
   5:     
   6:     private static WeakHashMap ALL_CONNECTION_MANAGERS = new WeakHashMap();
   7:     
   8:         ...
   9:                     connManagers = (MultiThreadedHttpConnectionManager[])
  10:                     ALL_CONNECTION_MANAGERS.keySet().toArray(
  11:                         new MultiThreadedHttpConnectionManager
  12:         ...
  13:             
  14:             Iterator referenceIter = REFERENCE_TO_CONNECTION_SOURCE.keySet().iterator();

View Full Code Here

put

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.
Specified by:
put in interface Map<K,V>
Overrides:
put in interface AbstractMap<K,V>
Parameters:
key - key with which the specified value is to be associated.
value - value to be associated with the specified key.
Returns:
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.
Usages and Demos :

View More Examples of put(K key,V value)
   1: import java.io.FileNotFoundException;
   2: import java.util.WeakHashMap;
   3: 
   4:         ...
   5: 
   6:     private static WeakHashMap envMap = new WeakHashMap();
   7: 
   8:         ...
   9:                 myEnv = new DataEnvironment(env);
  10:                 envMap.put(env, myEnv);
  11:             }

View Full Code Here
   1: import java.util.Hashtable;
   2: import java.util.WeakHashMap;
   3: import java.security.AccessController;
   4:         ...
   5:     
   6:    private static WeakHashMap encs = new WeakHashMap();
   7:    private static ClassLoader topLoader;
   8:         ...
   9:                compCtx = new NamingContext(environment, null, srv);
  10:                encs.put(ctxClassLoader, compCtx);
  11:             }

View Full Code Here
   1: 
   2: import java.util.WeakHashMap;
   3: import java.util.Collection;
   4:         ...
   5:     
   6:     private WeakHashMap mappingColleagueToMediator = new WeakHashMap();
   7: 
   8:         ...
   9:     public void    setMediator(Colleague colleague, Mediator mediator) { 
  10:         mappingColleagueToMediator.put(colleague, mediator); 
  11:     }

View Full Code Here
   1:                     correspondingDescriptor = new PluginDescriptor(bundle);
   2:                     descriptors.put(bundle, correspondingDescriptor);
   3:                 }
   4:                 return correspondingDescriptor;
   5:             }

View Full Code Here
   1:  
   2: import java.util.WeakHashMap;
   3: import java.util.Collection;
   4:         ...
   5:     
   6:     private WeakHashMap mappingInvokerToCommand = new WeakHashMap();
   7:     
   8:         ...
   9:     public Object setCommand(CommandInvoker invoker, Command command) { 
  10:         return mappingInvokerToCommand.put(invoker, command); 
  11:     }
  12:         ...
  13:     public Object setReceiver(Command command, CommandReceiver receiver) { 
  14:         return mappingCommandToReceiver.put(command, receiver); 

View Full Code Here

putAll

public void putAll(extends K,
                   V> 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.

Specified by:
putAll in interface Map<K,V>
Overrides:
putAll in interface AbstractMap<K,V>
Parameters:
m - mappings to be stored in this map.
Throws:
NullPointerException - if the specified map is null.

remove

public V remove(Object key)
Removes the mapping for this key from this map if present.
Specified by:
remove in interface Map<K,V>
Overrides:
remove in interface AbstractMap<K,V>
Parameters:
key - key whose mapping is to be removed from the map.
Returns:
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.
Usages and Demos :

View More Examples of remove(Object key)
   1: import java.util.Iterator;
   2: import java.util.WeakHashMap;
   3: 
   4:         ...
   5: 
   6:     private WeakHashMap listeners = new WeakHashMap();
   7: 
   8:         ...
   9:     public void removeListener(Listener listener) {
  10:         listeners.remove(listener);
  11:     }

View Full Code Here
   1: 
   2:     WeakHashMap resources = new WeakHashMap();
   3: 
   4:         ...
   5:        public synchronized void invalidate(Object key){
   6:        resources.remove(key);
   7:     }
   8: 
   9:     public synchronized void reset(){

View Full Code Here
   1: 
   2: import java.util.WeakHashMap;
   3: 
   4:         ...
   5: {
   6:     private static WeakHashMap registry = new WeakHashMap();
   7: 
   8:         ...
   9:         if(object != null)
  10:             registry.remove(object);
  11:     }

View Full Code Here
   1: 
   2: import java.util.WeakHashMap;
   3: 
   4:         ...
   5: {
   6:     private static WeakHashMap registry = new WeakHashMap();
   7:     
   8:         ...
   9:         if(object != null)
  10:             registry.remove(object);
  11:     }

View Full Code Here
   1: import java.util.HashMap;
   2: import java.util.WeakHashMap;
   3: import java.util.Iterator;
   4:         ...
   5:         HashMap map = (HashMap) currentBrokerMap.get();
   6:         WeakHashMap set = null;
   7:         if (map == null)
   8:         ...
   9:         {
  10:             set = (WeakHashMap) map.get(key);
  11:         }
  12:         ...
  13:             {
  14:                 set.remove(broker);

View Full Code Here

size

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.
Specified by:
size in interface Map<K,V>
Overrides:
size in interface AbstractMap<K,V>

values

public Collection values()
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.
Specified by:
values in interface Map<K,V>
Overrides:
values in interface AbstractMap<K,V>
Returns:
a collection view of the values contained in this map.