- Type Parameters:
K- the class of key elements.V- the class of value elements.
- All Implemented Interfaces:
Map<K,V>
WeakValueHashMap
will automatically be removed when its value is no longer in ordinary use. This class is
similar to the standard WeakHashMap class, except that weak references
apply to values rather than keys.
Note that this class is not a cache, because the entries are discarded
as soon as the garbage collector determines that they are no longer in use. If caching
service are wanted, or if concurrency are wanted, consider using Cache instead.
This class is convenient for avoiding the creation of duplicated elements, as in the example below:
K key = ...
V value;
synchronized (map) {
value = map.get(key);
if (value == null) {
value = ...; // Create the value here.
map.put(key, value);
}
}
WeakValueHashMap instance
is not expected to be used in a highly concurrent environment.
WeakValueHashMap works with array keys as one would expect. For example, arrays of int[] are
compared using the Arrays.equals(int[], int[]) method.
Thread safety
The sameWeakValueHashMap instance can be safely used by many threads without synchronization on the part
of the caller. But if a sequence of two or more method calls need to appear atomic from other threads perspective,
then the caller can synchronize on this.- Since:
- 0.3
- See Also:
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Nested Class Summary
Nested classes/interfaces inherited from class AbstractMap
AbstractMap.SimpleEntry<K,V>, AbstractMap.SimpleImmutableEntry<K, V> -
Constructor Summary
ConstructorsConstructorDescriptionWeakValueHashMap(Class<K> keyType) Creates a newWeakValueHashMap.WeakValueHashMap(Class<K> keyType, boolean identity) Creates a newWeakValueHashMap, optionally using reference-equality in place of object-equality.WeakValueHashMap(Class<K> keyType, ToIntFunction<Object> hashFunction, BiPredicate<Object, Object> comparator) Creates a newWeakValueHashMapusing the given functions for computing hash code and equality. -
Method Summary
Modifier and TypeMethodDescriptionvoidclear()Removes all of the elements from this map.booleancontainsKey(Object key) Returnstrueif this map contains a mapping for the specified key.booleancontainsValue(Object value) Returnstrueif this map maps one or more keys to the specified value.Set<Map.Entry<K, V>> Returns a set view of the mappings contained in this map.Returns the value to which this map maps the specified key.getOrDefault(Object key, V defaultValue) Returns the value to which this map maps the specified key.If this map contains the specified key, returns the instance contained in this map.Associates the specified value with the specified key in this map.putIfAbsent(K key, V value) Associates the specified value with the specified key in this map if no value were previously associated.Removes the mapping for this key from this map if present.booleanRemoves the entry for the specified key only if it is currently mapped to the specified value.Replaces the entry for the specified key only if it is currently mapped to some value.booleanReplaces the entry for the specified key only if currently mapped to the specified value.intsize()Returns the number of key-value mappings in this map.Methods inherited from class AbstractMap
clone, equals, hashCode, isEmpty, keySet, putAll, toString, valuesMethods inherited from interface Map
compute, computeIfAbsent, computeIfPresent, forEach, merge, replaceAll
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Constructor Details
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WeakValueHashMap
Creates a newWeakValueHashMap.- Parameters:
keyType- the type of keys in the map.
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WeakValueHashMap
Creates a newWeakValueHashMap, optionally using reference-equality in place of object-equality. Ifidentityistrue, then two keysk1andk2are considered equal if and only if(k1 == k2)instead of ifk1.equals(k2).Reference-equality semantic is rarely used. See the
IdentityHashMapclass javadoc for a discussion about drawbacks and use cases when reference-equality semantic is useful.- Parameters:
keyType- the type of keys in the map.identity-trueif the map shall use reference-equality in place of object-equality when comparing keys, orfalsefor the standard behavior.- Since:
- 0.4
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WeakValueHashMap
public WeakValueHashMap(Class<K> keyType, ToIntFunction<Object> hashFunction, BiPredicate<Object, Object> comparator) Creates a newWeakValueHashMapusing the given functions for computing hash code and equality. Commonly used functions are:
The functions do not need to be null-safe.Frequently used hash and comparison functions Desired behavior Hash function Comparator Standard (like HashMap)Object.hashCode()Object.equals(Object)Robust to arrays Utilities.deepHashCode(Object)Objects.deepEquals(Object, Object)Identity (like IdentityHashMap)System.identityHashCode(Object)(o1,o2) -> o1 == o2WeakValueHashMapwill never invokeToIntFunction.applyAsInt(Object)orBiPredicate.test(Object, Object)with null arguments.- Parameters:
keyType- the type of keys in the map.hashFunction- the function to invoke for computing hash codes.comparator- the function to invoke for comparing two objects.- Since:
- 1.4
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Method Details
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size
public int size()Returns the number of key-value mappings in this map. -
intern
If this map contains the specified key, returns the instance contained in this map. Otherwise returns the givenkeyinstance.This method can be useful when the keys are potentially large objects. It allows to opportunistically share existing instances, a little bit like when using
WeakHashSetexcept that this method does not add the given key to this map if not present.- Parameters:
key- key to look for in this map.- Returns:
- the key instance in this map which is equal to the specified key, or
keyif none. - Since:
- 1.4
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containsKey
Returnstrueif this map contains a mapping for the specified key. Null keys are considered never present.- Specified by:
containsKeyin interfaceMap<K,V> - Overrides:
containsKeyin classAbstractMap<K,V> - Parameters:
key- key whose presence in this map is to be tested.- Returns:
trueif this map contains a mapping for the specified key.
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containsValue
Returnstrueif this map maps one or more keys to the specified value. Null values are considered never present.- Specified by:
containsValuein interfaceMap<K,V> - Overrides:
containsValuein classAbstractMap<K,V> - Parameters:
value- value whose presence in this map is to be tested.- Returns:
trueif this map maps one or more keys to the specified value.
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get
Returns the value to which this map maps the specified key. Returnsnullif the map contains no mapping for this key. Null keys are considered never present. -
getOrDefault
Returns the value to which this map maps the specified key. ReturnsdefaultValueif the map contains no mapping for this key. Null keys are considered never present.- Parameters:
key- key whose associated value is to be returned.defaultValue- the default mapping of the key.- Returns:
- the value to which this map maps the specified key.
- Since:
- 1.4
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put
Associates the specified value with the specified key in this map. The value is associated using aWeakReference.- Specified by:
putin interfaceMap<K,V> - Overrides:
putin classAbstractMap<K,V> - Parameters:
key- key with which the specified value is to be associated.value- value to be associated with the specified key.- Returns:
- the previous value associated with specified key, or
nullif there was no mapping for the key. - Throws:
NullPointerException- if the key or the value isnull.
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putIfAbsent
Associates the specified value with the specified key in this map if no value were previously associated. If another value is already associated to the given key, then the map is left unchanged and the current value is returned. Otherwise the specified value is associated to the key using aWeakReferenceandnullis returned.- Parameters:
key- key with which the specified value is to be associated.value- value to be associated with the specified key.- Returns:
- the current value associated with specified key, or
nullif there was no mapping for the key. - Throws:
NullPointerException- if the key or the value isnull.- Since:
- 0.7
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replace
Replaces the entry for the specified key only if it is currently mapped to some value.- Parameters:
key- key with which the specified value is to be associated.value- value to be associated with the specified key.- Returns:
- the previous value associated with specified key, or
nullif there was no mapping for the key. - Throws:
NullPointerException- if the value isnull.- Since:
- 1.2
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replace
Replaces the entry for the specified key only if currently mapped to the specified value.- Parameters:
key- key with which the specified value is to be associated.oldValue- value expected to be associated with the specified key.newValue- value to be associated with the specified key.- Returns:
trueif the value was replaced.- Throws:
NullPointerException- if the new value isnull.- Since:
- 1.2
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remove
Removes the mapping for this key from this map if present. -
remove
Removes the entry for the specified key only if it is currently mapped to the specified value.- Parameters:
key- key whose mapping is to be removed from the map.value- value expected to be associated with the specified key.- Returns:
trueif the value was removed.- Since:
- 1.2
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clear
public void clear()Removes all of the elements from this map. -
entrySet
Returns a set view of the mappings contained in this map. Each element in this set is aMap.Entry.
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