Package org.apache.sis.util.collection
Class WeakValueHashMap<K,V>
Object
AbstractMap<K,V>
WeakValueHashMap<K,V>
- Type Parameters:
K
- the class of key elements.V
- the class of value elements.
- All Implemented Interfaces:
Map<K,
V>
A hashtable-based map implementation that uses weak references,
leaving memory when an entry is not used anymore. An entry in a
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:
In the above example, the calculation of a new value needs to be fast because it is performed inside a synchronized statement blocking all other access to the map. This is okay if that particularK 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:
Defined in the sis-utility
module
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Nested Class Summary
Nested classes/interfaces inherited from class AbstractMap
AbstractMap.SimpleEntry<K extends Object,
V extends Object>, AbstractMap.SimpleImmutableEntry<K extends Object, V extends Object> -
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. -
Method Summary
Modifier and TypeMethodDescriptionvoid
clear()
Removes all of the elements from this map.boolean
containsKey
(Object key) Returnstrue
if this map contains a mapping for the specified key.boolean
containsValue
(Object value) Returnstrue
if this map maps one or more keys to this 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.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.boolean
Removes 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.boolean
Replaces the entry for the specified key only if currently mapped to the specified value.int
size()
Returns the number of key-value mappings in this map.Methods inherited from class AbstractMap
clone, equals, hashCode, isEmpty, keySet, putAll, toString, values
Methods inherited from interface Map
compute, computeIfAbsent, computeIfPresent, forEach, getOrDefault, merge, replaceAll
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Constructor Details
-
WeakValueHashMap
Creates a newWeakValueHashMap
.- Parameters:
keyType
- the type of keys in the map.
-
WeakValueHashMap
Creates a newWeakValueHashMap
, optionally using reference-equality in place of object-equality. Ifidentity
istrue
, then two keysk1
andk2
are considered equal if and only if(k1 == k2)
instead of ifk1.equals(k2)
.Reference-equality semantic is rarely used. See the
IdentityHashMap
class 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
-true
if the map shall use reference-equality in place of object-equality when comparing keys, orfalse
for the standard behavior.- Since:
- 0.4
-
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Method Details
-
size
public int size()Returns the number of key-value mappings in this map. -
containsKey
Returnstrue
if this map contains a mapping for the specified key. Null keys are considered never present.- Specified by:
containsKey
in interfaceMap<K,
V> - Overrides:
containsKey
in classAbstractMap<K,
V> - Parameters:
key
- key whose presence in this map is to be tested.- Returns:
true
if this map contains a mapping for the specified key.
-
containsValue
Returnstrue
if this map maps one or more keys to this value. Null values are considered never present.- Specified by:
containsValue
in interfaceMap<K,
V> - Overrides:
containsValue
in classAbstractMap<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 this value.
-
get
Returns the value to which this map maps the specified key. Returnsnull
if the map contains no mapping for this key. Null keys are considered never present. -
put
Associates the specified value with the specified key in this map. The value is associated using aWeakReference
.- Specified by:
put
in interfaceMap<K,
V> - Overrides:
put
in 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
null
if there was no mapping for the key. - Throws:
NullArgumentException
- if the key or the value isnull
.
-
putIfAbsent
Associates the specified value with the specified key in this map if no value were previously associated. If an other 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 aWeakReference
andnull
is 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
null
if there was no mapping for the key. - Throws:
NullArgumentException
- if the key or the value isnull
.- Since:
- 0.7
-
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
null
if there was no mapping for the key. - Throws:
NullArgumentException
- if the value isnull
.- Since:
- 1.2
-
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:
true
if the value was replaced.- Throws:
NullArgumentException
- if the new value isnull
.- Since:
- 1.2
-
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:
true
if the value was removed.- Since:
- 1.2
-
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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