Module org.apache.sis.util
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>
public class WeakValueHashMap<K,V> extends AbstractMap<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:

K key = ...
V value;
synchronized (map) {
    value = map.get(key);
    if (value == null) {
        value = ...; // Create the value here.
        map.put(key, value);
    }
}
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 particular 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 same Weak­Value­Hash­Map 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:

  • Constructor Details

    • WeakValueHashMap

      public WeakValueHashMap(Class<K> keyType)
      Creates a new Weak­Value­Hash­Map.
      Parameters:
      key­Type - the type of keys in the map.

    • WeakValueHashMap

      public WeakValueHashMap(Class<K> keyType, boolean identity)
      Creates a new Weak­Value­Hash­Map, optionally using reference-equality in place of object-equality. If identity is true, then two keys k1 and k2 are considered equal if and only if (k1 == k2) instead of if k1​.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, or false for the standard behavior.
      Since:
      0.4

    • WeakValueHashMap

      public WeakValueHashMap(Class<K> keyType, ToIntFunction<Object> hashFunction, BiPredicate<Object,Object> comparator)
      Creates a new Weak­Value­Hash­Map using the given functions for computing hash code and equality. Commonly used functions are:
      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 == o2
      The functions do not need to be null-safe. Weak­Value­Hash­Map will never invoke To­Int­Function​.apply­As­Int(Object) or Bi­Predicate​.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

  • Method Details

    • size

      public int size()
      Returns the number of key-value mappings in this map.
      Specified by:
      size in interface Map<K,V>
      Overrides:
      size in class Abstract­Map<K,V>
      Returns:
      the number of entries in this map.

    • intern

      public K intern(K key)
      If this map contains the specified key, returns the instance contained in this map. Otherwise returns the given key instance.

      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 Weak­Hash­Set except 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 key if none.
      Since:
      1.4

    • containsKey

      public boolean containsKey(Object key)
      Returns true if this map contains a mapping for the specified key. Null keys are considered never present.
      Specified by:
      contains­Key in interface Map<K,V>
      Overrides:
      contains­Key in class Abstract­Map<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

      public boolean containsValue(Object value)
      Returns true if this map maps one or more keys to the specified value. Null values are considered never present.
      Specified by:
      contains­Value in interface Map<K,V>
      Overrides:
      contains­Value in class 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.

    • get

      public V get(Object key)
      Returns the value to which this map maps the specified key. Returns null if the map contains no mapping for this key. Null keys are considered never present.
      Specified by:
      get in interface Map<K,V>
      Overrides:
      get in class Abstract­Map<K,V>
      Parameters:
      key - key whose associated value is to be returned.
      Returns:
      the value to which this map maps the specified key.

    • getOrDefault

      public V getOrDefault(Object key, V defaultValue)
      Returns the value to which this map maps the specified key. Returns default­Value if 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

    • put

      public V put(K key, V value)
      Associates the specified value with the specified key in this map. The value is associated using a Weak­Reference.
      Specified by:
      put in interface Map<K,V>
      Overrides:
      put in class 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:
      the previous value associated with specified key, or null if there was no mapping for the key.
      Throws:
      Null­Pointer­Exception - if the key or the value is null.

    • putIfAbsent

      public V putIfAbsent(K key, V value)
      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 a Weak­Reference and null 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:
      Null­Pointer­Exception - if the key or the value is null.
      Since:
      0.7

    • replace

      public V replace(K key, V value)
      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:
      Null­Pointer­Exception - if the value is null.
      Since:
      1.2

    • replace

      public boolean replace(K key, V oldValue, V newValue)
      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:
      Null­Pointer­Exception - if the new value is null.
      Since:
      1.2

    • 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 class 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 entry for the key.

    • remove

      public boolean remove(Object key, Object value)
      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.
      Specified by:
      clear in interface Map<K,V>
      Overrides:
      clear in class Abstract­Map<K,V>

    • entrySet

      public Set<Map.Entry<K,V>> entrySet()
      Returns a set view of the mappings contained in this map. Each element in this set is a Map​.Entry.
      Specified by:
      entry­Set in interface Map<K,V>
      Specified by:
      entry­Set in class Abstract­Map<K,V>
      Returns:
      a set view of the mappings contained in this map.