Package org.plumelib.util

Class WeakIdentityHashMap<K,V>

java.lang.Object
java.util.AbstractMap<K,V>
org.plumelib.util.WeakIdentityHashMap<K,V>
Type Parameters:
K - the type of map keys
V - the type of map values
All Implemented Interfaces:
Map<K,V>
public class WeakIdentityHashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>
This class combines the features of WeakHashMap and IdentityHashMap. That is, keys are weak and are compared using reference equality.

The implementation is a modified version of WeakHashMap from JDK 1.5, which differs from the original in two ways:

  • uses of hashCode() are replaced by System.identityHashCode()
  • uses of object equality (.equals()) are replaced by identity checks (==)
See IdentityHashMap for more information on the changes made in an identity hash map.

The original documentation follows:

A hashtable-based Map implementation with weak keys. An entry in a WeakIdentityHashMap 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 WeakIdentityHashMap may be constructed using the Collections.synchronizedMap method.

The behavior of the WeakIdentityHashMap 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 WeakIdentityHashMap may behave as though an unknown thread is silently removing entries. In particular, even if you synchronize on a WeakIdentityHashMap 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 WeakIdentityHashMap 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 WeakIdentityHashMap 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 WeakIdentityHashMap 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.

Since:
1.2
See Also:

  • Nested Class Summary

    Nested classes/interfaces inherited from class java.util.AbstractMap

    AbstractMap.SimpleEntry<K,V>, AbstractMap.SimpleImmutableEntry<K,V>

  • Constructor Summary

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

  • Method Summary

    Modifier and Type
    Method
    Description
    void
    clear()
    Removes all mappings from this map.
    boolean
    containsKey(@Nullable Object key)
    Returns true if this map contains a mapping for the specified key.
    boolean
    containsValue(@Nullable Object value)
    Returns true if this map maps one or more keys to the specified value.
    Set<Map.Entry<K,V>>
    entrySet()
    Returns a collection view of the mappings contained in this map.
    @Nullable V
    get(@Nullable @UnknownSignedness @GuardSatisfied 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<K>
    keySet()
    Returns a set view of the keys contained in this map.
    @Nullable V
    put(K key, V value)
    Associates the specified value with the specified key in this map.
    void
    putAll(Map<? extends K,? extends 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.
    @Nullable 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<V>
    values()
    Returns a collection view of the values contained in this map.

    Methods inherited from class java.util.AbstractMap

    clone, equals, hashCode, toString

    Methods inherited from class java.lang.Object

    finalize, getClass, notify, notifyAll, wait, wait, wait

    Methods inherited from interface java.util.Map

    compute, computeIfAbsent, computeIfPresent, equals, forEach, getOrDefault, hashCode, merge, putIfAbsent, remove, replace, replace, replaceAll

  • Constructor Details

    • WeakIdentityHashMap

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

    • WeakIdentityHashMap

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

    • WeakIdentityHashMap

      public WeakIdentityHashMap()
      Constructs a new, empty WeakIdentityHashMap with the default initial capacity (16) and the default load factor (0.75).

    • WeakIdentityHashMap

      public WeakIdentityHashMap(Map<? extends K,? extends V> t)
      Constructs a new WeakIdentityHashMap with the same mappings as the specified Map. The WeakIdentityHashMap 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

  • Method Details

    • size

      @Pure 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 class AbstractMap<K,V>

    • isEmpty

      @Pure 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 class AbstractMap<K,V>

    • get

      @Pure public @Nullable V get(@Nullable @UnknownSignedness @GuardSatisfied 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 class 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:

    • containsKey

      @Pure public boolean containsKey(@Nullable 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 class 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

    • put

      public @Nullable 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 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:
      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.

    • putAll

      public void putAll(Map<? extends K,? extends 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 class AbstractMap<K,V>
      Parameters:
      m - mappings to be stored in this map
      Throws:
      NullPointerException - if the specified map is null

    • remove

      public @Nullable 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 mapping for key. A null return can also indicate that the map previously associated null with the specified key.

    • clear

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

    • containsValue

      @Pure public boolean containsValue(@Nullable 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 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.

    • keySet

      @SideEffectFree public Set<K> 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 class AbstractMap<K,V>
      Returns:
      a set view of the keys contained in this map

    • values

      @SideEffectFree public Collection<V> 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 class AbstractMap<K,V>
      Returns:
      a collection view of the values contained in this map

    • entrySet

      @SideEffectFree public Set<Map.Entry<K,V>> 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.
      Specified by:
      entrySet in interface Map<K,V>
      Specified by:
      entrySet in class AbstractMap<K,V>
      Returns:
      a collection view of the mappings contained in this map
      See Also: