Package org.plumelib.util

Class WeakHasherMap<K,V>

java.lang.Object
java.util.AbstractMap<K,V>
org.plumelib.util.WeakHasherMap<K,V>
Type Parameters:
K - the type of the map keys
V - the type of the map values
All Implemented Interfaces:
Map<K,V>
public final class WeakHasherMap<K,V> extends AbstractMap<K,V> implements Map<K,V>
This is a modified version of WeakHashMap from JDK 1.2.2, that adds a constructor that takes a Hasher argument. I also replaced uses of hashCode() and equals() by uses of Hasher, where appropriate. I've changed the name so that this version is not used accidentally. It is slightly less efficient, in the absence of a Hasher, and has not been as extensively tested.

The original documentation follows.

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. This problem may be fixed in a future release.

Since:
JDK1.2
See Also:

  • Constructor Details

    • WeakHasherMap

      public WeakHasherMap(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 less than zero, or if the load factor is nonpositive

    • WeakHasherMap

      public WeakHasherMap(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 less than zero

    • WeakHasherMap

      public WeakHasherMap()
      Constructs a new, empty WeakHashMap with the default capacity and the default load factor, which is 0.75.

    • WeakHasherMap

      public WeakHasherMap(Hasher h)
      Constructs a new, empty WeakHashMap with the default capacity and the default load factor, which is 0.75. The WeakHashMap uses the specified hasher for hashing keys and comparing them for equality.
      Parameters:
      h - the Hasher to use when hashing values for this map

  • Method Details

    • size

      @Pure public int size()
      Returns the number of key-value mappings in this map. Note: In contrast to most implementations of the Map interface, the time required by this operation is linear in the size of the map.
      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.
      Specified by:
      isEmpty in interface Map<K,V>
      Overrides:
      isEmpty in class AbstractMap<K,V>

    • containsKey

      @Pure 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 class AbstractMap<K,V>
      Parameters:
      key - the key whose presence in this map is to be tested

    • get

      @Pure public @Nullable V get(@UnknownSignedness @GuardSatisfied Object key)
      Returns the value to which this map maps the specified key. If this map does not contain a value for this key, then return null.
      Specified by:
      get in interface Map<K,V>
      Overrides:
      get in class AbstractMap<K,V>
      Parameters:
      key - the key whose associated value, if any, is to be returned

    • put

      public V put(K key, V value)
      Updates this map so that the given key maps to the given value. If the map previously contained a mapping for key then that mapping is replaced and the previous value is returned.
      Specified by:
      put in interface Map<K,V>
      Overrides:
      put in class AbstractMap<K,V>
      Parameters:
      key - the key that is to be mapped to the given value
      value - the value to which the given key is to be mapped
      Returns:
      the previous value to which this key was mapped, or null if if there was no mapping for the key

    • remove

      public V remove(Object key)
      Removes the mapping for the given key from this map, if present.
      Specified by:
      remove in interface Map<K,V>
      Overrides:
      remove in class AbstractMap<K,V>
      Parameters:
      key - the key whose mapping is to be removed
      Returns:
      the value to which this key was mapped, or null if there was no mapping for the 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>

    • entrySet

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