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When you construct a ReferenceMap, you can specify what kind of references are used to store the map's keys and values. If non-hard references are used, then the garbage collector can remove mappings if a key or value becomes unreachable, or if the JVM's memory is running low. For information on how the different reference types behave, see java.lang.ref.Reference Reference .

Different types of references can be specified for keys and values. The keys can be configured to be weak but the values hard, in which case this class will behave like a WeakHashMap. However, you can also specify hard keys and weak values, or any other combination. The default constructor uses hard keys and soft values, providing a memory-sensitive cache.

This map is similar to org.apache.commons.collections.map.ReferenceIdentityMap ReferenceIdentityMap . It differs in that keys and values in this class are compared using equals().

This java.util.Map Map implementation does not allow null elements. Attempting to add a null key or value to the map will raise a NullPointerException.

This implementation is not synchronized. You can use java.util.Collections.synchronizedMap to provide synchronized access to a ReferenceMap. Remember that synchronization will not stop the garbage collecter removing entries.

All the available iterators can be reset back to the start by casting to ResettableIterator and calling reset().

Note that ReferenceMap is not synchronized and is not thread-safe. If you wish to use this map from multiple threads concurrently, you must use appropriate synchronization. The simplest approach is to wrap this map using java.util.Collections.synchronizedMap . This class may throw exceptions when accessed by concurrent threads without synchronization.

NOTE: As from Commons Collections 3.1 this map extends AbstractReferenceMap (previously it extended AbstractMap). As a result, the implementation is now extensible and provides a MapIterator.
See Also:   java.lang.ref.Reference
since:
   Commons Collections 3.0 (previously in main package v2.1)
version:
   $Revision: 348007 $ $Date: 2005-11-21 22:52:57 +0000 (Mon, 21 Nov 2005) $
author:
   Paul Jack
author:
   Stephen Colebourne