Package org.apache.commons.collections.map

Class ReferenceIdentityMap

  • All Implemented Interfaces:
    Serializable, Map, IterableMap
    public class ReferenceIdentityMap
extends AbstractReferenceMap
implements Serializable
    A Map implementation that allows mappings to be removed by the garbage collector and matches keys and values based on == not equals().

    When you construct a ReferenceIdentityMap, 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 Reference.

    Different types of references can be specified for keys and values. The default constructor uses hard keys and soft values, providing a memory-sensitive cache.

    This map is similar to ReferenceMap. It differs in that keys and values in this class are compared using ==.

    This map will violate the detail of various Map and map view contracts. As a general rule, don't compare this map to other maps.

    This 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 Collections.synchronizedMap(java.util.Map<K, V>) to provide synchronized access to a ReferenceIdentityMap. 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 ReferenceIdentityMap 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 Collections.synchronizedMap(java.util.Map<K, V>). This class may throw exceptions when accessed by concurrent threads without synchronization.

    Since:
    Commons Collections 3.0 (previously in main package v2.1)
    Version:
    $Revision: 646777 $ $Date: 2008-04-10 14:33:15 +0200 (Thu, 10 Apr 2008) $
    Author:
    Stephen Colebourne
    See Also:
    Reference, Serialized Form

    • Method Detail

      • hash

        protected int hash​(Object key)
        Gets the hash code for the key specified.

        This implementation uses the identity hash code.

        Overrides:
        hash in class AbstractHashedMap
        Parameters:
        key - the key to get a hash code for
        Returns:
        the hash code

      • hashEntry

        protected int hashEntry​(Object key,
                        Object value)
        Gets the hash code for a MapEntry.

        This implementation uses the identity hash code.

        Overrides:
        hashEntry in class AbstractReferenceMap
        Parameters:
        key - the key to get a hash code for, may be null
        value - the value to get a hash code for, may be null
        Returns:
        the hash code, as per the MapEntry specification

      • isEqualKey

        protected boolean isEqualKey​(Object key1,
                             Object key2)
        Compares two keys for equals.

        This implementation converts the key from the entry to a real reference before comparison and uses ==.

        Overrides:
        isEqualKey in class AbstractReferenceMap
        Parameters:
        key1 - the first key to compare passed in from outside
        key2 - the second key extracted from the entry via entry.key
        Returns:
        true if equal by identity

      • isEqualValue

        protected boolean isEqualValue​(Object value1,
                               Object value2)
        Compares two values for equals.

        This implementation uses ==.

        Overrides:
        isEqualValue in class AbstractHashedMap
        Parameters:
        value1 - the first value to compare passed in from outside
        value2 - the second value extracted from the entry via getValue()
        Returns:
        true if equal by identity