Package org.apache.commons.collections4.map

Class ReferenceIdentityMap<K,​V>

  • Type Parameters:
    K - the type of the keys in this map
    V - the type of the values in this map
    All Implemented Interfaces:
    Serializable, Map<K,​V>, Get<K,​V>, IterableGet<K,​V>, IterableMap<K,​V>, Put<K,​V>
    public class ReferenceIdentityMap<K,​V>
extends AbstractReferenceMap<K,​V>
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 collector 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:
    3.0 (previously in main package v2.1)
    See Also:
    Reference, Serialized Form

    • Constructor Detail

      • ReferenceIdentityMap

        public ReferenceIdentityMap()
        Constructs a new ReferenceIdentityMap that will use hard references to keys and soft references to values.

      • ReferenceIdentityMap

        public ReferenceIdentityMap​(AbstractReferenceMap.ReferenceStrength keyType,
                            AbstractReferenceMap.ReferenceStrength valueType,
                            boolean purgeValues)
        Constructs a new ReferenceIdentityMap that will use the specified types of references.
        Parameters:
        keyType - the type of reference to use for keys; must be HARD, SOFT, WEAK
        valueType - the type of reference to use for values; must be HARD, SOFT, WEAK
        purgeValues - should the value be automatically purged when the key is garbage collected

      • ReferenceIdentityMap

        public ReferenceIdentityMap​(AbstractReferenceMap.ReferenceStrength keyType,
                            AbstractReferenceMap.ReferenceStrength valueType,
                            int capacity,
                            float loadFactor)
        Constructs a new ReferenceIdentityMap with the specified reference types, load factor and initial capacity.
        Parameters:
        keyType - the type of reference to use for keys; must be HARD, SOFT, WEAK
        valueType - the type of reference to use for values; must be HARD, SOFT, WEAK
        capacity - the initial capacity for the map
        loadFactor - the load factor for the map

      • ReferenceIdentityMap

        public ReferenceIdentityMap​(AbstractReferenceMap.ReferenceStrength keyType,
                            AbstractReferenceMap.ReferenceStrength valueType,
                            int capacity,
                            float loadFactor,
                            boolean purgeValues)
        Constructs a new ReferenceIdentityMap with the specified reference types, load factor and initial capacity.
        Parameters:
        keyType - the type of reference to use for keys; must be HARD, SOFT, WEAK
        valueType - the type of reference to use for values; must be HARD, SOFT, WEAK
        capacity - the initial capacity for the map
        loadFactor - the load factor for the map
        purgeValues - should the value be automatically purged when the key is garbage collected

    • 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<K,​V>
        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<K,​V>
        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<K,​V>
        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<K,​V>
        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