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IdentityHashMap


java.util

Class IdentityHashMap<K,V>

  • All Implemented Interfaces:
    Serializable, Cloneable, Map<K,V>


    public class IdentityHashMap<K,V>
    extends AbstractMap<K,V>
    implements Map<K,V>, Serializable, Cloneable
    IdentityHashMap is a variant on HashMap which tests equality by reference instead of equality by value. Basically, keys and values are compared for equality by checking if their references are equal rather than by calling the "equals" function.

    Note: This class intentionally violates the general contract of Map's on comparing objects by their equals method.

    IdentityHashMap uses open addressing (linear probing in particular) for collision resolution. This is different from HashMap which uses Chaining.

    Like HashMap, IdentityHashMap is not thread safe, so access by multiple threads must be synchronized by an external mechanism such as Collections.synchronizedMap.

    Since:
    1.4
    See Also:
    Serialized Form
    • Constructor Summary

      Constructors
      Constructor and Description
      IdentityHashMap()
      Creates an IdentityHashMap with default expected maximum size.
      IdentityHashMap(int maxSize)
      Creates an IdentityHashMap with the specified maximum size parameter.
      IdentityHashMap(Map<? extends K,? extends V> map)
      Creates an IdentityHashMap using the given map as initial values.
    • Method Summary

      Methods
      Modifier and Type Method and Description
      void clear()
      Removes all elements from this map, leaving it empty.
      Object clone()
      Returns a new IdentityHashMap with the same mappings and size as this one.
      boolean containsKey(Object key)
      Returns whether this map contains the specified key.
      boolean containsValue(Object value)
      Returns whether this map contains the specified value.
      Set<Map.Entry<K,V>> entrySet()
      Returns a set containing all of the mappings in this map.
      boolean equals(Object object)
      Compares this map with other objects.
      V get(Object key)
      Returns the value of the mapping with the specified key.
      boolean isEmpty()
      Returns whether this IdentityHashMap has no elements.
      Set<K> keySet()
      Returns a set of the keys contained in this map.
      V put(K key, V value)
      Maps the specified key to the specified value.
      void putAll(Map<? extends K,? extends V> map)
      Copies all the mappings in the specified map to this map.
      V remove(Object key)
      Removes the mapping with the specified key from this map.
      int size()
      Returns the number of mappings in this IdentityHashMap.
      Collection<V> values()
      Returns a collection of the values contained in this map.
    • Constructor Detail

      • IdentityHashMap

        public IdentityHashMap()
        Creates an IdentityHashMap with default expected maximum size.
      • IdentityHashMap

        public IdentityHashMap(int maxSize)
        Creates an IdentityHashMap with the specified maximum size parameter.
        Parameters:
        maxSize - The estimated maximum number of entries that will be put in this map.
      • IdentityHashMap

        public IdentityHashMap(Map<? extends K,? extends V> map)
        Creates an IdentityHashMap using the given map as initial values.
        Parameters:
        map - A map of (key,value) pairs to copy into the IdentityHashMap.
    • Method Detail

      • containsKey

        public boolean containsKey(Object key)
        Returns whether this map contains the specified key.
        Specified by:
        containsKey in interface Map<K,V>
        Overrides:
        containsKey in class AbstractMap<K,V>
        Parameters:
        key - the key to search for.
        Returns:
        true if this map contains the specified key, false otherwise.
      • containsValue

        public boolean containsValue(Object value)
        Returns whether this map contains the specified value.
        Specified by:
        containsValue in interface Map<K,V>
        Overrides:
        containsValue in class AbstractMap<K,V>
        Parameters:
        value - the value to search for.
        Returns:
        true if this map contains the specified value, false otherwise.
      • get

        public V get(Object key)
        Returns the value of the mapping with the specified key.
        Specified by:
        get in interface Map<K,V>
        Overrides:
        get in class AbstractMap<K,V>
        Parameters:
        key - the key.
        Returns:
        the value of the mapping with the specified key.
      • put

        public V put(K key,
            V value)
        Maps the specified key to the specified value.
        Specified by:
        put in interface Map<K,V>
        Overrides:
        put in class AbstractMap<K,V>
        Parameters:
        key - the key.
        value - the value.
        Returns:
        the value of any previous mapping with the specified key or null if there was no such mapping.
      • putAll

        public void putAll(Map<? extends K,? extends V> map)
        Copies all the mappings in the specified map to this map. These mappings will replace all mappings that this map had for any of the keys currently in the given map.
        Specified by:
        putAll in interface Map<K,V>
        Overrides:
        putAll in class AbstractMap<K,V>
        Parameters:
        map - the map to copy mappings from.
        Throws:
        NullPointerException - if map is null.
      • remove

        public V remove(Object key)
        Removes the mapping with the specified key from this map.
        Specified by:
        remove in interface Map<K,V>
        Overrides:
        remove in class AbstractMap<K,V>
        Parameters:
        key - the key of the mapping to remove.
        Returns:
        the value of the removed mapping, or null if no mapping for the specified key was found.
      • entrySet

        public Set<Map.Entry<K,V>> entrySet()
        Returns a set containing all of the mappings in this map. Each mapping is an instance of Map.Entry. As the set is backed by this map, changes in one will be reflected in the other.
        Specified by:
        entrySet in interface Map<K,V>
        Specified by:
        entrySet in class AbstractMap<K,V>
        Returns:
        a set of the mappings.
      • keySet

        public Set<K> keySet()
        Returns a set of the keys contained in this map. The set is backed by this map so changes to one are reflected by the other. The set does not support adding.
        Specified by:
        keySet in interface Map<K,V>
        Overrides:
        keySet in class AbstractMap<K,V>
        Returns:
        a set of the keys.
      • values

        public Collection<V> values()
        Returns a collection of the values contained in this map. The collection is backed by this map so changes to one are reflected by the other. The collection supports remove, removeAll, retainAll and clear operations, and it does not support add or addAll operations.

        This method returns a collection which is the subclass of AbstractCollection. The iterator method of this subclass returns a "wrapper object" over the iterator of map's entrySet(). The size method wraps the map's size method and the contains method wraps the map's containsValue method.

        The collection is created when this method is called for the first time and returned in response to all subsequent calls. This method may return different collections when multiple concurrent calls occur, since no synchronization is performed.

        Specified by:
        values in interface Map<K,V>
        Overrides:
        values in class AbstractMap<K,V>
        Returns:
        a collection of the values contained in this map.
      • equals

        public boolean equals(Object object)
        Compares this map with other objects. This map is equal to another map is it represents the same set of mappings. With this map, two mappings are the same if both the key and the value are equal by reference. When compared with a map that is not an IdentityHashMap, the equals method is neither necessarily symmetric (a.equals(b) implies b.equals(a)) nor transitive (a.equals(b) and b.equals(c) implies a.equals(c)).
        Specified by:
        equals in interface Map<K,V>
        Overrides:
        equals in class AbstractMap<K,V>
        Parameters:
        object - the object to compare to.
        Returns:
        whether the argument object is equal to this object.
        See Also:
        Object.hashCode()
      • clone

        public Object clone()
        Returns a new IdentityHashMap with the same mappings and size as this one.
        Overrides:
        clone in class AbstractMap<K,V>
        Returns:
        a shallow copy of this IdentityHashMap.
        See Also:
        Cloneable
      • isEmpty

        public boolean isEmpty()
        Returns whether this IdentityHashMap has no elements.
        Specified by:
        isEmpty in interface Map<K,V>
        Overrides:
        isEmpty in class AbstractMap<K,V>
        Returns:
        true if this IdentityHashMap has no elements, false otherwise.
        See Also:
        size()
      • size

        public int size()
        Returns the number of mappings in this IdentityHashMap.
        Specified by:
        size in interface Map<K,V>
        Overrides:
        size in class AbstractMap<K,V>
        Returns:
        the number of mappings in this IdentityHashMap.


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