public IdentityHashMap()

Constructs a new, empty identity hash map with a default expected maximum size (21).

public IdentityHashMap(int expectedMaxSize)

Constructs a new, empty map with the specified expected maximum size. Putting more than the expected number of key-value mappings into the map may cause the internal data structure to grow, which may be somewhat time-consuming.

public IdentityHashMap(Map<? extends K,? extends V> m)

Constructs a new identity hash map containing the keys-value mappings in the specified map.

public int size()

Returns the number of key-value mappings in this identity hash map.

public boolean isEmpty()

Returns true if this identity hash map contains no key-value mappings.

public V get(Object key)

Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.

More formally, if this map contains a mapping from a key k to a value v such that (key == k), then this method returns v; otherwise it returns null. (There can be at most one such mapping.)

A return value of null does not necessarily indicate that the map contains no mapping for the key; it's also possible that the map explicitly maps the key to null. The containsKey operation may be used to distinguish these two cases.

public boolean containsKey(Object key)

Tests whether the specified object reference is a key in this identity hash map. Returns true if and only if this map contains a mapping with key k such that (key == k).

public boolean containsValue(Object value)

Tests whether the specified object reference is a value in this identity hash map. Returns true if and only if this map contains a mapping with value v such that (value == v).

public V put(K key, V value)

Associates the specified value with the specified key in this identity hash map. If this map already contains a mapping for the key, the old value is replaced, otherwise, a new mapping is inserted into this map.

public void putAll(Map<? extends K,? extends V> m)

Copies all of the mappings from the specified map to this map. For each mapping in the specified map, if this map already contains a mapping for the key, its value is replaced with the value from the specified map; otherwise, a new mapping is inserted into this map.

public V remove(Object key)

Removes the mapping for this key from this map if present. The mapping is removed if and only if the mapping has a key k such that (key == k).

public void clear()

Removes all of the mappings from this map. The map will be empty after this call returns.

public boolean equals(Object o)

Compares the specified object with this map for equality. Returns true if the given object is also a map and the two maps represent identical object-reference mappings. More formally, this map is equal to another map m if and only if this.entrySet().equals(m.entrySet()). See the entrySet method for the specification of equality of this map's entries.

Owing to the reference-equality-based semantics of this map it is possible that the symmetry and transitivity requirements of the Object.equals contract may be violated if this map is compared to a normal map. However, the Object.equals contract is guaranteed to hold among IdentityHashMap instances.

public int hashCode()

Returns the hash code value for this map. The hash code of a map is defined to be the sum of the hash codes of each entry of this map. See the entrySet method for a specification of the hash code of this map's entries.

This specification ensures that m1.equals(m2) implies that m1.hashCode()==m2.hashCode() for any two IdentityHashMap instances m1 and m2, as required by the general contract of Object.hashCode().

Owing to the reference-equality-based semantics of the Map.Entry instances in the set returned by this map's entrySet method, it is possible that the contractual requirement of Object.hashCode mentioned in the previous paragraph will be violated if one of the two objects being compared is an IdentityHashMap instance and the other is a normal map.

public Object clone()

Returns a shallow copy of this identity hash map: the keys and values themselves are not cloned.

public Set<K> keySet()

Returns an identity-based set view of the keys contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress, the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Set.remove, removeAll, retainAll, and clear methods. It does not support the add or addAll methods.

While the object returned by this method implements the Set interface, it does not obey Set's general contract. Like its backing map, the set returned by this method defines element equality as reference-equality rather than object-equality. This affects the behavior of its contains, remove, containsAll, equals, and hashCode methods.

The equals method of the returned set returns true only if the specified object is a set containing exactly the same object references as the returned set. The symmetry and transitivity requirements of the Object.equals contract may be violated if the set returned by this method is compared to a normal set. However, the Object.equals contract is guaranteed to hold among sets returned by this method.

The hashCode method of the returned set returns the sum of the identity hashcodes of the elements in the set, rather than the sum of their hashcodes. This is mandated by the change in the semantics of the equals method, in order to enforce the general contract of the Object.hashCode method among sets returned by this method.

public Collection<V> values()

Returns a Collection view of the values contained in this map. The collection is backed by the map, so changes to the map are reflected in the collection, and vice-versa. If the map is modified while an iteration over the collection is in progress, the results of the iteration are undefined. The collection supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Collection.remove, removeAll, retainAll and clear methods. It does not support the add or addAll methods.

While the object returned by this method implements the Collection interface, it does not obey Collection's general contract. Like its backing map, the collection returned by this method defines element equality as reference-equality rather than object-equality. This affects the behavior of its contains, remove and containsAll methods.

public Set<Map.Entry<K,V>> entrySet()

Returns a Set view of the mappings contained in this map. Each element in the returned set is a reference-equality-based Map.Entry. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress, the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Set.remove, removeAll, retainAll and clear methods. It does not support the add or addAll methods.

Like the backing map, the Map.Entry objects in the set returned by this method define key and value equality as reference-equality rather than object-equality. This affects the behavior of the equals and hashCode methods of these Map.Entry objects. A reference-equality based Map.Entry e is equal to an object o if and only if o is a Map.Entry and e.getKey()==o.getKey() && e.getValue()==o.getValue(). To accommodate these equals semantics, the hashCode method returns System.identityHashCode(e.getKey()) ^ System.identityHashCode(e.getValue()). (While the keys and values are compared using reference equality, the Map.Entry objects themselves are not.)

Owing to the reference-equality-based semantics of the Map.Entry instances in the set returned by this method, it is possible that the symmetry and transitivity requirements of the Object.equals(Object) contract may be violated if any of the entries in the set is compared to a normal map entry, or if the set returned by this method is compared to a set of normal map entries (such as would be returned by a call to this method on a normal map). However, the Object.equals contract is guaranteed to hold among identity-based map entries, and among sets of such entries.

public boolean remove(Object key, Object value)

Removes the entry for the specified key only if it is currently mapped to the specified value.

More formally, if this map contains a mapping from a key k to a value v such that (key == k) and (value == v), then this method removes the mapping for this key and returns true; otherwise it returns false.

public boolean replace(K key, V oldValue, V newValue)

Replaces the entry for the specified key only if currently mapped to the specified value.

More formally, if this map contains a mapping from a key k to a value v such that (key == k) and (oldValue == v), then this method associates k with newValue and returns true; otherwise it returns false.