public class IdentityHashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Serializable, Cloneable
This class is not a general-purpose Map implementation! While this class implements the Map interface, it intentionally violates Map's general contract, which mandates the use of the equals method when comparing objects. This class is designed for use only in the rare cases wherein reference-equality semantics are required.
A typical use of this class is topology-preserving object graph transformations, such as serialization or deep-copying. To perform such a transformation, a program must maintain a "node table" that keeps track of all the object references that have already been processed. The node table must not equate distinct objects even if they happen to be equal. Another typical use of this class is to maintain proxy objects. For example, a debugging facility might wish to maintain a proxy object for each object in the program being debugged.
This class provides all of the optional map operations, and permits null values and the null key. This class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time.
This class provides constant-time performance for the basic
operations (get and put), assuming the system
identity hash function (System.identityHashCode(Object))
disperses elements properly among the buckets.
This class has one tuning parameter (which affects performance but not semantics): expected maximum size. This parameter is the maximum number of key-value mappings that the map is expected to hold. Internally, this parameter is used to determine the number of buckets initially comprising the hash table. The precise relationship between the expected maximum size and the number of buckets is unspecified.
If the size of the map (the number of key-value mappings) sufficiently exceeds the expected maximum size, the number of buckets is increased Increasing the number of buckets ("rehashing") may be fairly expensive, so it pays to create identity hash maps with a sufficiently large expected maximum size. On the other hand, iteration over collection views requires time proportional to the number of buckets in the hash table, so it pays not to set the expected maximum size too high if you are especially concerned with iteration performance or memory usage.
Note that this implementation is not synchronized.
If multiple threads access an identity hash map concurrently, and at
least one of the threads modifies the map structurally, it must
be synchronized externally. (A structural modification is any operation
that adds or deletes one or more mappings; merely changing the value
associated with a key that an instance already contains is not a
structural modification.) This is typically accomplished by
synchronizing on some object that naturally encapsulates the map.
If no such object exists, the map should be "wrapped" using the
Collections.synchronizedMap
method. This is best done at creation time, to prevent accidental
unsynchronized access to the map:
Map m = Collections.synchronizedMap(new IdentityHashMap(...));
The iterators returned by the iterator method of the
collections returned by all of this class's "collection view
methods" are fail-fast: if the map is structurally modified
at any time after the iterator is created, in any way except
through the iterator's own remove method, the iterator
will throw a ConcurrentModificationException. Thus, in the
face of concurrent modification, the iterator fails quickly and
cleanly, rather than risking arbitrary, non-deterministic behavior
at an undetermined time in the future.
Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: fail-fast iterators should be used only to detect bugs.
Implementation note: This is a simple linear-probe hash table,
as described for example in texts by Sedgewick and Knuth. The array
alternates holding keys and values. (This has better locality for large
tables than does using separate arrays.) For many JRE implementations
and operation mixes, this class will yield better performance than
HashMap (which uses chaining rather than linear-probing).
This class is a member of the Java Collections Framework.
System.identityHashCode(Object),
Object.hashCode(),
Collection,
Map,
HashMap,
TreeMap,
Serialized FormAbstractMap.SimpleEntry<K,V>, AbstractMap.SimpleImmutableEntry<K,V>| Constructor and Description |
|---|
IdentityHashMap()
Constructs a new, empty identity hash map with a default expected
maximum size (21).
|
IdentityHashMap(int expectedMaxSize)
Constructs a new, empty map with the specified expected maximum size.
|
IdentityHashMap(Map<? extends K,? extends V> m)
Constructs a new identity hash map containing the keys-value mappings
in the specified map.
|
| Modifier and Type | Method and Description |
|---|---|
void |
clear()
Removes all of the mappings from this map.
|
Object |
clone()
Returns a shallow copy of this identity hash map: the keys and values
themselves are not cloned.
|
boolean |
containsKey(Object key)
Tests whether the specified object reference is a key in this identity
hash map.
|
boolean |
containsValue(Object value)
Tests whether the specified object reference is a value in this identity
hash map.
|
Set<Map.Entry<K,V>> |
entrySet()
Returns a
Set view of the mappings contained in this map. |
boolean |
equals(Object o)
Compares the specified object with this map for equality.
|
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. |
int |
hashCode()
Returns the hash code value for this map.
|
boolean |
isEmpty()
Returns true if this identity hash map contains no key-value
mappings.
|
Set<K> |
keySet()
Returns an identity-based set view of the keys contained in this map.
|
V |
put(K key,
V value)
Associates the specified value with the specified key in this identity
hash map.
|
void |
putAll(Map<? extends K,? extends V> m)
Copies all of the mappings from the specified map to this map.
|
V |
remove(Object key)
Removes the mapping for this key from this map if present.
|
int |
size()
Returns the number of key-value mappings in this identity hash map.
|
Collection<V> |
values()
Returns a
Collection view of the values contained in this map. |
toStringpublic IdentityHashMap()
public IdentityHashMap(int expectedMaxSize)
expectedMaxSize - the expected maximum size of the mapIllegalArgumentException - if expectedMaxSize is negativepublic IdentityHashMap(Map<? extends K,? extends V> m)
m - the map whose mappings are to be placed into this mapNullPointerException - if the specified map is nullpublic int size()
public boolean isEmpty()
public V get(Object key)
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.
get in interface Map<K,V>get in class AbstractMap<K,V>key - the key whose associated value is to be returnednull if this map contains no mapping for the keyput(Object, Object)public boolean containsKey(Object key)
containsKey in interface Map<K,V>containsKey in class AbstractMap<K,V>key - possible keytrue if the specified object reference is a key
in this mapcontainsValue(Object)public boolean containsValue(Object value)
containsValue in interface Map<K,V>containsValue in class AbstractMap<K,V>value - value whose presence in this map is to be testedcontainsKey(Object)public V put(K key, V value)
put in interface Map<K,V>put in class AbstractMap<K,V>key - the key with which the specified value is to be associatedvalue - the value to be associated with the specified keyObject.equals(Object),
get(Object),
containsKey(Object)public void putAll(Map<? extends K,? extends V> m)
putAll in interface Map<K,V>putAll in class AbstractMap<K,V>m - mappings to be stored in this mapNullPointerException - if the specified map is nullpublic V remove(Object key)
remove in interface Map<K,V>remove in class AbstractMap<K,V>key - key whose mapping is to be removed from the mappublic void clear()
public boolean equals(Object o)
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.
equals in interface Map<K,V>equals in class AbstractMap<K,V>o - object to be compared for equality with this mapObject.equals(Object)public int hashCode()
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.
hashCode in interface Map<K,V>hashCode in class AbstractMap<K,V>Object.equals(Object),
equals(Object)public Object clone()
clone in class AbstractMap<K,V>Cloneablepublic Set<K> keySet()
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.
keySet in interface Map<K,V>keySet in class AbstractMap<K,V>Object.equals(Object),
System.identityHashCode(Object)public Collection<V> values()
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()
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()).
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.
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