| java.util.IdentityHashMap
| IdentityHashMap | | public class IdentityHashMap extends AbstractMap implements Map<K, V>,java.io.Serializable,Cloneable(Code) | | This class implements the Map interface with a hash table, using
reference-equality in place of object-equality when comparing keys (and
values). In other words, in an IdentityHashMap, two keys
k1 and k2 are considered equal if and only if
(k1==k2). (In normal Map implementations (like
HashMap) two keys k1 and k2 are considered equal
if and only if (k1==null ? k2==null : k1.equals(k2)).)
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 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.
See Also: System.identityHashCode(Object)
See Also: Object.hashCode
See Also: Collection
See Also: Map
See Also: HashMap
See Also: TreeMap
author:
Doug Lea and Josh Bloch
since:
1.4 |
| Constructor Summary | |
| 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. | | public | IdentityHashMap(Map<? extends K, ? extends V> m)
Constructs a new identity hash map containing the keys-value mappings
in the specified map. |
| Method Summary | |
| public void | clear()
Removes all of the mappings from this map. | | public Object | clone()
Returns a shallow copy of this identity hash map: the keys and values
themselves are not cloned. | | public boolean | containsKey(Object key)
Tests whether the specified object reference is a key in this identity
hash map. | | public boolean | containsValue(Object value)
Tests whether the specified object reference is a value in this identity
hash map. | | 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. | | public boolean | equals(Object o)
Compares the specified object with this map for equality. | | 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 . | | public int | hashCode()
Returns the hash code value for this map. | | public boolean | isEmpty()
Returns true if this identity hash map contains no key-value
mappings. | | 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. | | public V | put(K key, V value)
Associates the specified value with the specified key in this identity
hash map. | | public void | putAll(Map<? extends K, ? extends V> m)
Copies all of the mappings from the specified map to this map. | | public V | remove(Object key)
Removes the mapping for this key from this map if present. | | public int | size()
Returns the number of key-value mappings in this identity hash map. | | 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. |
| IdentityHashMap | | public IdentityHashMap()(Code) | | Constructs a new, empty identity hash map with a default expected
maximum size (21).
|
| IdentityHashMap | | public IdentityHashMap(int expectedMaxSize)(Code) | | 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.
Parameters:
expectedMaxSize - the expected maximum size of the map
throws:
IllegalArgumentException - if expectedMaxSize is negative |
| IdentityHashMap | | public IdentityHashMap(Map<? extends K, ? extends V> m)(Code) | | Constructs a new identity hash map containing the keys-value mappings
in the specified map.
Parameters:
m - the map whose mappings are to be placed into this map
throws:
NullPointerException - if the specified map is null |
| clear | | public void clear()(Code) | | Removes all of the mappings from this map.
The map will be empty after this call returns.
|
| clone | | public Object clone()(Code) | | Returns a shallow copy of this identity hash map: the keys and values
themselves are not cloned.
a shallow copy of this map |
| containsKey | | public boolean containsKey(Object key)(Code) | | Tests whether the specified object reference is a key in this identity
hash map.
Parameters:
key - possible key true if the specified object reference is a keyin this map
See Also: IdentityHashMap.containsValue(Object) |
| containsValue | | public boolean containsValue(Object value)(Code) | | Tests whether the specified object reference is a value in this identity
hash map.
Parameters:
value - value whose presence in this map is to be tested true if this map maps one or more keys to thespecified object reference
See Also: IdentityHashMap.containsKey(Object) |
| entrySet | | public Set<Map.Entry<K, V>> entrySet()(Code) | | 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()).
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.
a set view of the identity-mappings contained in this map |
| equals | | public boolean equals(Object o)(Code) | | 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()).
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.
Parameters:
o - object to be compared for equality with this map true if the specified object is equal to this map
See Also: Object.equals(Object) |
| get | | public V get(Object key)(Code) | | 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
IdentityHashMap.containsKey containsKey operation may be used to
distinguish these two cases.
See Also: IdentityHashMap.put(Object,Object) |
| hashCode | | public int hashCode()(Code) | | 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 in the map's
entrySet() view. This 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.
the hash code value for this map
See Also: Object.equals(Object)
See Also: IdentityHashMap.equals(Object) |
| isEmpty | | public boolean isEmpty()(Code) | | Returns true if this identity hash map contains no key-value
mappings.
true if this identity hash map contains no key-valuemappings |
| keySet | | public Set<K> keySet()(Code) | | 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.
an identity-based set view of the keys contained in this map
See Also: Object.equals(Object)
See Also: System.identityHashCode(Object) |
| put | | public V put(K key, V value)(Code) | | Associates the specified value with the specified key in this identity
hash map. If the map previously contained a mapping for the key, the
old value is replaced.
Parameters:
key - the key with which the specified value is to be associated
Parameters:
value - the value to be associated with the specified key the previous value associated with key, ornull if there was no mapping for key.(A null return can also indicate that the mappreviously associated null with key.)
See Also: Object.equals(Object)
See Also: IdentityHashMap.get(Object)
See Also: IdentityHashMap.containsKey(Object) |
| putAll | | public void putAll(Map<? extends K, ? extends V> m)(Code) | | Copies all of the mappings from the specified map to this map.
These mappings will replace any mappings that this map had for
any of the keys currently in the specified map.
Parameters:
m - mappings to be stored in this map
throws:
NullPointerException - if the specified map is null |
| remove | | public V remove(Object key)(Code) | | Removes the mapping for this key from this map if present.
Parameters:
key - key whose mapping is to be removed from the map the previous value associated with key, ornull if there was no mapping for key.(A null return can also indicate that the mappreviously associated null with key.) |
| size | | public int size()(Code) | | Returns the number of key-value mappings in this identity hash map.
the number of key-value mappings in this map |
| values | | public Collection<V> values()(Code) | | 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.
|
|
|