java.util
Class IdentityHashMap<K,V>
- Cloneable, Map<K,V>, Serializable
This class provides a hashtable-backed implementation of the
Map interface, but uses object identity to do its hashing. In fact,
it uses object identity for comparing values, as well. It uses a
linear-probe hash table, which may have faster performance
than the chaining employed by HashMap.
WARNING: This is not a general purpose map. Because it uses
System.identityHashCode and ==, instead of hashCode and equals, for
comparison, it violated Map's general contract, and may cause
undefined behavior when compared to other maps which are not
IdentityHashMaps. This is designed only for the rare cases when
identity semantics are needed. An example use is
topology-preserving graph transformations, such as deep cloning,
or as proxy object mapping such as in debugging.
This map permits null keys and values, and does not
guarantee that elements will stay in the same order over time. The
basic operations (get and put) take
constant time, provided System.identityHashCode is decent. You can
tune the behavior by specifying the expected maximum size. As more
elements are added, the map may need to allocate a larger table,
which can be expensive.
This implementation is unsynchronized. If you want multi-thread
access to be consistent, you must synchronize it, perhaps by using
Collections.synchronizedMap(new IdentityHashMap(...));.
The iterators are fail-fast, meaning that a structural modification
made to the map outside of an iterator's remove method cause the
iterator, and in the case of the entrySet, the Map.Entry, to
fail with a ConcurrentModificationException.
IdentityHashMap()- Create a new IdentityHashMap with the default capacity (21 entries).
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IdentityHashMap(extends K, V> m)- Create a new IdentityHashMap whose contents are taken from the
given Map.
|
IdentityHashMap(int max)- Create a new IdentityHashMap with the indicated number of
entries.
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void | clear()- Remove all mappings from this map.
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Object | clone()- Creates a shallow copy where keys and values are not cloned.
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boolean | containsKey(Object key)- Tests whether the specified key is in this map.
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boolean | containsValue(Object value)- Returns true if this HashMap contains the value.
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Set> | entrySet()- Returns a "set view" of this Map's entries.
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boolean | equals(Object o)- Compares two maps for equality.
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V | get(Object key)- Return the value in this Map associated with the supplied key, or
null if the key maps to nothing.
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int | hashCode()- Returns the hashcode of this map.
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boolean | isEmpty()- Returns true if there are no key-value mappings currently in this Map
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Set | keySet()- Returns a "set view" of this Map's keys.
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V | put(K key, V value)- Puts the supplied value into the Map, mapped by the supplied key.
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void | putAll(extends K, V> m)- Copies all of the mappings from the specified map to this.
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V | remove(Object key)- Removes from the HashMap and returns the value which is mapped by
the supplied key.
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int | size()- Returns the number of kay-value mappings currently in this Map
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Collection | values()- Returns a "collection view" (or "bag view") of this Map's values.
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V>> entrySet, clear, clone, containsKey, containsValue, equals, get, hashCode, isEmpty, keySet, put, putAll, remove, size, toString, values |
clone, equals, extends Object> getClass, finalize, hashCode, notify, notifyAll, toString, wait, wait, wait |
IdentityHashMap
public IdentityHashMap(extends K,
V> m) Create a new IdentityHashMap whose contents are taken from the
given Map.
m - The map whose elements are to be put in this map
IdentityHashMap
public IdentityHashMap(int max)
Create a new IdentityHashMap with the indicated number of
entries. If the number of elements added to this hash map
exceeds this maximum, the map will grow itself; however, that
incurs a performance penalty.
entrySet
public Set> entrySet()
Returns a "set view" of this Map's entries. The set is backed by
the Map, so changes in one show up in the other. The set supports
element removal, but not element addition.
The semantics of this set, and of its contained entries, are
different from the contract of Set and Map.Entry in order to make
IdentityHashMap work. This means that while you can compare these
objects between IdentityHashMaps, comparing them with regular sets
or entries is likely to have undefined behavior. The entries
in this set are reference-based, rather than the normal object
equality. Therefore, e1.equals(e2) returns
e1.getKey() == e2.getKey() && e1.getValue() == e2.getValue(),
and e.hashCode() returns
System.identityHashCode(e.getKey()) ^
System.identityHashCode(e.getValue()).
Note that the iterators for all three views, from keySet(), entrySet(),
and values(), traverse the Map in the same sequence.
- a set view of the entries
equals
public boolean equals(Object o)
Compares two maps for equality. This returns true only if both maps
have the same reference-identity comparisons. While this returns
this.entrySet().equals(m.entrySet()) as specified by Map,
this will not work with normal maps, since the entry set compares
with == instead of .equals.
- equals in interface Map<K,V>
- equals in interface AbstractMap<K,V>
o - the object to compare to
get
public V get(Object key)
Return the value in this Map associated with the supplied key, or
null if the key maps to nothing.
NOTE: Since the value could also be null, you must use
containsKey to see if this key actually maps to something.
Unlike normal maps, this tests for the key with entry ==
key instead of entry == null ? key == null :
entry.equals(key).
- get in interface Map<K,V>
- get in interface AbstractMap<K,V>
key - the key for which to fetch an associated value
- what the key maps to, if present
hashCode
public int hashCode()
Returns the hashcode of this map. This guarantees that two
IdentityHashMaps that compare with equals() will have the same hash code,
but may break with comparison to normal maps since it uses
System.identityHashCode() instead of hashCode().
- hashCode in interface Map<K,V>
- hashCode in interface AbstractMap<K,V>
keySet
public Set keySet()
Returns a "set view" of this Map's keys. The set is backed by the
Map, so changes in one show up in the other. The set supports
element removal, but not element addition.
The semantics of this set are different from the contract of Set
in order to make IdentityHashMap work. This means that while you can
compare these objects between IdentityHashMaps, comparing them with
regular sets is likely to have undefined behavior. The hashCode
of the set is the sum of the identity hash codes, instead of the
regular hashCodes, and equality is determined by reference instead
of by the equals method.
- keySet in interface Map<K,V>
- keySet in interface AbstractMap<K,V>
put
public V put(K key,
V value) Puts the supplied value into the Map, mapped by the supplied key.
The value may be retrieved by any object which equals()
this key. NOTE: Since the prior value could also be null, you must
first use containsKey if you want to see if you are replacing the
key's mapping. Unlike normal maps, this tests for the key
with entry == key instead of
entry == null ? key == null : entry.equals(key).
- put in interface Map<K,V>
- put in interface AbstractMap<K,V>
key - the key used to locate the valuevalue - the value to be stored in the HashMap
- the prior mapping of the key, or null if there was none
remove
public V remove(Object key)
Removes from the HashMap and returns the value which is mapped by
the supplied key. If the key maps to nothing, then the HashMap
remains unchanged, and null is returned.
NOTE: Since the value could also be null, you must use
containsKey to see if you are actually removing a mapping.
Unlike normal maps, this tests for the key with entry ==
key instead of entry == null ? key == null :
entry.equals(key).
- remove in interface Map<K,V>
- remove in interface AbstractMap<K,V>
key - the key used to locate the value to remove
- whatever the key mapped to, if present
values
public Collection values()
Returns a "collection view" (or "bag view") of this Map's values.
The collection is backed by the Map, so changes in one show up
in the other. The collection supports element removal, but not element
addition.
The semantics of this set are different from the contract of
Collection in order to make IdentityHashMap work. This means that
while you can compare these objects between IdentityHashMaps, comparing
them with regular sets is likely to have undefined behavior.
Likewise, contains and remove go by == instead of equals().
- values in interface Map<K,V>
- values in interface AbstractMap<K,V>
IdentityHashMap.java -- a class providing a hashtable data structure,
mapping Object --> Object, which uses object identity for hashing.
Copyright (C) 2001, 2002, 2004, 2005 Free Software Foundation, Inc.
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