Class Overview
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.
Summary
| Public Constructors |
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IdentityHashMap()
Creates an IdentityHashMap with default expected maximum size.
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IdentityHashMap(int maxSize)
Creates an IdentityHashMap with the specified maximum size parameter.
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IdentityHashMap(Map<? extends K, ? extends V> map)
Creates an IdentityHashMap using the given map as initial values.
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| Public Methods |
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void
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clear()
Removes all elements from this map, leaving it empty.
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Object
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clone()
Returns a new IdentityHashMap with the same mappings and size as this
one.
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boolean
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containsKey(Object key)
Returns whether this map contains the specified key.
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boolean
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containsValue(Object value)
Returns whether this map contains the specified value.
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Set<Entry<K, V>>
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entrySet()
Returns a set containing all of the mappings in this map.
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boolean
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equals(Object object)
Compares this map with other objects.
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V
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get(Object key)
Returns the value of the mapping with the specified key.
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boolean
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isEmpty()
Returns whether this IdentityHashMap has no elements.
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Set<K>
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keySet()
Returns a set of the keys contained in this map.
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V
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put(K key, V value)
Maps the specified key to the specified value.
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void
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putAll(Map<? extends K, ? extends V> map)
Copies all the mappings in the specified map to this map.
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V
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remove(Object key)
Removes the mapping with the specified key from this map.
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int
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size()
Returns the number of mappings in this IdentityHashMap.
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Collection<V>
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values()
Returns a collection of the values contained in this map.
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[Expand]
Inherited Methods |
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From class
java.util.AbstractMap
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void
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clear()
Removes all elements from this Map, leaving it empty.
This implementation calls entrySet().clear(). |
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Object
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clone()
Creates and returns a copy of this Object.
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boolean
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containsKey(Object key)
Returns whether this Map contains the specified key.
This implementation iterates its key set, looking for a key that
key equals. |
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boolean
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containsValue(Object value)
Returns whether this Map contains the specified value.
This implementation iterates its entry set, looking for an entry with
a value that value equals. |
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abstract
Set<Entry<K, V>>
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entrySet()
Returns a Set containing all of the mappings in this Map.
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boolean
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equals(Object object)
Compares this instance with the specified object and indicates if they
are equal.
This implementation first checks the structure of object. |
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V
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get(Object key)
Returns the value of the mapping with the specified key.
This implementation iterates its entry set, looking for an entry with
a key that key equals. |
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int
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hashCode()
Returns an integer hash code for this object.
This implementation iterates its entry set, summing the hashcodes of
its entries. |
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boolean
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isEmpty()
Returns whether this map is empty.
This implementation compares size() to 0. |
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Set<K>
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keySet()
Returns a set of the keys contained in this Map.
This implementation returns a view that calls through this to map. |
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V
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put(K key, V value)
Maps the specified key to the specified value.
This base implementation throws UnsupportedOperationException. |
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void
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putAll(Map<? extends K, ? extends V> map)
Copies every mapping in the specified Map to this Map.
This implementation iterates through map's entry set, calling
put() for each. |
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V
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remove(Object key)
Removes a mapping with the specified key from this Map.
This implementation iterates its entry set, removing the entry with
a key that key equals. |
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int
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size()
Returns the number of mappings in this Map.
This implementation returns its entry set's size. |
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String
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toString()
Returns a string containing a concise, human-readable description of this
object.
This implementation composes a string by iterating its entry set. |
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Collection<V>
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values()
Returns a Collection of the values contained in this Map.
This implementation returns a view that calls through this to map. |
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From class
java.lang.Object
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Object
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clone()
Creates and returns a copy of this Object.
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boolean
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equals(Object o)
Compares this instance with the specified object and indicates if they
are equal.
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void
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finalize()
Invoked when the garbage collector has detected that this instance is no longer reachable.
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final
Class<?>
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getClass()
Returns the unique instance of Class that represents this
object's class. |
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int
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hashCode()
Returns an integer hash code for this object.
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final
void
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notify()
Causes a thread which is waiting on this object's monitor (by means of
calling one of the wait() methods) to be woken up.
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final
void
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notifyAll()
Causes all threads which are waiting on this object's monitor (by means
of calling one of the wait() methods) to be woken up.
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String
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toString()
Returns a string containing a concise, human-readable description of this
object.
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final
void
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wait()
Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object.
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final
void
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wait(long millis, int nanos)
Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the
specified timeout expires.
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final
void
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wait(long millis)
Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the
specified timeout expires.
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From interface
java.util.Map
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abstract
void
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clear()
Removes all elements from this Map, leaving it empty.
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abstract
boolean
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containsKey(Object key)
Returns whether this Map contains the specified key.
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abstract
boolean
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containsValue(Object value)
Returns whether this Map contains the specified value.
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abstract
Set<Entry<K, V>>
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entrySet()
Returns a Set containing all of the mappings in this Map.
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abstract
boolean
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equals(Object object)
Compares the argument to the receiver, and returns true if the
specified object is a Map and both Maps contain the same mappings.
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abstract
V
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get(Object key)
Returns the value of the mapping with the specified key.
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abstract
int
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hashCode()
Returns an integer hash code for the receiver.
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abstract
boolean
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isEmpty()
Returns whether this map is empty.
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abstract
Set<K>
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keySet()
Returns a set of the keys contained in this Map.
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abstract
V
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put(K key, V value)
Maps the specified key to the specified value.
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abstract
void
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putAll(Map<? extends K, ? extends V> map)
Copies every mapping in the specified Map to this Map.
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abstract
V
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remove(Object key)
Removes a mapping with the specified key from this Map.
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abstract
int
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size()
Returns the number of mappings in this Map.
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abstract
Collection<V>
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values()
Returns a Collection of the values contained in this Map.
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Public Constructors
public
IdentityHashMap
()
Creates an IdentityHashMap with default expected maximum size.
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.
|
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.
|
Public Methods
public
void
clear
()
Removes all elements from this map, leaving it empty.
public
Object
clone
()
Returns a new IdentityHashMap with the same mappings and size as this
one.
Returns
- a shallow copy of this IdentityHashMap.
public
boolean
containsKey
(Object key)
Returns whether this map contains the specified key.
Parameters
| key
| the key to search for. |
Returns
true if this map contains the specified key,
false otherwise.
public
boolean
containsValue
(Object value)
Returns whether this map contains the specified value.
Parameters
| value
| the value to search for. |
Returns
true if this map contains the specified value,
false otherwise.
public
Set<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.
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)).
Parameters
| object
| the object to compare to. |
Returns
- whether the argument object is equal to this object.
public
V
get
(Object key)
Returns the value of the mapping with the specified key.
Returns
- the value of the mapping with the specified key.
public
boolean
isEmpty
()
Returns whether this IdentityHashMap has no elements.
Returns
true if this IdentityHashMap has no elements,
false otherwise.
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.
public
V
put
(K key, V value)
Maps the specified key to the specified value.
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.
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.
Parameters
| map
| the map to copy mappings from. |
public
V
remove
(Object key)
Removes the mapping with the specified key from this map.
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.
public
int
size
()
Returns the number of mappings in this IdentityHashMap.
Returns
- the number of mappings in this IdentityHashMap.
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.
Returns
- a collection of the values contained in this map.
