 Known Direct Subclasses
| LinkedHashMap<K, V> |
LinkedHashMap is an implementation of Map that guarantees iteration order. |
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Class Overview
HashMap is an implementation of Map. All optional operations are supported.
All elements are permitted as keys or values, including null.
Note that the iteration order for HashMap is non-deterministic. If you want
deterministic iteration, use LinkedHashMap.
Note: the implementation of HashMap is not synchronized.
If one thread of several threads accessing an instance modifies the map
structurally, access to the map needs to be synchronized. A structural
modification is an operation that adds or removes an entry. Changes in
the value of an entry are not structural changes.
The Iterator created by calling the iterator method
may throw a ConcurrentModificationException if the map is structurally
changed while an iterator is used to iterate over the elements. Only the
remove method that is provided by the iterator allows for removal of
elements during iteration. It is not possible to guarantee that this
mechanism works in all cases of unsynchronized concurrent modification. It
should only be used for debugging purposes.
Summary
| Public Constructors |
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HashMap()
Constructs a new empty HashMap instance.
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HashMap(int capacity)
Constructs a new HashMap instance with the specified capacity.
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HashMap(int capacity, float loadFactor)
Constructs a new HashMap instance with the specified capacity and
load factor.
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HashMap(Map<? extends K, ? extends V> map)
Constructs a new HashMap instance containing the mappings from
the specified map.
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| Public Methods |
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void
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clear()
Removes all mappings from this hash map, leaving it empty.
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Object
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clone()
Returns a shallow copy of this map.
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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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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 map is empty.
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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 elements in this map.
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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()
Called before the object's memory is reclaimed by the VM.
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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
HashMap
()
Constructs a new empty HashMap instance.
public
HashMap
(int capacity)
Constructs a new HashMap instance with the specified capacity.
Parameters
| capacity
| the initial capacity of this hash map. |
public
HashMap
(int capacity, float loadFactor)
Constructs a new HashMap instance with the specified capacity and
load factor.
Parameters
| capacity
| the initial capacity of this hash map. |
| loadFactor
| the initial load factor. |
public
HashMap
(Map<? extends K, ? extends V> map)
Constructs a new HashMap instance containing the mappings from
the specified map.
Public Methods
public
void
clear
()
Removes all mappings from this hash map, leaving it empty.
public
Object
clone
()
Returns a shallow copy of this map.
Returns
- a shallow copy of this map.
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
V
get
(Object key)
Returns the value of the mapping with the specified key.
Returns
- the value of the mapping with the specified key, or
null
if no mapping for the specified key is found.
public
boolean
isEmpty
()
Returns whether this map is empty.
Returns
true if this map 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.
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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 elements in this map.
Returns
- the number of elements in this map.
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.
