java.lang.Object | ||
↳ | java.util.AbstractMap <K, V> | |
↳ | java.util.HashMap<K, V> |
Known Direct Subclasses
LinkedHashMap
<K, V>
|
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.
Public Constructors | |||||||||||
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Constructs a new empty
HashMap
instance.
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Constructs a new
HashMap
instance with the specified capacity.
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Constructs a new
HashMap
instance with the specified capacity and
load factor.
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Constructs a new
HashMap
instance containing the mappings from
the specified map.
|
Public Methods | |||||||||||
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Removes all mappings from this hash map, leaving it empty.
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Returns a shallow copy of this map.
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Returns whether this map contains the specified key.
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Returns whether this map contains the specified value.
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Returns a set containing all of the mappings in this map.
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Returns the value of the mapping with the specified key.
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Returns whether this map is empty.
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Returns a set of the keys contained in this map.
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Maps the specified key to the specified value.
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Copies all the mappings in the specified map to this map.
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Removes the mapping with the specified key from this map.
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Returns the number of elements in this map.
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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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From class
java.lang.Object
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From interface
java.util.Map
|
Constructs a new
HashMap
instance with the specified capacity.
capacity | the initial capacity of this hash map. |
---|
IllegalArgumentException | when the capacity is less than zero. |
---|
Constructs a new
HashMap
instance with the specified capacity and
load factor.
capacity | the initial capacity of this hash map. |
---|---|
loadFactor | the initial load factor. |
IllegalArgumentException | when the capacity is less than zero or the load factor is less or equal to zero or NaN. |
---|
Constructs a new
HashMap
instance containing the mappings from
the specified map.
map | the mappings to add. |
---|
Returns a shallow copy of this map.
Returns whether this map contains the specified key.
key | the key to search for. |
---|
true
if this map contains the specified key,
false
otherwise.
Returns whether this map contains the specified value.
value | the value to search for. |
---|
true
if this map contains the specified value,
false
otherwise.
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.
Returns the value of the mapping with the specified key.
key | the key. |
---|
null
if no mapping for the specified key is found.
Returns whether this map is empty.
true
if this map has no elements,
false
otherwise.
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.
Maps the specified key to the specified value.
key | the key. |
---|---|
value | the value. |
null
if there was no such mapping.
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.
map | the map to copy mappings from. |
---|
Removes the mapping with the specified key from this map.
key | the key of the mapping to remove. |
---|
null
if no mapping
for the specified key was found.
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.