java.util.AbstractMap<K,V>
java.util.HashMap<K,V>
is new.
java.lang.Object
Type Parameters:
K - the type of keys maintained by this map
V - the type of mapped values
public class HashMap<K,V>
Hash table based implementation of the Map interface. This implementation provides all of the optional map operations, and permits null values and the null key. (The HashMap class is roughly equivalent to Hashtable , except that it is unsynchronized and permits nulls.) 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 implementation provides constant-time performance for the basic operations ( get and put ), assuming the hash function disperses the elements properly among the buckets. Iteration over collection views requires time proportional to the "capacity" of the HashMap instance (the number of buckets) plus its size (the number of key-value mappings). Thus, it's very important not to set the initial capacity too high (or the load factor too low) if iteration performance is important.
An instance of
HashMap
has two parameters that affect its performance:
initial capacity
and
load factor
. The
capacity
is the number of buckets in the hash table, and the initial capacity is simply the capacity at the time the hash table is created. The
load factor
is a measure of how full the hash table is allowed to get before its capacity is automatically increased. When the number of entries in the hash table exceeds the product of the load factor and the current capacity, the
hash table
capacity
is
rehashed
(that is, internal data structures are rebuilt) so that
roughly doubled by calling
the
hash table has approximately twice the number of buckets.
rehash
method.
As a general rule, the default load factor (.75) offers a good tradeoff between time and space costs. Higher values decrease the space overhead but increase the lookup cost (reflected in most of the operations of the HashMap class, including get and put ). The expected number of entries in the map and its load factor should be taken into account when setting its initial capacity, so as to minimize the number of rehash operations. If the initial capacity is greater than the maximum number of entries divided by the load factor, no rehash operations will ever occur.
If many mappings are to be stored in a HashMap instance, creating it with a sufficiently large capacity will allow the mappings to be stored more efficiently than letting it perform automatic rehashing as needed to grow the table.
Note that this implementation is not synchronized.
If multiple threads access
a hash
this
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 HashMap(...));
The iterators 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
or
add
methods, 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: the fail-fast behavior of iterators should be used only to detect bugs.
This class is a member of the Java Collections Framework .
|
Nested Class Summary
|
|---|
|
Nested classes/interfaces inherited from class java.util.
AbstractMap
|
|---|
|
AbstractMap.SimpleEntry
<
K
,
V
>,
AbstractMap.SimpleImmutableEntry
<
K
,
V
|
| Constructor Summary | |
|---|---|
|
HashMap
() Constructs an empty HashMap with the default initial capacity (16) and the default load factor (0.75). |
|
|
HashMap
(int initialCapacity) Constructs an empty HashMap with the specified initial capacity and the default load factor (0.75). |
|
|
HashMap
(int initialCapacity, float loadFactor) Constructs an empty HashMap with the specified initial capacity and load factor. |
|
|
HashMap
(
Map
<? extends
K
,? extends
V
> m) Constructs a new HashMap with the same mappings as the specified Map . |
|
| Method Summary | |
|---|---|
| void |
clear
() Removes all
of the
mappings from this map. |
| Object |
clone
() Returns a shallow copy of this HashMap instance: the keys and values themselves are not cloned. |
| boolean |
containsKey
(
Object
key) Returns true if this map contains a mapping for the specified key. |
| boolean |
containsValue
(
Object
value) Returns true if this map maps one or more keys to the specified value. |
| Set < Map.Entry < K , V |
entrySet
() Returns a
Set
|
| V |
get
(
Object
Returns the value to which the specified key is
mapped,
this
the
|
| boolean |
isEmpty
() Returns true if this map contains no key-value mappings. |
| Set < K |
keySet
() Returns a
Set
|
| V |
put
(
K
key,
V
value) Associates the specified value with the specified key in this map. |
| void |
putAll
(
Map
<? extends
K
,? extends
V
Copies all of the mappings from the specified map to this
|
| V |
remove
(
Object
Removes the mapping for
the specified
|
| int |
size
() Returns the number of key-value mappings in this map. |
| Collection < V |
values
() Returns a
Collection
|
| Methods inherited from class java.util. AbstractMap |
|---|
| equals , hashCode , toString |
| Methods inherited from class java.lang. Object |
|---|
| finalize , getClass , notify , notifyAll , wait , wait , wait |
| Methods inherited from interface java.util. Map |
|---|
| equals , hashCode |
| Constructor Detail |
|---|
public HashMap(int initialCapacity,
float loadFactor)
the
capacity
the
factor
nonpositive
public HashMap(int initialCapacity)
public HashMap()
public HashMap(Map<? extends K,? extends V> m)
map
null
| Method Detail |
|---|
public int size()
map
public boolean isEmpty()
mappings
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==null ? k==null : key.equals(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
Returns the value to which the specified key is mapped in this identity hash map, or
null
if the map contains no mapping for this key. A return value of
null
does not
necessarily
indicate that the map contains no mapping for the key;
it's
it is
also possible that the map explicitly maps the key to
null.
null
.
The
containsKey
operation
containsKey
method
may be used to distinguish these two cases.
returned
key is mapped,
this
the key
public boolean containsKey(Object key)
public V put(K key,
V value)
the
associated
key
the
previous value associated with
key
, or
null
if there was no mapping for
key
. (A
null
return can also indicate that the
map
with
key
.)
public void putAll(Map<? extends K,? extends V> m)
map.
map
null
public V remove(Object key)
the specified
map
the
previous value associated with
key
, or
null
if there was no mapping for
key
. (A
null
return can also indicate that the map previously associated
null
with
key
.)
public void clear()
of the
mappings from this map.
The map will be empty after this call returns.
public boolean containsValue(Object value)
tested
value
public Object clone()
map
public Set<K> keySet()
Set
If the map is modified while an iteration over the set is in progress (except through the iterator's own
remove
operation), 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
operations. It does not support the
add
or
addAll
operations.
map
public Collection<V> values()
Collection
If the map is modified while an iteration over the collection is in progress (except through the iterator's own
remove
operation), 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
map
public Set<Map.Entry<K,V>> entrySet()
Returns a
Set
view of the mappings 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 (except through the iterator's own
remove
operation, or through the
setValue
operation on a map entry returned by the iterator) 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
set
map