K - the type of keys maintained by this mapV - the type of mapped valuespublic class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable
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 is rehashed (that is, internal data structures are rebuilt) so that the hash table has approximately twice the number of buckets.
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 using
many keys with the same hashCode() is a sure way to slow
down performance of any hash table. To ameliorate impact, when keys
are Comparable, this class may use comparison order among
keys to help break ties.
Note that this implementation is not synchronized.
If multiple threads access a hash 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
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. 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.
Object.hashCode(),
Collection,
Map,
TreeMap,
Hashtable,
Serialized FormAbstractMap.SimpleEntry<K,V>, AbstractMap.SimpleImmutableEntry<K,V>| Constructor and Description |
|---|
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.
|
| Modifier and Type | Method and Description |
|---|---|
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.
|
V |
compute(K key,
BiFunction<? super K,? super V,? extends V> remappingFunction)
Attempts to compute a mapping for the specified key and its current
mapped value (or
null if there is no current mapping). |
V |
computeIfAbsent(K key,
Function<? super K,? extends V> mappingFunction)
If the specified key is not already associated with a value (or is mapped
to
null), attempts to compute its value using the given mapping
function and enters it into this map unless null. |
V |
computeIfPresent(K key,
BiFunction<? super K,? super V,? extends V> remappingFunction)
If the value for the specified key is present and non-null, attempts to
compute a new mapping given the key and its current mapped value.
|
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 view of the mappings contained in this map. |
void |
forEach(BiConsumer<? super K,? super V> action)
Performs the given action for each entry in this map until all entries
have been processed or the action throws an exception.
|
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. |
V |
getOrDefault(Object key,
V defaultValue)
Returns the value to which the specified key is mapped, or
defaultValue if this map contains no mapping for the key. |
boolean |
isEmpty()
Returns true if this map contains no key-value mappings.
|
Set<K> |
keySet()
Returns a
Set view of the keys contained in this map. |
V |
merge(K key,
V value,
BiFunction<? super V,? super V,? extends V> remappingFunction)
If the specified key is not already associated with a value or is
associated with null, associates it with the given non-null value.
|
V |
put(K key,
V value)
Associates the specified value with the specified key in this map.
|
void |
putAll(Map<? extends K,? extends V> m)
Copies all of the mappings from the specified map to this map.
|
V |
putIfAbsent(K key,
V value)
If the specified key is not already associated with a value (or is mapped
to
null) associates it with the given value and returns
null, else returns the current value. |
V |
remove(Object key)
Removes the mapping for the specified key from this map if present.
|
boolean |
remove(Object key,
Object value)
Removes the entry for the specified key only if it is currently
mapped to the specified value.
|
V |
replace(K key,
V value)
Replaces the entry for the specified key only if it is
currently mapped to some value.
|
boolean |
replace(K key,
V oldValue,
V newValue)
Replaces the entry for the specified key only if currently
mapped to the specified value.
|
void |
replaceAll(BiFunction<? super K,? super V,? extends V> function)
Replaces each entry's value with the result of invoking the given
function on that entry until all entries have been processed or the
function throws an exception.
|
int |
size()
Returns the number of key-value mappings in this map.
|
Collection<V> |
values()
Returns a
Collection view of the values contained in this map. |
equals, hashCode, toStringpublic HashMap(int initialCapacity,
float loadFactor)
initialCapacity - the initial capacityloadFactor - the load factorIllegalArgumentException - if the initial capacity is negative
or the load factor is nonpositivepublic HashMap(int initialCapacity)
initialCapacity - the initial capacity.IllegalArgumentException - if the initial capacity is negative.public HashMap()
public HashMap(Map<? extends K,? extends V> m)
m - the map whose mappings are to be placed in this mapNullPointerException - if the specified map is nullpublic int size()
public boolean isEmpty()
public V get(Object key)
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 necessarily
indicate that the map contains no mapping for the key; it's also
possible that the map explicitly maps the key to null.
The containsKey operation may be used to
distinguish these two cases.
get in interface Map<K,V>get in class AbstractMap<K,V>key - the key whose associated value is to be returnednull if this map contains no mapping for the keyput(Object, Object)public boolean containsKey(Object key)
containsKey in interface Map<K,V>containsKey in class AbstractMap<K,V>key - The key whose presence in this map is to be testedpublic V put(K key, V value)
put in interface Map<K,V>put in class AbstractMap<K,V>key - key with which the specified value is to be associatedvalue - value to be associated with the specified keypublic void putAll(Map<? extends K,? extends V> m)
putAll in interface Map<K,V>putAll in class AbstractMap<K,V>m - mappings to be stored in this mapNullPointerException - if the specified map is nullpublic V remove(Object key)
remove in interface Map<K,V>remove in class AbstractMap<K,V>key - key whose mapping is to be removed from the mappublic void clear()
public boolean containsValue(Object value)
containsValue in interface Map<K,V>containsValue in class AbstractMap<K,V>value - value whose presence in this map is to be testedpublic Set<K> keySet()
Set view of the keys 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), 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.public Collection<V> values()
Collection view of the values contained in this map.
The collection is backed by the map, so changes to the map are
reflected in the collection, and vice-versa. 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 and clear operations. It does not
support the add or addAll operations.public Set<Map.Entry<K,V>> entrySet()
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, removeAll, retainAll and
clear operations. It does not support the
add or addAll operations.public V getOrDefault(Object key, V defaultValue)
MapdefaultValue if this map contains no mapping for the key.getOrDefault in interface Map<K,V>key - the key whose associated value is to be returneddefaultValue - the default mapping of the keydefaultValue if this map contains no mapping for the keypublic V putIfAbsent(K key, V value)
Mapnull) associates it with the given value and returns
null, else returns the current value.putIfAbsent in interface Map<K,V>key - key with which the specified value is to be associatedvalue - value to be associated with the specified keynull if there was no mapping for the key.
(A null return can also indicate that the map
previously associated null with the key,
if the implementation supports null values.)public boolean remove(Object key, Object value)
Mappublic boolean replace(K key, V oldValue, V newValue)
Mappublic V replace(K key, V value)
Mapreplace in interface Map<K,V>key - key with which the specified value is associatedvalue - value to be associated with the specified keynull if there was no mapping for the key.
(A null return can also indicate that the map
previously associated null with the key,
if the implementation supports null values.)public V computeIfAbsent(K key, Function<? super K,? extends V> mappingFunction)
Mapnull), attempts to compute its value using the given mapping
function and enters it into this map unless null.
If the function returns null no mapping is recorded. If
the function itself throws an (unchecked) exception, the
exception is rethrown, and no mapping is recorded. The most
common usage is to construct a new object serving as an initial
mapped value or memoized result, as in:
map.computeIfAbsent(key, k -> new Value(f(k)));
Or to implement a multi-value map, Map<K,Collection<V>>,
supporting multiple values per key:
map.computeIfAbsent(key, k -> new HashSet<V>()).add(v);
computeIfAbsent in interface Map<K,V>key - key with which the specified value is to be associatedmappingFunction - the function to compute a valuepublic V computeIfPresent(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)
MapIf the function returns null, the mapping is removed. If the
function itself throws an (unchecked) exception, the exception is
rethrown, and the current mapping is left unchanged.
computeIfPresent in interface Map<K,V>key - key with which the specified value is to be associatedremappingFunction - the function to compute a valuepublic V compute(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)
Mapnull if there is no current mapping). For
example, to either create or append a String msg to a value
mapping:
map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))
(Method merge() is often simpler to use for such purposes.)
If the function returns null, the mapping is removed (or
remains absent if initially absent). If the function itself throws an
(unchecked) exception, the exception is rethrown, and the current mapping
is left unchanged.
public V merge(K key, V value, BiFunction<? super V,? super V,? extends V> remappingFunction)
Mapnull. This
method may be of use when combining multiple mapped values for a key.
For example, to either create or append a String msg to a
value mapping:
map.merge(key, msg, String::concat)
If the function returns null the mapping is removed. If the
function itself throws an (unchecked) exception, the exception is
rethrown, and the current mapping is left unchanged.
merge in interface Map<K,V>key - key with which the resulting value is to be associatedvalue - the non-null value to be merged with the existing value
associated with the key or, if no existing value or a null value
is associated with the key, to be associated with the keyremappingFunction - the function to recompute a value if presentpublic void forEach(BiConsumer<? super K,? super V> action)
Mappublic void replaceAll(BiFunction<? super K,? super V,? extends V> function)
MapreplaceAll in interface Map<K,V>function - the function to apply to each entry Submit a bug or feature
For further API reference and developer documentation, see Java SE Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.
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