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Java™ Platform
Standard Ed. 8
compact1, compact2, compact3
java.util
Class HashMap<K,V>
-
- Type Parameters:
K - the type of keys maintained by this mapV - the type of mapped values
- All Implemented Interfaces:
- Serializable, Cloneable, Map<K,V>
- Direct Known Subclasses:
- LinkedHashMap, PrinterStateReasons
public class HashMap<K,V>
extends AbstractMap<K,V>
implements Map<K,V>, Cloneable, Serializable
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 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.
- Since:
- 1.2
- See Also:
Object.hashCode(),
Collection,
Map,
TreeMap,
Hashtable,
Serialized Form
-
-
Nested Class Summary
-
Nested classes/interfaces inherited from interface java.util.Map
Map.Entry<K,V>
-
Constructor Summary
Constructors
| Constructor |
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.
|
-
Method Summary
All Methods Instance Methods Concrete Methods
| Modifier and Type |
Method |
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. |
-
-
Constructor Detail
-
HashMap
public HashMap(int initialCapacity,
float loadFactor)
Constructs an empty HashMap with the specified initial
capacity and load factor.
- Parameters:
initialCapacity - the initial capacityloadFactor - the load factor- Throws:
IllegalArgumentException - if the initial capacity is negative
or the load factor is nonpositive
-
HashMap
public HashMap(Map<? extends K,? extends V> m)
Constructs a new HashMap with the same mappings as the
specified Map. The HashMap is created with
default load factor (0.75) and an initial capacity sufficient to
hold the mappings in the specified Map.
- Parameters:
m - the map whose mappings are to be placed in this map- Throws:
NullPointerException - if the specified map is null
-
Method Detail
-
get
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 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.
- Specified by:
get in interface Map<K,V>- Overrides:
get in class AbstractMap<K,V>- Parameters:
key - the key whose associated value is to be returned- Returns:
- the value to which the specified key is mapped, or
null if this map contains no mapping for the key
- See Also:
put(Object, Object)
-
put
public V put(K key,
V value)
Associates the specified value with the specified key in this map.
If the map previously contained a mapping for the key, the old
value is replaced.
- Specified by:
put in interface Map<K,V>- Overrides:
put in class AbstractMap<K,V>- Parameters:
key - key with which the specified value is to be associatedvalue - value to be associated with the specified key- Returns:
- 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.)
-
putAll
public void putAll(Map<? extends K,? extends V> m)
Copies all of the mappings from the specified map to this map.
These mappings will replace any mappings that this map had for
any of the keys currently in the specified map.
- Specified by:
putAll in interface Map<K,V>- Overrides:
putAll in class AbstractMap<K,V>- Parameters:
m - mappings to be stored in this map- Throws:
NullPointerException - if the specified map is null
-
keySet
public Set<K> keySet()
Returns a 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.
- Specified by:
keySet in interface Map<K,V>- Overrides:
keySet in class AbstractMap<K,V>- Returns:
- a set view of the keys contained in this map
-
getOrDefault
public V getOrDefault(Object key,
V defaultValue)
Description copied from interface: Map
Returns the value to which the specified key is mapped, or
defaultValue if this map contains no mapping for the key.
- Specified by:
getOrDefault in interface Map<K,V>- Parameters:
key - the key whose associated value is to be returneddefaultValue - the default mapping of the key- Returns:
- the value to which the specified key is mapped, or
defaultValue if this map contains no mapping for the key
-
putIfAbsent
public V putIfAbsent(K key,
V value)
Description copied from interface: Map
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.
- Specified by:
putIfAbsent in interface Map<K,V>- Parameters:
key - key with which the specified value is to be associatedvalue - value to be associated with the specified key- Returns:
- the previous value associated with the specified key, or
null 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.)
-
remove
public boolean remove(Object key,
Object value)
Description copied from interface: Map
Removes the entry for the specified key only if it is currently
mapped to the specified value.
- Specified by:
remove in interface Map<K,V>- Parameters:
key - key with which the specified value is associatedvalue - value expected to be associated with the specified key- Returns:
true if the value was removed
-
replace
public boolean replace(K key,
V oldValue,
V newValue)
Description copied from interface: Map
Replaces the entry for the specified key only if currently
mapped to the specified value.
- Specified by:
replace in interface Map<K,V>- Parameters:
key - key with which the specified value is associatedoldValue - value expected to be associated with the specified keynewValue - value to be associated with the specified key- Returns:
true if the value was replaced
-
replace
public V replace(K key,
V value)
Description copied from interface: Map
Replaces the entry for the specified key only if it is
currently mapped to some value.
- Specified by:
replace in interface Map<K,V>- Parameters:
key - key with which the specified value is associatedvalue - value to be associated with the specified key- Returns:
- the previous value associated with the specified key, or
null 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.)
-
merge
public V merge(K key,
V value,
BiFunction<? super V,? super V,? extends V> remappingFunction)
Description copied from interface: Map
If the specified key is not already associated with a value or is
associated with null, associates it with the given non-null value.
Otherwise, replaces the associated value with the results of the given
remapping function, or removes if the result is null. 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.
- Specified by:
merge in interface Map<K,V>- Parameters:
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 present- Returns:
- the new value associated with the specified key, or null if no
value is associated with the key
-
forEach
public void forEach(BiConsumer<? super K,? super V> action)
Description copied from interface: Map
Performs the given action for each entry in this map until all entries
have been processed or the action throws an exception. Unless
otherwise specified by the implementing class, actions are performed in
the order of entry set iteration (if an iteration order is specified.)
Exceptions thrown by the action are relayed to the caller.
- Specified by:
forEach in interface Map<K,V>- Parameters:
action - The action to be performed for each entry
-
replaceAll
public void replaceAll(BiFunction<? super K,? super V,? extends V> function)
Description copied from interface: Map
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. Exceptions thrown by the function are
relayed to the caller.
- Specified by:
replaceAll in interface Map<K,V>- Parameters:
function - the function to apply to each entry
Java™ Platform
Standard Ed. 8
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