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
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:
-
Nested Class Summary
Nested classes/interfaces inherited from class java.util.AbstractMap
AbstractMap.SimpleEntry<K,
V>, AbstractMap.SimpleImmutableEntry<K, V> -
Constructor Summary
ConstructorDescriptionHashMap()
Constructs an emptyHashMap
with the default initial capacity (16) and the default load factor (0.75).HashMap
(int initialCapacity) Constructs an emptyHashMap
with the specified initial capacity and the default load factor (0.75).HashMap
(int initialCapacity, float loadFactor) Constructs an emptyHashMap
with the specified initial capacity and load factor.Constructs a newHashMap
with the same mappings as the specifiedMap
. -
Method Summary
Modifier and TypeMethodDescriptionvoid
clear()
Removes all of the mappings from this map.clone()
Returns a shallow copy of thisHashMap
instance: the keys and values themselves are not cloned.Attempts to compute a mapping for the specified key and its current mapped value, ornull
if there is no current mapping (optional operation).computeIfAbsent
(K key, Function<? super K, ? extends V> mappingFunction) If the specified key is not already associated with a value (or is mapped tonull
), attempts to compute its value using the given mapping function and enters it into this map unlessnull
(optional operation).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 (optional operation).boolean
containsKey
(Object key) Returnstrue
if this map contains a mapping for the specified key.boolean
containsValue
(Object value) Returnstrue
if this map maps one or more keys to the specified value.entrySet()
Returns aSet
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.Returns the value to which the specified key is mapped, ornull
if this map contains no mapping for the key.getOrDefault
(Object key, V defaultValue) Returns the value to which the specified key is mapped, ordefaultValue
if this map contains no mapping for the key.boolean
isEmpty()
Returnstrue
if this map contains no key-value mappings.keySet()
Returns aSet
view of the keys contained in this 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 (optional operation).static <K,
V> HashMap <K, V> newHashMap
(int numMappings) Creates a new, empty HashMap suitable for the expected number of mappings.Associates the specified value with the specified key in this map.void
Copies all of the mappings from the specified map to this map.putIfAbsent
(K key, V value) If the specified key is not already associated with a value (or is mapped tonull
) associates it with the given value and returnsnull
, else returns the current value (optional operation).Removes the mapping for the specified key from this map if present.boolean
Removes the entry for the specified key only if it is currently mapped to the specified value (optional operation).Replaces the entry for the specified key only if it is currently mapped to some value (optional operation).boolean
Replaces the entry for the specified key only if currently mapped to the specified value (optional operation).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 (optional operation).int
size()
Returns the number of key-value mappings in this map.values()
Returns aCollection
view of the values contained in this map.Methods inherited from class java.util.AbstractMap
equals, hashCode, toString
-
Constructor Details
-
HashMap
public HashMap(int initialCapacity, float loadFactor) Constructs an emptyHashMap
with the specified initial capacity and load factor.- API Note:
- To create a
HashMap
with an initial capacity that accommodates an expected number of mappings, usenewHashMap
. - 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(int initialCapacity) Constructs an emptyHashMap
with the specified initial capacity and the default load factor (0.75).- API Note:
- To create a
HashMap
with an initial capacity that accommodates an expected number of mappings, usenewHashMap
. - Parameters:
initialCapacity
- the initial capacity.- Throws:
IllegalArgumentException
- if the initial capacity is negative.
-
HashMap
public HashMap()Constructs an emptyHashMap
with the default initial capacity (16) and the default load factor (0.75). -
HashMap
Constructs a newHashMap
with the same mappings as the specifiedMap
. TheHashMap
is created with default load factor (0.75) and an initial capacity sufficient to hold the mappings in the specifiedMap
.- Parameters:
m
- the map whose mappings are to be placed in this map- Throws:
NullPointerException
- if the specified map is null
-
-
Method Details
-
size
-
isEmpty
-
get
Returns the value to which the specified key is mapped, ornull
if this map contains no mapping for the key.More formally, if this map contains a mapping from a key
k
to a valuev
such that(key==null ? k==null : key.equals(k))
, then this method returnsv
; otherwise it returnsnull
. (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 tonull
. ThecontainsKey
operation may be used to distinguish these two cases. -
containsKey
Returnstrue
if this map contains a mapping for the specified key.- Specified by:
containsKey
in interfaceMap<K,
V> - Overrides:
containsKey
in classAbstractMap<K,
V> - Parameters:
key
- The key whose presence in this map is to be tested- Returns:
true
if this map contains a mapping for the specified key.
-
put
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 interfaceMap<K,
V> - Overrides:
put
in classAbstractMap<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
, ornull
if there was no mapping forkey
. (Anull
return can also indicate that the map previously associatednull
withkey
.)
-
putAll
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 interfaceMap<K,
V> - Overrides:
putAll
in classAbstractMap<K,
V> - Parameters:
m
- mappings to be stored in this map- Throws:
NullPointerException
- if the specified map is null
-
remove
Removes the mapping for the specified key from this map if present.- Specified by:
remove
in interfaceMap<K,
V> - Overrides:
remove
in classAbstractMap<K,
V> - Parameters:
key
- key whose mapping is to be removed from the map- Returns:
- the previous value associated with
key
, ornull
if there was no mapping forkey
. (Anull
return can also indicate that the map previously associatednull
withkey
.)
-
clear
-
containsValue
Returnstrue
if this map maps one or more keys to the specified value.- Specified by:
containsValue
in interfaceMap<K,
V> - Overrides:
containsValue
in classAbstractMap<K,
V> - Parameters:
value
- value whose presence in this map is to be tested- Returns:
true
if this map maps one or more keys to the specified value
-
keySet
Returns aSet
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 ownremove
operation), the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via theIterator.remove
,Set.remove
,removeAll
,retainAll
, andclear
operations. It does not support theadd
oraddAll
operations. -
values
Returns aCollection
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 ownremove
operation), the results of the iteration are undefined. The collection supports element removal, which removes the corresponding mapping from the map, via theIterator.remove
,Collection.remove
,removeAll
,retainAll
andclear
operations. It does not support theadd
oraddAll
operations. -
entrySet
Returns aSet
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 ownremove
operation, or through thesetValue
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 theIterator.remove
,Set.remove
,removeAll
,retainAll
andclear
operations. It does not support theadd
oraddAll
operations. -
getOrDefault
Description copied from interface:Map
Returns the value to which the specified key is mapped, ordefaultValue
if this map contains no mapping for the key.- Specified by:
getOrDefault
in interfaceMap<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
Description copied from interface:Map
If the specified key is not already associated with a value (or is mapped tonull
) associates it with the given value and returnsnull
, else returns the current value (optional operation).- Specified by:
putIfAbsent
in interfaceMap<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. (Anull
return can also indicate that the map previously associatednull
with the key, if the implementation supports null values.)
-
remove
Description copied from interface:Map
Removes the entry for the specified key only if it is currently mapped to the specified value (optional operation). -
replace
Description copied from interface:Map
Replaces the entry for the specified key only if currently mapped to the specified value (optional operation). -
replace
Description copied from interface:Map
Replaces the entry for the specified key only if it is currently mapped to some value (optional operation).- Specified by:
replace
in interfaceMap<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. (Anull
return can also indicate that the map previously associatednull
with the key, if the implementation supports null values.)
-
computeIfAbsent
If the specified key is not already associated with a value (or is mapped tonull
), attempts to compute its value using the given mapping function and enters it into this map unlessnull
(optional operation).If the mapping function returns
null
, no mapping is recorded. If the mapping 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);
The mapping function should not modify this map during computation.
This method will, on a best-effort basis, throw a
ConcurrentModificationException
if it is detected that the mapping function modifies this map during computation.- Specified by:
computeIfAbsent
in interfaceMap<K,
V> - Parameters:
key
- key with which the specified value is to be associatedmappingFunction
- the mapping function to compute a value- Returns:
- the current (existing or computed) value associated with the specified key, or null if the computed value is null
- Throws:
ConcurrentModificationException
- if it is detected that the mapping function modified this map
-
computeIfPresent
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 (optional operation).If the remapping function returns
null
, the mapping is removed. If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.The remapping function should not modify this map during computation.
This method will, on a best-effort basis, throw a
ConcurrentModificationException
if it is detected that the remapping function modifies this map during computation.- Specified by:
computeIfPresent
in interfaceMap<K,
V> - Parameters:
key
- key with which the specified value is to be associatedremappingFunction
- the remapping function to compute a value- Returns:
- the new value associated with the specified key, or null if none
- Throws:
ConcurrentModificationException
- if it is detected that the remapping function modified this map
-
compute
Attempts to compute a mapping for the specified key and its current mapped value, ornull
if there is no current mapping (optional operation). For example, to either create or append aString
msg to a value mapping:map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))
merge()
is often simpler to use for such purposes.)If the remapping function returns
null
, the mapping is removed (or remains absent if initially absent). If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.The remapping function should not modify this map during computation.
This method will, on a best-effort basis, throw a
ConcurrentModificationException
if it is detected that the remapping function modifies this map during computation.- Specified by:
compute
in interfaceMap<K,
V> - Parameters:
key
- key with which the specified value is to be associatedremappingFunction
- the remapping function to compute a value- Returns:
- the new value associated with the specified key, or null if none
- Throws:
ConcurrentModificationException
- if it is detected that the remapping function modified this map
-
merge
If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value (optional operation). Otherwise, replaces the associated value with the results of the given remapping function, or removes if the result isnull
. This method may be of use when combining multiple mapped values for a key. For example, to either create or append aString msg
to a value mapping:map.merge(key, msg, String::concat)
If the remapping function returns
null
, the mapping is removed. If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.The remapping function should not modify this map during computation.
This method will, on a best-effort basis, throw a
ConcurrentModificationException
if it is detected that the remapping function modifies this map during computation.- Specified by:
merge
in interfaceMap<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 remapping 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
- Throws:
ConcurrentModificationException
- if it is detected that the remapping function modified this map
-
forEach
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. -
replaceAll
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 (optional operation). Exceptions thrown by the function are relayed to the caller.- Specified by:
replaceAll
in interfaceMap<K,
V> - Parameters:
function
- the function to apply to each entry
-
clone
Returns a shallow copy of thisHashMap
instance: the keys and values themselves are not cloned.- Overrides:
clone
in classAbstractMap<K,
V> - Returns:
- a shallow copy of this map
- See Also:
-
newHashMap
Creates a new, empty HashMap suitable for the expected number of mappings. The returned map uses the default load factor of 0.75, and its initial capacity is generally large enough so that the expected number of mappings can be added without resizing the map.- Type Parameters:
K
- the type of keys maintained by the new mapV
- the type of mapped values- Parameters:
numMappings
- the expected number of mappings- Returns:
- the newly created map
- Throws:
IllegalArgumentException
- if numMappings is negative- Since:
- 19
-