- Type Parameters:
K
- the type of keys maintained by this mapV
- the type of mapped values
- All Known Subinterfaces:
Bindings
,ConcurrentMap<K,
,V> ConcurrentNavigableMap<K,
,V> NavigableMap<K,
,V> SequencedMap<K,
,V> SortedMap<K,
V>
- All Known Implementing Classes:
AbstractMap
,Attributes
,AuthProvider
,ConcurrentHashMap
,ConcurrentSkipListMap
,EnumMap
,HashMap
,Hashtable
,Headers
,IdentityHashMap
,LinkedHashMap
,PrinterStateReasons
,Properties
,Provider
,RenderingHints
,SimpleBindings
,TabularDataSupport
,TreeMap
,UIDefaults
,WeakHashMap
This interface takes the place of the Dictionary
class, which
was a totally abstract class rather than an interface.
The Map
interface provides three collection views, which
allow a map's contents to be viewed as a set of keys, collection of values,
or set of key-value mappings. The order of a map is defined as
the order in which the iterators on the map's collection views return their
elements. Some map implementations, like the TreeMap
class, make
specific guarantees as to their encounter order; others, like the
HashMap
class, do not. Maps with a defined
encounter order
are generally subtypes of the SequencedMap
interface.
Note: great care must be exercised if mutable objects are used as map
keys. The behavior of a map is not specified if the value of an object is
changed in a manner that affects equals
comparisons while the
object is a key in the map. A special case of this prohibition is that it
is not permissible for a map to contain itself as a key. While it is
permissible for a map to contain itself as a value, extreme caution is
advised: the equals
and hashCode
methods are no longer
well defined on such a map.
All general-purpose map implementation classes should provide two
"standard" constructors: a void (no arguments) constructor which creates an
empty map, and a constructor with a single argument of type Map
,
which creates a new map with the same key-value mappings as its argument.
In effect, the latter constructor allows the user to copy any map,
producing an equivalent map of the desired class. There is no way to
enforce this recommendation (as interfaces cannot contain constructors) but
all of the general-purpose map implementations in the JDK comply.
The "destructive" methods contained in this interface, that is, the
methods that modify the map on which they operate, are specified to throw
UnsupportedOperationException
if this map does not support the
operation. If this is the case, these methods may, but are not required
to, throw an UnsupportedOperationException
if the invocation would
have no effect on the map. For example, invoking the putAll(Map)
method on an unmodifiable map may, but is not required to, throw the
exception if the map whose mappings are to be "superimposed" is empty.
Some map implementations have restrictions on the keys and values they
may contain. For example, some implementations prohibit null keys and
values, and some have restrictions on the types of their keys. Attempting
to insert an ineligible key or value throws an unchecked exception,
typically NullPointerException
or ClassCastException
.
Attempting to query the presence of an ineligible key or value may throw an
exception, or it may simply return false; some implementations will exhibit
the former behavior and some will exhibit the latter. More generally,
attempting an operation on an ineligible key or value whose completion
would not result in the insertion of an ineligible element into the map may
throw an exception or it may succeed, at the option of the implementation.
Such exceptions are marked as "optional" in the specification for this
interface.
Many methods in Collections Framework interfaces are defined
in terms of the equals
method. For
example, the specification for the containsKey(Object key)
method says: "returns true
if and
only if this map contains a mapping for a key k
such that
(key==null ? k==null : key.equals(k))
." This specification should
not be construed to imply that invoking Map.containsKey
with a non-null argument key
will cause key.equals(k)
to
be invoked for any key k
. Implementations are free to
implement optimizations whereby the equals
invocation is avoided,
for example, by first comparing the hash codes of the two keys. (The
Object.hashCode()
specification guarantees that two objects with
unequal hash codes cannot be equal.) More generally, implementations of
the various Collections Framework interfaces are free to take advantage of
the specified behavior of underlying Object
methods wherever the
implementor deems it appropriate.
Some map operations which perform recursive traversal of the map may fail
with an exception for self-referential instances where the map directly or
indirectly contains itself. This includes the clone()
,
equals()
, hashCode()
and toString()
methods.
Implementations may optionally handle the self-referential scenario, however
most current implementations do not do so.
Unmodifiable Maps
The Map.of
,
Map.ofEntries
, and
Map.copyOf
static factory methods provide a convenient way to create unmodifiable maps.
The Map
instances created by these methods have the following characteristics:
- They are unmodifiable. Keys and values
cannot be added, removed, or updated. Calling any mutator method on the Map
will always cause
UnsupportedOperationException
to be thrown. However, if the contained keys or values are themselves mutable, this may cause the Map to behave inconsistently or its contents to appear to change. - They disallow
null
keys and values. Attempts to create them withnull
keys or values result inNullPointerException
. - They are serializable if all keys and values are serializable.
- They reject duplicate keys at creation time. Duplicate keys
passed to a static factory method result in
IllegalArgumentException
. - The iteration order of mappings is unspecified and is subject to change.
- They are value-based. Programmers should treat instances that are equal as interchangeable and should not use them for synchronization, or unpredictable behavior may occur. For example, in a future release, synchronization may fail. Callers should make no assumptions about the identity of the returned instances. Factories are free to create new instances or reuse existing ones.
- They are serialized as specified on the Serialized Form page.
This interface is a member of the Java Collections Framework.
- Since:
- 1.2
- See Also:
-
Nested Class Summary
-
Method Summary
Modifier and TypeMethodDescriptionvoid
clear()
Removes all of the mappings from this map (optional operation).default V
Attempts to compute a mapping for the specified key and its current mapped value (ornull
if there is no current mapping).default V
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
.default 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) 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.static <K,
V> Map <K, V> Returns an unmodifiable Map containing the entries of the given Map.static <K,
V> Map.Entry <K, V> entry
(K k, V v) Returns an unmodifiableMap.Entry
containing the given key and value.entrySet()
Returns aSet
view of the mappings contained in this map.boolean
Compares the specified object with this map for equality.default 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.default V
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.int
hashCode()
Returns the hash code value for this map.boolean
isEmpty()
Returnstrue
if this map contains no key-value mappings.keySet()
Returns aSet
view of the keys contained in this map.default V
If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value.static <K,
V> Map <K, V> of()
Returns an unmodifiable map containing zero mappings.static <K,
V> Map <K, V> of
(K k1, V v1) Returns an unmodifiable map containing a single mapping.static <K,
V> Map <K, V> of
(K k1, V v1, K k2, V v2) Returns an unmodifiable map containing two mappings.static <K,
V> Map <K, V> of
(K k1, V v1, K k2, V v2, K k3, V v3) Returns an unmodifiable map containing three mappings.static <K,
V> Map <K, V> of
(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4) Returns an unmodifiable map containing four mappings.static <K,
V> Map <K, V> of
(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5) Returns an unmodifiable map containing five mappings.static <K,
V> Map <K, V> of
(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6) Returns an unmodifiable map containing six mappings.static <K,
V> Map <K, V> of
(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7) Returns an unmodifiable map containing seven mappings.static <K,
V> Map <K, V> of
(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7, K k8, V v8) Returns an unmodifiable map containing eight mappings.static <K,
V> Map <K, V> of
(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7, K k8, V v8, K k9, V v9) Returns an unmodifiable map containing nine mappings.static <K,
V> Map <K, V> of
(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7, K k8, V v8, K k9, V v9, K k10, V v10) Returns an unmodifiable map containing ten mappings.static <K,
V> Map <K, V> Returns an unmodifiable map containing keys and values extracted from the given entries.Associates the specified value with the specified key in this map (optional operation).void
Copies all of the mappings from the specified map to this map (optional operation).default V
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.Removes the mapping for a key from this map if it is present (optional operation).default boolean
Removes the entry for the specified key only if it is currently mapped to the specified value.default V
Replaces the entry for the specified key only if it is currently mapped to some value.default boolean
Replaces the entry for the specified key only if currently mapped to the specified value.default 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.values()
Returns aCollection
view of the values contained in this map.
-
Method Details
-
size
int size()Returns the number of key-value mappings in this map. If the map contains more thanInteger.MAX_VALUE
elements, returnsInteger.MAX_VALUE
.- Returns:
- the number of key-value mappings in this map
-
isEmpty
boolean isEmpty()Returnstrue
if this map contains no key-value mappings.- Returns:
true
if this map contains no key-value mappings
-
containsKey
Returnstrue
if this map contains a mapping for the specified key. More formally, returnstrue
if and only if this map contains a mapping for a keyk
such thatObjects.equals(key, k)
. (There can be at most one such mapping.)- Parameters:
key
- key whose presence in this map is to be tested- Returns:
true
if this map contains a mapping for the specified key- Throws:
ClassCastException
- if the key is of an inappropriate type for this map (optional)NullPointerException
- if the specified key is null and this map does not permit null keys (optional)
-
containsValue
Returnstrue
if this map maps one or more keys to the specified value. More formally, returnstrue
if and only if this map contains at least one mapping to a valuev
such thatObjects.equals(value, v)
. This operation will probably require time linear in the map size for most implementations of theMap
interface.- 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- Throws:
ClassCastException
- if the value is of an inappropriate type for this map (optional)NullPointerException
- if the specified value is null and this map does not permit null values (optional)
-
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 thatObjects.equals(key, k)
, then this method returnsv
; otherwise it returnsnull
. (There can be at most one such mapping.)If this map permits null values, then 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.- 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 - Throws:
ClassCastException
- if the key is of an inappropriate type for this map (optional)NullPointerException
- if the specified key is null and this map does not permit null keys (optional)
-
put
Associates the specified value with the specified key in this map (optional operation). If the map previously contained a mapping for the key, the old value is replaced by the specified value. (A mapm
is said to contain a mapping for a keyk
if and only ifm.containsKey(k)
would returntrue
.)- 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
, if the implementation supportsnull
values.) - Throws:
UnsupportedOperationException
- if theput
operation is not supported by this mapClassCastException
- if the class of the specified key or value prevents it from being stored in this mapNullPointerException
- if the specified key or value is null and this map does not permit null keys or valuesIllegalArgumentException
- if some property of the specified key or value prevents it from being stored in this map
-
remove
Removes the mapping for a key from this map if it is present (optional operation). More formally, if this map contains a mapping from keyk
to valuev
such thatObjects.equals(key, k)
, that mapping is removed. (The map can contain at most one such mapping.)Returns the value to which this map previously associated the key, or
null
if the map contained no mapping for the key.If this map permits null values, then a return value of
null
does not necessarily indicate that the map contained no mapping for the key; it's also possible that the map explicitly mapped the key tonull
.The map will not contain a mapping for the specified key once the call returns.
- 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
. - Throws:
UnsupportedOperationException
- if theremove
operation is not supported by this mapClassCastException
- if the key is of an inappropriate type for this map (optional)NullPointerException
- if the specified key is null and this map does not permit null keys (optional)
-
putAll
Copies all of the mappings from the specified map to this map (optional operation). The effect of this call is equivalent to that of callingput(k, v)
on this map once for each mapping from keyk
to valuev
in the specified map. The behavior of this operation is undefined if the specified map is modified while the operation is in progress. If the specified map has a defined encounter order, processing of its mappings generally occurs in that order.- Parameters:
m
- mappings to be stored in this map- Throws:
UnsupportedOperationException
- if theputAll
operation is not supported by this mapClassCastException
- if the class of a key or value in the specified map prevents it from being stored in this mapNullPointerException
- if the specified map is null, or if this map does not permit null keys or values, and the specified map contains null keys or valuesIllegalArgumentException
- if some property of a key or value in the specified map prevents it from being stored in this map
-
clear
void clear()Removes all of the mappings from this map (optional operation). The map will be empty after this call returns.- Throws:
UnsupportedOperationException
- if theclear
operation is not supported by this map
-
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.- Returns:
- a set view of the keys contained in this map
-
values
Collection<V> 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.- Returns:
- a collection view of the values contained in this map
-
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.- Returns:
- a set view of the mappings contained in this map
-
equals
Compares the specified object with this map for equality. Returnstrue
if the given object is also a map and the two maps represent the same mappings. More formally, two mapsm1
andm2
represent the same mappings ifm1.entrySet().equals(m2.entrySet())
. This ensures that theequals
method works properly across different implementations of theMap
interface. -
hashCode
int hashCode()Returns the hash code value for this map. The hash code of a map is defined to be the sum of the hash codes of each entry in the map'sentrySet()
view. This ensures thatm1.equals(m2)
implies thatm1.hashCode()==m2.hashCode()
for any two mapsm1
andm2
, as required by the general contract ofObject.hashCode()
. -
getOrDefault
Returns the value to which the specified key is mapped, ordefaultValue
if this map contains no mapping for the key.- Implementation Requirements:
- The default implementation makes no guarantees about synchronization or atomicity properties of this method. Any implementation providing atomicity guarantees must override this method and document its concurrency properties.
- 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 - Throws:
ClassCastException
- if the key is of an inappropriate type for this map (optional)NullPointerException
- if the specified key is null and this map does not permit null keys (optional)- Since:
- 1.8
-
forEach
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.- Implementation Requirements:
- The default implementation is equivalent to, for this
map
:for (Map.Entry<K, V> entry : map.entrySet()) action.accept(entry.getKey(), entry.getValue());
- Parameters:
action
- The action to be performed for each entry- Throws:
NullPointerException
- if the specified action is nullConcurrentModificationException
- if an entry is found to be removed during iteration- Since:
- 1.8
-
replaceAll
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.- Implementation Requirements:
The default implementation is equivalent to, for this
map
:for (Map.Entry<K, V> entry : map.entrySet()) entry.setValue(function.apply(entry.getKey(), entry.getValue()));
The default implementation makes no guarantees about synchronization or atomicity properties of this method. Any implementation providing atomicity guarantees must override this method and document its concurrency properties.
- Parameters:
function
- the function to apply to each entry- Throws:
UnsupportedOperationException
- if theset
operation is not supported by this map's entry set iterator.ClassCastException
- if the class of a replacement value prevents it from being stored in this map (optional)NullPointerException
- if the specified function is null, or if a replacement value is null and this map does not permit null values (optional)IllegalArgumentException
- if some property of a replacement value prevents it from being stored in this map (optional)ConcurrentModificationException
- if an entry is found to be removed during iteration- Since:
- 1.8
-
putIfAbsent
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.- Implementation Requirements:
- The default implementation is equivalent to, for this
map
:V v = map.get(key); if (v == null) v = map.put(key, value); return v;
The default implementation makes no guarantees about synchronization or atomicity properties of this method. Any implementation providing atomicity guarantees must override this method and document its concurrency properties.
- 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.) - Throws:
UnsupportedOperationException
- if theput
operation is not supported by this map (optional)ClassCastException
- if the key or value is of an inappropriate type for this map (optional)NullPointerException
- if the specified key or value is null, and this map does not permit null keys or values (optional)IllegalArgumentException
- if some property of the specified key or value prevents it from being stored in this map (optional)- Since:
- 1.8
-
remove
Removes the entry for the specified key only if it is currently mapped to the specified value.- Implementation Requirements:
- The default implementation is equivalent to, for this
map
:if (map.containsKey(key) && Objects.equals(map.get(key), value)) { map.remove(key); return true; } else return false;
The default implementation makes no guarantees about synchronization or atomicity properties of this method. Any implementation providing atomicity guarantees must override this method and document its concurrency properties.
- 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- Throws:
UnsupportedOperationException
- if theremove
operation is not supported by this map (optional)ClassCastException
- if the key or value is of an inappropriate type for this map (optional)NullPointerException
- if the specified key or value is null, and this map does not permit null keys or values (optional)- Since:
- 1.8
-
replace
Replaces the entry for the specified key only if currently mapped to the specified value.- Implementation Requirements:
- The default implementation is equivalent to, for this
map
:if (map.containsKey(key) && Objects.equals(map.get(key), oldValue)) { map.put(key, newValue); return true; } else return false;
The default implementation makes no guarantees about synchronization or atomicity properties of this method. Any implementation providing atomicity guarantees must override this method and document its concurrency properties.
- 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- Throws:
UnsupportedOperationException
- if theput
operation is not supported by this map (optional)ClassCastException
- if the class of a specified key or value prevents it from being stored in this mapNullPointerException
- if a specified key or newValue is null, and this map does not permit null keys or valuesNullPointerException
- if oldValue is null and this map does not permit null values (optional)IllegalArgumentException
- if some property of a specified key or value prevents it from being stored in this map- Since:
- 1.8
-
replace
Replaces the entry for the specified key only if it is currently mapped to some value.- Implementation Requirements:
- The default implementation is equivalent to, for this
map
:if (map.containsKey(key)) { return map.put(key, value); } else return null;
The default implementation makes no guarantees about synchronization or atomicity properties of this method. Any implementation providing atomicity guarantees must override this method and document its concurrency properties.
- 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.) - Throws:
UnsupportedOperationException
- if theput
operation is not supported by this map (optional)ClassCastException
- if the class of the specified key or value prevents it from being stored in this map (optional)NullPointerException
- if the specified key or value is null, and this map does not permit null keys or valuesIllegalArgumentException
- if some property of the specified key or value prevents it from being stored in this map- Since:
- 1.8
-
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
.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.
- Implementation Requirements:
- The default implementation is equivalent to the following steps for this
map
, then returning the current value ornull
if now absent:if (map.get(key) == null) { V newValue = mappingFunction.apply(key); if (newValue != null) map.put(key, newValue); }
The default implementation makes no guarantees about detecting if the mapping function modifies this map during computation and, if appropriate, reporting an error. Non-concurrent implementations should override this method and, on a best-effort basis, throw a
ConcurrentModificationException
if it is detected that the mapping function modifies this map during computation. Concurrent implementations should override this method and, on a best-effort basis, throw anIllegalStateException
if it is detected that the mapping function modifies this map during computation and as a result computation would never complete.The default implementation makes no guarantees about synchronization or atomicity properties of this method. Any implementation providing atomicity guarantees must override this method and document its concurrency properties. In particular, all implementations of subinterface
ConcurrentMap
must document whether the mapping function is applied once atomically only if the value is not present. - 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:
NullPointerException
- if the specified key is null and this map does not support null keys, or the mappingFunction is nullUnsupportedOperationException
- if theput
operation is not supported by this map (optional)ClassCastException
- if the class of the specified key or value prevents it from being stored in this map (optional)IllegalArgumentException
- if some property of the specified key or value prevents it from being stored in this map (optional)- Since:
- 1.8
-
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.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.
- Implementation Requirements:
- The default implementation is equivalent to performing the following
steps for this
map
, then returning the current value ornull
if now absent:if (map.get(key) != null) { V oldValue = map.get(key); V newValue = remappingFunction.apply(key, oldValue); if (newValue != null) map.put(key, newValue); else map.remove(key); }
The default implementation makes no guarantees about detecting if the remapping function modifies this map during computation and, if appropriate, reporting an error. Non-concurrent implementations should override this method and, on a best-effort basis, throw a
ConcurrentModificationException
if it is detected that the remapping function modifies this map during computation. Concurrent implementations should override this method and, on a best-effort basis, throw anIllegalStateException
if it is detected that the remapping function modifies this map during computation and as a result computation would never complete.The default implementation makes no guarantees about synchronization or atomicity properties of this method. Any implementation providing atomicity guarantees must override this method and document its concurrency properties. In particular, all implementations of subinterface
ConcurrentMap
must document whether the remapping function is applied once atomically only if the value is not present. - 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:
NullPointerException
- if the specified key is null and this map does not support null keys, or the remappingFunction is nullUnsupportedOperationException
- if theput
operation is not supported by this map (optional)ClassCastException
- if the class of the specified key or value prevents it from being stored in this map (optional)IllegalArgumentException
- if some property of the specified key or value prevents it from being stored in this map (optional)- Since:
- 1.8
-
compute
Attempts to compute a mapping for the specified key and its current mapped value (ornull
if there is no current mapping). 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.
- Implementation Requirements:
- The default implementation is equivalent to performing the following
steps for this
map
:V oldValue = map.get(key); V newValue = remappingFunction.apply(key, oldValue); if (newValue != null) { map.put(key, newValue); } else if (oldValue != null || map.containsKey(key)) { map.remove(key); } return newValue;
The default implementation makes no guarantees about detecting if the remapping function modifies this map during computation and, if appropriate, reporting an error. Non-concurrent implementations should override this method and, on a best-effort basis, throw a
ConcurrentModificationException
if it is detected that the remapping function modifies this map during computation. Concurrent implementations should override this method and, on a best-effort basis, throw anIllegalStateException
if it is detected that the remapping function modifies this map during computation and as a result computation would never complete.The default implementation makes no guarantees about synchronization or atomicity properties of this method. Any implementation providing atomicity guarantees must override this method and document its concurrency properties. In particular, all implementations of subinterface
ConcurrentMap
must document whether the remapping function is applied once atomically only if the value is not present. - 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:
NullPointerException
- if the specified key is null and this map does not support null keys, or the remappingFunction is nullUnsupportedOperationException
- if theput
operation is not supported by this map (optional)ClassCastException
- if the class of the specified key or value prevents it from being stored in this map (optional)IllegalArgumentException
- if some property of the specified key or value prevents it from being stored in this map (optional)- Since:
- 1.8
-
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. 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.
- Implementation Requirements:
- The default implementation is equivalent to performing the following
steps for this
map
, then returning the current value ornull
if absent:V oldValue = map.get(key); V newValue = (oldValue == null) ? value : remappingFunction.apply(oldValue, value); if (newValue == null) map.remove(key); else map.put(key, newValue);
The default implementation makes no guarantees about detecting if the remapping function modifies this map during computation and, if appropriate, reporting an error. Non-concurrent implementations should override this method and, on a best-effort basis, throw a
ConcurrentModificationException
if it is detected that the remapping function modifies this map during computation. Concurrent implementations should override this method and, on a best-effort basis, throw anIllegalStateException
if it is detected that the remapping function modifies this map during computation and as a result computation would never complete.The default implementation makes no guarantees about synchronization or atomicity properties of this method. Any implementation providing atomicity guarantees must override this method and document its concurrency properties. In particular, all implementations of subinterface
ConcurrentMap
must document whether the remapping function is applied once atomically only if the value is not present. - 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:
UnsupportedOperationException
- if theput
operation is not supported by this map (optional)ClassCastException
- if the class of the specified key or value prevents it from being stored in this map (optional)IllegalArgumentException
- if some property of the specified key or value prevents it from being stored in this map (optional)NullPointerException
- if the specified key is null and this map does not support null keys or the value or remappingFunction is null- Since:
- 1.8
-
of
Returns an unmodifiable map containing zero mappings. See Unmodifiable Maps for details.- Type Parameters:
K
- theMap
's key typeV
- theMap
's value type- Returns:
- an empty
Map
- Since:
- 9
-
of
Returns an unmodifiable map containing a single mapping. See Unmodifiable Maps for details.- Type Parameters:
K
- theMap
's key typeV
- theMap
's value type- Parameters:
k1
- the mapping's keyv1
- the mapping's value- Returns:
- a
Map
containing the specified mapping - Throws:
NullPointerException
- if the key or the value isnull
- Since:
- 9
-
of
Returns an unmodifiable map containing two mappings. See Unmodifiable Maps for details.- Type Parameters:
K
- theMap
's key typeV
- theMap
's value type- Parameters:
k1
- the first mapping's keyv1
- the first mapping's valuek2
- the second mapping's keyv2
- the second mapping's value- Returns:
- a
Map
containing the specified mappings - Throws:
IllegalArgumentException
- if the keys are duplicatesNullPointerException
- if any key or value isnull
- Since:
- 9
-
of
Returns an unmodifiable map containing three mappings. See Unmodifiable Maps for details.- Type Parameters:
K
- theMap
's key typeV
- theMap
's value type- Parameters:
k1
- the first mapping's keyv1
- the first mapping's valuek2
- the second mapping's keyv2
- the second mapping's valuek3
- the third mapping's keyv3
- the third mapping's value- Returns:
- a
Map
containing the specified mappings - Throws:
IllegalArgumentException
- if there are any duplicate keysNullPointerException
- if any key or value isnull
- Since:
- 9
-
of
Returns an unmodifiable map containing four mappings. See Unmodifiable Maps for details.- Type Parameters:
K
- theMap
's key typeV
- theMap
's value type- Parameters:
k1
- the first mapping's keyv1
- the first mapping's valuek2
- the second mapping's keyv2
- the second mapping's valuek3
- the third mapping's keyv3
- the third mapping's valuek4
- the fourth mapping's keyv4
- the fourth mapping's value- Returns:
- a
Map
containing the specified mappings - Throws:
IllegalArgumentException
- if there are any duplicate keysNullPointerException
- if any key or value isnull
- Since:
- 9
-
of
Returns an unmodifiable map containing five mappings. See Unmodifiable Maps for details.- Type Parameters:
K
- theMap
's key typeV
- theMap
's value type- Parameters:
k1
- the first mapping's keyv1
- the first mapping's valuek2
- the second mapping's keyv2
- the second mapping's valuek3
- the third mapping's keyv3
- the third mapping's valuek4
- the fourth mapping's keyv4
- the fourth mapping's valuek5
- the fifth mapping's keyv5
- the fifth mapping's value- Returns:
- a
Map
containing the specified mappings - Throws:
IllegalArgumentException
- if there are any duplicate keysNullPointerException
- if any key or value isnull
- Since:
- 9
-
of
Returns an unmodifiable map containing six mappings. See Unmodifiable Maps for details.- Type Parameters:
K
- theMap
's key typeV
- theMap
's value type- Parameters:
k1
- the first mapping's keyv1
- the first mapping's valuek2
- the second mapping's keyv2
- the second mapping's valuek3
- the third mapping's keyv3
- the third mapping's valuek4
- the fourth mapping's keyv4
- the fourth mapping's valuek5
- the fifth mapping's keyv5
- the fifth mapping's valuek6
- the sixth mapping's keyv6
- the sixth mapping's value- Returns:
- a
Map
containing the specified mappings - Throws:
IllegalArgumentException
- if there are any duplicate keysNullPointerException
- if any key or value isnull
- Since:
- 9
-
of
static <K,V> Map<K,V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7) Returns an unmodifiable map containing seven mappings. See Unmodifiable Maps for details.- Type Parameters:
K
- theMap
's key typeV
- theMap
's value type- Parameters:
k1
- the first mapping's keyv1
- the first mapping's valuek2
- the second mapping's keyv2
- the second mapping's valuek3
- the third mapping's keyv3
- the third mapping's valuek4
- the fourth mapping's keyv4
- the fourth mapping's valuek5
- the fifth mapping's keyv5
- the fifth mapping's valuek6
- the sixth mapping's keyv6
- the sixth mapping's valuek7
- the seventh mapping's keyv7
- the seventh mapping's value- Returns:
- a
Map
containing the specified mappings - Throws:
IllegalArgumentException
- if there are any duplicate keysNullPointerException
- if any key or value isnull
- Since:
- 9
-
of
static <K,V> Map<K,V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7, K k8, V v8) Returns an unmodifiable map containing eight mappings. See Unmodifiable Maps for details.- Type Parameters:
K
- theMap
's key typeV
- theMap
's value type- Parameters:
k1
- the first mapping's keyv1
- the first mapping's valuek2
- the second mapping's keyv2
- the second mapping's valuek3
- the third mapping's keyv3
- the third mapping's valuek4
- the fourth mapping's keyv4
- the fourth mapping's valuek5
- the fifth mapping's keyv5
- the fifth mapping's valuek6
- the sixth mapping's keyv6
- the sixth mapping's valuek7
- the seventh mapping's keyv7
- the seventh mapping's valuek8
- the eighth mapping's keyv8
- the eighth mapping's value- Returns:
- a
Map
containing the specified mappings - Throws:
IllegalArgumentException
- if there are any duplicate keysNullPointerException
- if any key or value isnull
- Since:
- 9
-
of
static <K,V> Map<K,V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7, K k8, V v8, K k9, V v9) Returns an unmodifiable map containing nine mappings. See Unmodifiable Maps for details.- Type Parameters:
K
- theMap
's key typeV
- theMap
's value type- Parameters:
k1
- the first mapping's keyv1
- the first mapping's valuek2
- the second mapping's keyv2
- the second mapping's valuek3
- the third mapping's keyv3
- the third mapping's valuek4
- the fourth mapping's keyv4
- the fourth mapping's valuek5
- the fifth mapping's keyv5
- the fifth mapping's valuek6
- the sixth mapping's keyv6
- the sixth mapping's valuek7
- the seventh mapping's keyv7
- the seventh mapping's valuek8
- the eighth mapping's keyv8
- the eighth mapping's valuek9
- the ninth mapping's keyv9
- the ninth mapping's value- Returns:
- a
Map
containing the specified mappings - Throws:
IllegalArgumentException
- if there are any duplicate keysNullPointerException
- if any key or value isnull
- Since:
- 9
-
of
static <K,V> Map<K,V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7, K k8, V v8, K k9, V v9, K k10, V v10) Returns an unmodifiable map containing ten mappings. See Unmodifiable Maps for details.- Type Parameters:
K
- theMap
's key typeV
- theMap
's value type- Parameters:
k1
- the first mapping's keyv1
- the first mapping's valuek2
- the second mapping's keyv2
- the second mapping's valuek3
- the third mapping's keyv3
- the third mapping's valuek4
- the fourth mapping's keyv4
- the fourth mapping's valuek5
- the fifth mapping's keyv5
- the fifth mapping's valuek6
- the sixth mapping's keyv6
- the sixth mapping's valuek7
- the seventh mapping's keyv7
- the seventh mapping's valuek8
- the eighth mapping's keyv8
- the eighth mapping's valuek9
- the ninth mapping's keyv9
- the ninth mapping's valuek10
- the tenth mapping's keyv10
- the tenth mapping's value- Returns:
- a
Map
containing the specified mappings - Throws:
IllegalArgumentException
- if there are any duplicate keysNullPointerException
- if any key or value isnull
- Since:
- 9
-
ofEntries
Returns an unmodifiable map containing keys and values extracted from the given entries. The entries themselves are not stored in the map. See Unmodifiable Maps for details.- API Note:
- It is convenient to create the map entries using the
Map.entry()
method. For example,import static java.util.Map.entry; Map<Integer,String> map = Map.ofEntries( entry(1, "a"), entry(2, "b"), entry(3, "c"), ... entry(26, "z"));
- Type Parameters:
K
- theMap
's key typeV
- theMap
's value type- Parameters:
entries
-Map.Entry
s containing the keys and values from which the map is populated- Returns:
- a
Map
containing the specified mappings - Throws:
IllegalArgumentException
- if there are any duplicate keysNullPointerException
- if any entry, key, or value isnull
, or if theentries
array isnull
- Since:
- 9
- See Also:
-
entry
Returns an unmodifiableMap.Entry
containing the given key and value. These entries are suitable for populatingMap
instances using theMap.ofEntries()
method. TheEntry
instances created by this method have the following characteristics:- They disallow
null
keys and values. Attempts to create them using anull
key or value result inNullPointerException
. - They are unmodifiable. Calls to
Entry.setValue()
on a returnedEntry
result inUnsupportedOperationException
. - They are not serializable.
- They are value-based. Programmers should treat instances that are equal as interchangeable and should not use them for synchronization, or unpredictable behavior may occur. For example, in a future release, synchronization may fail. Callers should make no assumptions about the identity of the returned instances. This method is free to create new instances or reuse existing ones.
- API Note:
- For a serializable
Entry
, seeAbstractMap.SimpleEntry
orAbstractMap.SimpleImmutableEntry
. - Type Parameters:
K
- the key's typeV
- the value's type- Parameters:
k
- the keyv
- the value- Returns:
- an
Entry
containing the specified key and value - Throws:
NullPointerException
- if the key or value isnull
- Since:
- 9
- See Also:
- They disallow
-
copyOf
Returns an unmodifiable Map containing the entries of the given Map. The given Map must not be null, and it must not contain any null keys or values. If the given Map is subsequently modified, the returned Map will not reflect such modifications.- Implementation Note:
- If the given Map is an unmodifiable Map, calling copyOf will generally not create a copy.
- Type Parameters:
K
- theMap
's key typeV
- theMap
's value type- Parameters:
map
- aMap
from which entries are drawn, must be non-null- Returns:
- a
Map
containing the entries of the givenMap
- Throws:
NullPointerException
- if map is null, or if it contains any null keys or values- Since:
- 10
-