K - the type of keys maintained by this mapV - the type of mapped valuespublic interface Map<K,V>
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 order; others, like the HashMap
class, do not.
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.
The Map.of() and
Map.ofEntries()
static factory methods provide a convenient way to create immutable maps.
The Map
instances created by these methods have the following characteristics:
UnsupportedOperationException.
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.
null keys and values. Attempts to create them with
null keys or values result in NullPointerException.
IllegalArgumentException.
==),
identity hash code, and synchronization) are unreliable and should be avoided.
This interface is a member of the Java Collections Framework.
| Modifier and Type | Interface and Description |
|---|---|
static interface |
Map.Entry<K,V>
A map entry (key-value pair).
|
| Modifier and Type | Method and Description |
|---|---|
void |
clear()
Removes all of the mappings from this map (optional operation).
|
default 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). |
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
to
null), attempts to compute its value using the given mapping
function and enters it into this map unless null. |
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)
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. |
static <K,V> Map.Entry<K,V> |
entry(K k,
V v)
Returns an immutable
Map.Entry containing the given key and value. |
Set<Map.Entry<K,V>> |
entrySet()
Returns a
Set view of the mappings contained in this map. |
boolean |
equals(Object o)
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.
|
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. |
default 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. |
int |
hashCode()
Returns the hash code value for this map.
|
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. |
default 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.
|
static <K,V> Map<K,V> |
of()
Returns an immutable map containing zero mappings.
|
static <K,V> Map<K,V> |
of(K k1,
V v1)
Returns an immutable map containing a single mapping.
|
static <K,V> Map<K,V> |
of(K k1,
V v1,
K k2,
V v2)
Returns an immutable 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 immutable 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 immutable 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 immutable 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 immutable 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 immutable 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 immutable 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 immutable 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 immutable map containing ten mappings.
|
static <K,V> Map<K,V> |
ofEntries(Map.Entry<? extends K,? extends V>... entries)
Returns an immutable map containing keys and values extracted from the given entries.
|
V |
put(K key,
V value)
Associates the specified value with the specified key in this map
(optional operation).
|
void |
putAll(Map<? extends K,? extends V> m)
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
to
null) associates it with the given value and returns
null, else returns the current value. |
V |
remove(Object key)
Removes the mapping for a key from this map if it is present
(optional operation).
|
default boolean |
remove(Object key,
Object value)
Removes the entry for the specified key only if it is currently
mapped to the specified value.
|
default V |
replace(K key,
V value)
Replaces the entry for the specified key only if it is
currently mapped to some value.
|
default boolean |
replace(K key,
V oldValue,
V newValue)
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.
|
Collection<V> |
values()
Returns a
Collection view of the values contained in this map. |
int size()
Integer.MAX_VALUE elements, returns
Integer.MAX_VALUE.boolean isEmpty()
true if this map contains no key-value mappings.true if this map contains no key-value mappingsboolean containsKey(Object key)
true if this map contains a mapping for the specified
key. More formally, returns true if and only if
this map contains a mapping for a key k such that
Objects.equals(key, k). (There can be
at most one such mapping.)key - key whose presence in this map is to be testedtrue if this map contains a mapping for the specified
keyClassCastException - 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)boolean containsValue(Object value)
true if this map maps one or more keys to the
specified value. More formally, returns true if and only if
this map contains at least one mapping to a value v such that
Objects.equals(value, v). This operation
will probably require time linear in the map size for most
implementations of the Map interface.value - value whose presence in this map is to be testedtrue if this map maps one or more keys to the
specified valueClassCastException - 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)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
Objects.equals(key, k),
then this method returns v; otherwise
it returns null. (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 to null. The containsKey operation may be used to distinguish these two cases.
key - the key whose associated value is to be returnednull if this map contains no mapping for the keyClassCastException - 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)V put(K key, V value)
m is said to contain a mapping for a key k if and only
if m.containsKey(k) would return
true.)key - key with which the specified value is to be associatedvalue - value to be associated with the specified keykey, or
null if there was no mapping for key.
(A null return can also indicate that the map
previously associated null with key,
if the implementation supports null values.)UnsupportedOperationException - if the put 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 mapV remove(Object key)
k to value v such that
Objects.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 to null.
The map will not contain a mapping for the specified key once the call returns.
key - key whose mapping is to be removed from the mapkey, or
null if there was no mapping for key.UnsupportedOperationException - if the remove 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)void putAll(Map<? extends K,? extends V> m)
put(k, v) on this map once
for each mapping from key k to value v in the
specified map. The behavior of this operation is undefined if the
specified map is modified while the operation is in progress.m - mappings to be stored in this mapUnsupportedOperationException - if the putAll 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 mapvoid clear()
UnsupportedOperationException - if the clear operation
is not supported by this mapSet<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.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.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.boolean equals(Object o)
true if the given object is also a map and the two maps
represent the same mappings. More formally, two maps m1 and
m2 represent the same mappings if
m1.entrySet().equals(m2.entrySet()). This ensures that the
equals method works properly across different implementations
of the Map interface.equals in class Objecto - object to be compared for equality with this maptrue if the specified object is equal to this mapObject.hashCode(),
HashMapint hashCode()
entrySet() view. This ensures that m1.equals(m2)
implies that m1.hashCode()==m2.hashCode() for any two maps
m1 and m2, as required by the general contract of
Object.hashCode().hashCode in class ObjectMap.Entry.hashCode(),
Object.equals(Object),
equals(Object)default V getOrDefault(Object key, V defaultValue)
defaultValue if this map contains no mapping for the key.key - the key whose associated value is to be returneddefaultValue - the default mapping of the keydefaultValue if this map contains no mapping for the keyClassCastException - 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)default void forEach(BiConsumer<? super K,? super V> action)
map:
for (Map.Entry<K, V> entry : map.entrySet())
action.accept(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.action - The action to be performed for each entryNullPointerException - if the specified action is nullConcurrentModificationException - if an entry is found to be
removed during iterationdefault void replaceAll(BiFunction<? super K,? super V,? extends V> function)
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.
function - the function to apply to each entryUnsupportedOperationException - if the set 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 mapNullPointerException - if the specified function is null, or the
specified replacement value is null, and this map does not permit null
valuesClassCastException - if a replacement value is of an inappropriate
type for this map
(optional)NullPointerException - if function or a replacement value is null,
and this map does not permit null keys or 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 iterationdefault V putIfAbsent(K key, V value)
null) associates it with the given value and returns
null, else returns the current value.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.
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.)UnsupportedOperationException - if the put 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)default boolean remove(Object key, Object value)
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.
key - key with which the specified value is associatedvalue - value expected to be associated with the specified keytrue if the value was removedUnsupportedOperationException - if the remove 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)default boolean replace(K key, V oldValue, V newValue)
map:
if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
map.put(key, newValue);
return true;
} else
return false;
The default implementation does not throw NullPointerException
for maps that do not support null values if oldValue is null unless
newValue is also 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.
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 keytrue if the value was replacedUnsupportedOperationException - if the put 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 mapdefault V replace(K key, V value)
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.
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.)UnsupportedOperationException - if the put 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 mapdefault V computeIfAbsent(K key, Function<? super K,? extends V> mappingFunction)
null), attempts to compute its value using the given mapping
function and enters it into this map unless null.
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.
map, then returning the current value or null 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 an IllegalStateException 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.
key - key with which the specified value is to be associatedmappingFunction - the mapping function to compute a valueNullPointerException - if the specified key is null and
this map does not support null keys, or the mappingFunction
is nullUnsupportedOperationException - if the put 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)default V computeIfPresent(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)
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.
map, then returning the current value or
null 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 an IllegalStateException 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.
key - key with which the specified value is to be associatedremappingFunction - the remapping function to compute a valueNullPointerException - if the specified key is null and
this map does not support null keys, or the
remappingFunction is nullUnsupportedOperationException - if the put 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)default V compute(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)
null 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 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.
map, then returning the current value or
null if absent:
V oldValue = map.get(key);
V newValue = remappingFunction.apply(key, oldValue);
if (oldValue != null ) {
if (newValue != null)
map.put(key, newValue);
else
map.remove(key);
} else {
if (newValue != null)
map.put(key, newValue);
else
return null;
}
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 an IllegalStateException 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.
key - key with which the specified value is to be associatedremappingFunction - the remapping function to compute a valueNullPointerException - if the specified key is null and
this map does not support null keys, or the
remappingFunction is nullUnsupportedOperationException - if the put 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)default V merge(K key, V value, BiFunction<? super V,? super V,? extends V> remappingFunction)
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 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.
map, then returning the current value or
null 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 an IllegalStateException 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.
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
presentUnsupportedOperationException - if the put 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 is null and this map
does not support null keys or the value or remappingFunction is
nullstatic <K,V> Map<K,V> of()
K - the Map's key typeV - the Map's value typeMapstatic <K,V> Map<K,V> of(K k1, V v1)
K - the Map's key typeV - the Map's value typek1 - the mapping's keyv1 - the mapping's valueMap containing the specified mappingNullPointerException - if the key or the value is nullstatic <K,V> Map<K,V> of(K k1, V v1, K k2, V v2)
K - the Map's key typeV - the Map's value typek1 - the first mapping's keyv1 - the first mapping's valuek2 - the second mapping's keyv2 - the second mapping's valueMap containing the specified mappingsIllegalArgumentException - if the keys are duplicatesNullPointerException - if any key or value is nullstatic <K,V> Map<K,V> of(K k1, V v1, K k2, V v2, K k3, V v3)
K - the Map's key typeV - the Map's value typek1 - 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 valueMap containing the specified mappingsIllegalArgumentException - if there are any duplicate keysNullPointerException - if any key or value is nullstatic <K,V> Map<K,V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4)
K - the Map's key typeV - the Map's value typek1 - 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 valueMap containing the specified mappingsIllegalArgumentException - if there are any duplicate keysNullPointerException - if any key or value is nullstatic <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 - the Map's key typeV - the Map's value typek1 - 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 valueMap containing the specified mappingsIllegalArgumentException - if there are any duplicate keysNullPointerException - if any key or value is nullstatic <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 - the Map's key typeV - the Map's value typek1 - 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 valueMap containing the specified mappingsIllegalArgumentException - if there are any duplicate keysNullPointerException - if any key or value is nullstatic <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 - the Map's key typeV - the Map's value typek1 - 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 valueMap containing the specified mappingsIllegalArgumentException - if there are any duplicate keysNullPointerException - if any key or value is nullstatic <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 - the Map's key typeV - the Map's value typek1 - 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 valueMap containing the specified mappingsIllegalArgumentException - if there are any duplicate keysNullPointerException - if any key or value is nullstatic <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 - the Map's key typeV - the Map's value typek1 - 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 valueMap containing the specified mappingsIllegalArgumentException - if there are any duplicate keysNullPointerException - if any key or value is nullstatic <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)
K - the Map's key typeV - the Map's value typek1 - 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 valueMap containing the specified mappingsIllegalArgumentException - if there are any duplicate keysNullPointerException - if any key or value is null@SafeVarargs static <K,V> Map<K,V> ofEntries(Map.Entry<? extends K,? extends V>... entries)
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"));
K - the Map's key typeV - the Map's value typeentries - Map.Entrys containing the keys and values from which the map is populatedMap containing the specified mappingsIllegalArgumentException - if there are any duplicate keysNullPointerException - if any entry, key, or value is null, or if
the entries array is nullMap.entry()static <K,V> Map.Entry<K,V> entry(K k, V v)
Map.Entry containing the given key and value.
These entries are suitable for populating Map instances using the
Map.ofEntries() method.
The Entry instances created by this method have the following characteristics:
null keys and values. Attempts to create them using a null
key or value result in NullPointerException.
Entry.setValue()
on a returned Entry result in UnsupportedOperationException.
==),
identity hash code, and synchronization) are unreliable and should be avoided.
Entry, see AbstractMap.SimpleEntry or
AbstractMap.SimpleImmutableEntry.K - the key's typeV - the value's typek - the keyv - the valueEntry containing the specified key and valueNullPointerException - if the key or value is nullMap.ofEntries() 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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DRAFT 9-internal+0-2016-01-26-133437.ivan.openjdk9onspinwait