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Type Parameters:
K - the type of keys maintained by this map
V - the type of mapped values
All Known Subinterfaces:
Bindings,ConcurrentMap<K,V>,ConcurrentNavigableMap<K,V>,LogicalMessageContext,MessageContext,NavigableMap<K,V>,SOAPMessageContext,SortedMap<K,V>
All Known Implementing Classes:
AbstractMap,Attributes,AuthProvider,ConcurrentHashMap,ConcurrentSkipListMap,EnumMap,HashMap,Hashtable,IdentityHashMap,LinkedHashMap,PrinterStateReasons,Properties,Provider,RenderingHints,SimpleBindings,TabularDataSupport,TreeMap,UIDefaults,WeakHashMap
```
public interfaceMap<K,V>
```
An object that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value.
This interface takes the place of theDictionary class, which was a totally abstract class rather than an interface.
TheMap interface provides threecollection views, which allow a map's contents to be viewed as a set of keys, collection of values, or set of key-value mappings. Theorder 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 theTreeMap class, make specific guarantees as to their order; others, like theHashMap 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 affectsequals 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: theequals andhashCode 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 typeMap, 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 throwUnsupportedOperationException if this map does not support the operation. If this is the case, these methods may, but are not required to, throw anUnsupportedOperationException if the invocation would have no effect on the map. For example, invoking theputAll(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, typicallyNullPointerException orClassCastException. 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 theequals method. For example, the specification for thecontainsKey(Object key) method says: "returnstrue if and only if this map contains a mapping for a keyk such that(key==null ? k==null : key.equals(k))." This specification shouldnot be construed to imply that invokingMap.containsKey with a non-null argumentkey will causekey.equals(k) to be invoked for any keyk. Implementations are free to implement optimizations whereby theequals invocation is avoided, for example, by first comparing the hash codes of the two keys. (TheObject.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 underlyingObject 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 theclone(),equals(),hashCode() andtoString() methods. Implementations may optionally handle the self-referential scenario, however most current implementations do not do so.
This interface is a member of the Java Collections Framework.
Since:
1.2
See Also:
HashMap,TreeMap,Hashtable,SortedMap,Collection,Set

Nested Class Summary
Nested Classes
Modifier and Type Interface Description
static interface Map.Entry<K,V>
A map entry (key-value pair).

Nested Classes
Modifier and Type	Interface	Description
`static interface`	`Map.Entry<K,V>`	A map entry (key-value pair).

Method Summary

All Methods Instance Methods Abstract Methods Default Methods
Modifier and Type	Method	Description
`void`	`clear()`	Removes all of the mappings from this map (optional operation).
`defaultV`	`compute(K key,BiFunction<? superK,? superV,? extendsV> remappingFunction)`	Attempts to compute a mapping for the specified key and its current mapped value (or`null` if there is no current mapping).
`defaultV`	`computeIfAbsent(K key,Function<? superK,? extendsV> 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`.
`defaultV`	`computeIfPresent(K key,BiFunction<? superK,? superV,? extendsV> remappingFunction)`	If the value for the specified key is present and non-null, attempts to compute a new mapping given the key and its current mapped value.
`boolean`	`containsKey(Object key)`	Returns`true` if this map contains a mapping for the specified key.
`boolean`	`containsValue(Object value)`	Returns`true` if this map maps one or more keys to the specified value.
`Set<Map.Entry<K,V>>`	`entrySet()`	Returns a`Set` view of the mappings contained in this map.
`boolean`	`equals(Object o)`	Compares the specified object with this map for equality.
`default void`	`forEach(BiConsumer<? superK,? superV> 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.
`defaultV`	`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.
`defaultV`	`merge(K key,V value,BiFunction<? superV,? superV,? extendsV> remappingFunction)`	If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value.
`V`	`put(K key,V value)`	Associates the specified value with the specified key in this map (optional operation).
`void`	`putAll(Map<? extendsK,? extendsV> m)`	Copies all of the mappings from the specified map to this map (optional operation).
`defaultV`	`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.
`defaultV`	`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<? superK,? superV,? extendsV> 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.

- Method Detail
  - 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
```
boolean containsKey(Object key)
```
    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 that(key==null ? k==null : key.equals(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
```
boolean containsValue(Object value)
```
    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 that(value==null ? v==null : value.equals(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
```
V get(Object key)
```
    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 keyk 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.)
    If this map permits null values, then a return value ofnull does notnecessarily 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, ornull 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
```
V put(K key,V value)
```
    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 associated
    value - value to be associated with the specified key
    Returns:
    the previous value associated withkey, 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 map
    ClassCastException - if the class of the specified key or value prevents it from being stored in this map
    NullPointerException - if the specified key or value is null and this map does not permit null keys or values
    IllegalArgumentException - if some property of the specified key or value prevents it from being stored in this map
  - remove
```
V remove(Object key)
```
    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 that(key==null ? k==null : key.equals(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, ornull if the map contained no mapping for the key.
    If this map permits null values, then a return value ofnull does notnecessarily 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 withkey, ornull if there was no mapping forkey.
    Throws:
    UnsupportedOperationException - if theremove operation is not supported by this map
    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)
  - putAll
```
void putAll(Map<? extendsK,? extendsV> m)
```
    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.
    Parameters:
    m - mappings to be stored in this map
    Throws:
    UnsupportedOperationException - if theputAll operation is not supported by this map
    ClassCastException - if the class of a key or value in the specified map prevents it from being stored in this map
    NullPointerException - 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 values
    IllegalArgumentException - 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
```
Set<K> 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
```
Set<Map.Entry<K,V>> 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
```
boolean equals(Object o)
```
    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.
    Overrides:
    equals in class Object
    Parameters:
    o - object to be compared for equality with this map
    Returns:
    true if the specified object is equal to this map
    See Also:
    Object.hashCode(),HashMap
  - 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().
    Overrides:
    hashCode in class Object
    Returns:
    the hash code value for this map
    See Also:
    Map.Entry.hashCode(),Object.equals(Object),equals(Object)
  - getOrDefault
```
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.
    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 returned
    defaultValue - the default mapping of the key
    Returns:
    the value to which the specified key is mapped, ordefaultValue 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
```
default void forEach(BiConsumer<? superK,? superV> action)
```
    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 thismap:
    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.
    Parameters:
    action - The action to be performed for each entry
    Throws:
    NullPointerException - if the specified action is null
    ConcurrentModificationException - if an entry is found to be removed during iteration
    Since:
    1.8
  - replaceAll
```
default void replaceAll(BiFunction<? superK,? superV,? extendsV> 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. Exceptions thrown by the function are relayed to the caller.
    Implementation Requirements:
    The default implementation is equivalent to, for thismap:
    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
    NullPointerException - if the specified function is null, or the specified replacement value is null, and this map does not permit null values
    ClassCastException - 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 iteration
    Since:
    1.8
  - putIfAbsent
```
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.
    Implementation Requirements:
    The default implementation is equivalent to, for thismap:
    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 associated
    value - value to be associated with the specified key
    Returns:
    the previous value associated with the specified key, ornull 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
```
default boolean remove(Object key,Object value)
```
    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 thismap:
    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 associated
    value - 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
```
default boolean replace(K key,V oldValue,V newValue)
```
    Replaces the entry for the specified key only if currently mapped to the specified value.
    Implementation Requirements:
    The default implementation is equivalent to, for thismap:
    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.
    Parameters:
    key - key with which the specified value is associated
    oldValue - value expected to be associated with the specified key
    newValue - 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 map
    NullPointerException - if a specified key or newValue is null, and this map does not permit null keys or values
    NullPointerException - 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
```
default V replace(K key,V value)
```
    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 thismap:
    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 associated
    value - value to be associated with the specified key
    Returns:
    the previous value associated with the specified key, ornull 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 values
    IllegalArgumentException - if some property of the specified key or value prevents it from being stored in this map
    Since:
    1.8
  - computeIfAbsent
```
default V computeIfAbsent(K key,Function<? superK,? extendsV> 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.
    If the function returnsnull no mapping is recorded. If the function itself throws an (unchecked) exception, the exception is rethrown, and no mapping is recorded. The most common usage is to construct a new object serving as an initial mapped value or memoized result, as in:
```
 map.computeIfAbsent(key, k -> new Value(f(k)));
```
    Or to implement a multi-value map,Map<K,Collection<V>>, supporting multiple values per key:
```
 map.computeIfAbsent(key, k -> new HashSet<V>()).add(v);
```
    Implementation Requirements:
    The default implementation is equivalent to the following steps for thismap, 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 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 subinterfaceConcurrentMap must document whether the function is applied once atomically only if the value is not present.
    Parameters:
    key - key with which the specified value is to be associated
    mappingFunction - the 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 null
    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)
    Since:
    1.8
  - computeIfPresent
```
default V computeIfPresent(K key,BiFunction<? superK,? superV,? extendsV> 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.
    If the function returnsnull, the mapping is removed. If the function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.
    Implementation Requirements:
    The default implementation is equivalent to performing the following steps for thismap, 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 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 subinterfaceConcurrentMap must document whether the function is applied once atomically only if the value is not present.
    Parameters:
    key - key with which the specified value is to be associated
    remappingFunction - the 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 null
    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)
    Since:
    1.8
  - compute
```
default V compute(K key,BiFunction<? superK,? superV,? extendsV> remappingFunction)
```
    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))
```
    (Methodmerge() is often simpler to use for such purposes.)
    If the function returnsnull, the mapping is removed (or remains absent if initially absent). If the function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.
    Implementation Requirements:
    The default implementation is equivalent to performing the following steps for thismap, then returning the current value ornull 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 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 subinterfaceConcurrentMap must document whether the function is applied once atomically only if the value is not present.
    Parameters:
    key - key with which the specified value is to be associated
    remappingFunction - the 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 null
    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)
    Since:
    1.8
  - merge
```
default V merge(K key,V value,BiFunction<? superV,? superV,? extendsV> 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. 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 function returnsnull the mapping is removed. If the function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.
    Implementation Requirements:
    The default implementation is equivalent to performing the following steps for thismap, 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 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 subinterfaceConcurrentMap must document whether the function is applied once atomically only if the value is not present.
    Parameters:
    key - key with which the resulting value is to be associated
    value - 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 key
    remappingFunction - the function to recompute a value if present
    Returns:
    the new value associated with the specified key, or null if no value is associated with the key
    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 is null and this map does not support null keys or the value or remappingFunction is null
    Since:
    1.8

Movatterモバイル変換

Interface Map<K,V>

Nested Class Summary

Method Summary

Method Detail

size

isEmpty

containsKey

containsValue

get

put

remove

putAll

clear

keySet

values

entrySet

equals

hashCode

getOrDefault

forEach

replaceAll

putIfAbsent

remove

replace

replace

computeIfAbsent

computeIfPresent

compute

merge