Interface Map<K,V>

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>,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
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 encounter order; others, like theHashMap class, do not. Maps with a definedencounter order are generally subtypes of theSequencedMap 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 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.

Unmodifiable Maps

TheMap.of,Map.ofEntries, andMap.copyOf static factory methods provide a convenient way to create unmodifiable maps. TheMap instances created by these methods have the following characteristics:

  • They areunmodifiable. Keys and values cannot be added, removed, or updated. Calling any mutator method on the Map will always causeUnsupportedOperationException 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 disallownull 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 inIllegalArgumentException.
  • The iteration order of mappings is unspecified and is subject to change.
  • They arevalue-based. Programmers should treat instances that areequal 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 theSerialized Form page.

This interface is a member of the Java Collections Framework.

Since:
1.2
See Also:

  • Nested Class Summary

    Nested Classes
    Modifier and Type
    Interface
    Description
    static interface 
    A map entry (key-value pair).

  • Method Summary

    Modifier and Type
    Method
    Description
    void
    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, ornull if there is no current mapping (optional operation).
    defaultV
    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 (optional operation).
    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 (optional operation).
    boolean
    Returnstrue if this map contains a mapping for the specified key.
    boolean
    Returnstrue if this map maps one or more keys to the specified value.
    static <K,V> Map<K,V>
    copyOf(Map<? extends K, ? extends V> map)
    Returns anunmodifiable 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.
    Returns aSet view of the mappings contained in this map.
    boolean
    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.
    get(Object key)
    Returns the value to which the specified key is mapped, ornull 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, ordefaultValue if this map contains no mapping for the key.
    int
    Returns the hash code value for this map.
    boolean
    Returnstrue if this map contains no key-value mappings.
    Returns aSet 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 (optional operation).
    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>
    ofEntries(Map.Entry<? extends K, ? extends V>... entries)
    Returns an unmodifiable map containing keys and values extracted from the given entries.
    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 tonull) associates it with the given value and returnsnull, else returns the current value (optional operation).
    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 (optional operation).
    defaultV
    replace(K key,V value)
    Replaces the entry for the specified key only if it is currently mapped to some value (optional operation).
    default boolean
    replace(K key,V oldValue,V newValue)
    Replaces the entry for the specified key only if currently mapped to the specified value (optional operation).
    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 (optional operation).
    int
    Returns the number of key-value mappings in this map.
    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

      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 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

      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 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

      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 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 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 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, 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. If the specified map has a definedencounter 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 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:

    • 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:

    • 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 (optional operation). 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 thereplaceAll operation is not supported by this map (optional)
      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

      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 (optional operation).
      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 theputIfAbsent 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 (optional operation).
      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 (optional operation).
      Implementation Requirements:
      The default implementation is equivalent to, for thismap:
       if (map.containsKey(key) && Objects.equals(map.get(key), oldValue)) {     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 thereplace 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 (optional operation).
      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 thereplace 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 (optional operation).

      If the mapping function returnsnull, 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 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 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 aConcurrentModificationException 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 subinterfaceConcurrentMap 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 associated
      mappingFunction - 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 null
      UnsupportedOperationException - if thecomputeIfAbsent 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

      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 (optional operation).

      If the remapping function returnsnull, 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 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 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 aConcurrentModificationException 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 subinterfaceConcurrentMap 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 associated
      remappingFunction - 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 null
      UnsupportedOperationException - if thecomputeIfPresent 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

      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 (optional operation). For example, to either create or append aString msg to a value mapping:
       map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))
      (Methodmerge() is often simpler to use for such purposes.)

      If the remapping function returnsnull, 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 thismap:
       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 aConcurrentModificationException 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 subinterfaceConcurrentMap 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 associated
      remappingFunction - 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 null
      UnsupportedOperationException - if thecompute 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

      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 (optional operation). Otherwise, replaces the associated value with the results of the given remapping function, or removes if the result isnull. This method may be of use when combining multiple mapped values for a key. For example, to either create or append aString msg to a value mapping:
       map.merge(key, msg, String::concat)

      If the remapping function returnsnull, 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 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 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 aConcurrentModificationException 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 subinterfaceConcurrentMap 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 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 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 themerge 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

      static <K,V> Map<K,V> of()
      Returns an unmodifiable map containing zero mappings. SeeUnmodifiable Maps for details.
      Type Parameters:
      K - theMap's key type
      V - theMap's value type
      Returns:
      an emptyMap
      Since:
      9

    • of

      static <K,V> Map<K,V> of(K k1, V v1)
      Returns an unmodifiable map containing a single mapping. SeeUnmodifiable Maps for details.
      Type Parameters:
      K - theMap's key type
      V - theMap's value type
      Parameters:
      k1 - the mapping's key
      v1 - the mapping's value
      Returns:
      aMap containing the specified mapping
      Throws:
      NullPointerException - if the key or the value isnull
      Since:
      9

    • of

      static <K,V> Map<K,V> of(K k1, V v1, K k2, V v2)
      Returns an unmodifiable map containing two mappings. SeeUnmodifiable Maps for details.
      Type Parameters:
      K - theMap's key type
      V - theMap's value type
      Parameters:
      k1 - the first mapping's key
      v1 - the first mapping's value
      k2 - the second mapping's key
      v2 - the second mapping's value
      Returns:
      aMap containing the specified mappings
      Throws:
      IllegalArgumentException - if the keys are duplicates
      NullPointerException - 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)
      Returns an unmodifiable map containing three mappings. SeeUnmodifiable Maps for details.
      Type Parameters:
      K - theMap's key type
      V - theMap's value type
      Parameters:
      k1 - the first mapping's key
      v1 - the first mapping's value
      k2 - the second mapping's key
      v2 - the second mapping's value
      k3 - the third mapping's key
      v3 - the third mapping's value
      Returns:
      aMap containing the specified mappings
      Throws:
      IllegalArgumentException - if there are any duplicate keys
      NullPointerException - 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)
      Returns an unmodifiable map containing four mappings. SeeUnmodifiable Maps for details.
      Type Parameters:
      K - theMap's key type
      V - theMap's value type
      Parameters:
      k1 - the first mapping's key
      v1 - the first mapping's value
      k2 - the second mapping's key
      v2 - the second mapping's value
      k3 - the third mapping's key
      v3 - the third mapping's value
      k4 - the fourth mapping's key
      v4 - the fourth mapping's value
      Returns:
      aMap containing the specified mappings
      Throws:
      IllegalArgumentException - if there are any duplicate keys
      NullPointerException - 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)
      Returns an unmodifiable map containing five mappings. SeeUnmodifiable Maps for details.
      Type Parameters:
      K - theMap's key type
      V - theMap's value type
      Parameters:
      k1 - the first mapping's key
      v1 - the first mapping's value
      k2 - the second mapping's key
      v2 - the second mapping's value
      k3 - the third mapping's key
      v3 - the third mapping's value
      k4 - the fourth mapping's key
      v4 - the fourth mapping's value
      k5 - the fifth mapping's key
      v5 - the fifth mapping's value
      Returns:
      aMap containing the specified mappings
      Throws:
      IllegalArgumentException - if there are any duplicate keys
      NullPointerException - 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)
      Returns an unmodifiable map containing six mappings. SeeUnmodifiable Maps for details.
      Type Parameters:
      K - theMap's key type
      V - theMap's value type
      Parameters:
      k1 - the first mapping's key
      v1 - the first mapping's value
      k2 - the second mapping's key
      v2 - the second mapping's value
      k3 - the third mapping's key
      v3 - the third mapping's value
      k4 - the fourth mapping's key
      v4 - the fourth mapping's value
      k5 - the fifth mapping's key
      v5 - the fifth mapping's value
      k6 - the sixth mapping's key
      v6 - the sixth mapping's value
      Returns:
      aMap containing the specified mappings
      Throws:
      IllegalArgumentException - if there are any duplicate keys
      NullPointerException - 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. SeeUnmodifiable Maps for details.
      Type Parameters:
      K - theMap's key type
      V - theMap's value type
      Parameters:
      k1 - the first mapping's key
      v1 - the first mapping's value
      k2 - the second mapping's key
      v2 - the second mapping's value
      k3 - the third mapping's key
      v3 - the third mapping's value
      k4 - the fourth mapping's key
      v4 - the fourth mapping's value
      k5 - the fifth mapping's key
      v5 - the fifth mapping's value
      k6 - the sixth mapping's key
      v6 - the sixth mapping's value
      k7 - the seventh mapping's key
      v7 - the seventh mapping's value
      Returns:
      aMap containing the specified mappings
      Throws:
      IllegalArgumentException - if there are any duplicate keys
      NullPointerException - 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. SeeUnmodifiable Maps for details.
      Type Parameters:
      K - theMap's key type
      V - theMap's value type
      Parameters:
      k1 - the first mapping's key
      v1 - the first mapping's value
      k2 - the second mapping's key
      v2 - the second mapping's value
      k3 - the third mapping's key
      v3 - the third mapping's value
      k4 - the fourth mapping's key
      v4 - the fourth mapping's value
      k5 - the fifth mapping's key
      v5 - the fifth mapping's value
      k6 - the sixth mapping's key
      v6 - the sixth mapping's value
      k7 - the seventh mapping's key
      v7 - the seventh mapping's value
      k8 - the eighth mapping's key
      v8 - the eighth mapping's value
      Returns:
      aMap containing the specified mappings
      Throws:
      IllegalArgumentException - if there are any duplicate keys
      NullPointerException - 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. SeeUnmodifiable Maps for details.
      Type Parameters:
      K - theMap's key type
      V - theMap's value type
      Parameters:
      k1 - the first mapping's key
      v1 - the first mapping's value
      k2 - the second mapping's key
      v2 - the second mapping's value
      k3 - the third mapping's key
      v3 - the third mapping's value
      k4 - the fourth mapping's key
      v4 - the fourth mapping's value
      k5 - the fifth mapping's key
      v5 - the fifth mapping's value
      k6 - the sixth mapping's key
      v6 - the sixth mapping's value
      k7 - the seventh mapping's key
      v7 - the seventh mapping's value
      k8 - the eighth mapping's key
      v8 - the eighth mapping's value
      k9 - the ninth mapping's key
      v9 - the ninth mapping's value
      Returns:
      aMap containing the specified mappings
      Throws:
      IllegalArgumentException - if there are any duplicate keys
      NullPointerException - 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. SeeUnmodifiable Maps for details.
      Type Parameters:
      K - theMap's key type
      V - theMap's value type
      Parameters:
      k1 - the first mapping's key
      v1 - the first mapping's value
      k2 - the second mapping's key
      v2 - the second mapping's value
      k3 - the third mapping's key
      v3 - the third mapping's value
      k4 - the fourth mapping's key
      v4 - the fourth mapping's value
      k5 - the fifth mapping's key
      v5 - the fifth mapping's value
      k6 - the sixth mapping's key
      v6 - the sixth mapping's value
      k7 - the seventh mapping's key
      v7 - the seventh mapping's value
      k8 - the eighth mapping's key
      v8 - the eighth mapping's value
      k9 - the ninth mapping's key
      v9 - the ninth mapping's value
      k10 - the tenth mapping's key
      v10 - the tenth mapping's value
      Returns:
      aMap containing the specified mappings
      Throws:
      IllegalArgumentException - if there are any duplicate keys
      NullPointerException - if any key or value isnull
      Since:
      9

    • ofEntries

      @SafeVarargsstatic <K,V> Map<K,V> ofEntries(Map.Entry<? extends K, ? extends V>... entries)
      Returns an unmodifiable map containing keys and values extracted from the given entries. The entries themselves are not stored in the map. SeeUnmodifiable Maps for details.
      API Note:
      It is convenient to create the map entries using theMap.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 type
      V - theMap's value type
      Parameters:
      entries -Map.Entrys containing the keys and values from which the map is populated
      Returns:
      aMap containing the specified mappings
      Throws:
      IllegalArgumentException - if there are any duplicate keys
      NullPointerException - if any entry, key, or value isnull, or if theentries array isnull
      Since:
      9
      See Also:

    • entry

      static <K,V> Map.Entry<K,V> entry(K k, V v)
      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 disallownull keys and values. Attempts to create them using anull key or value result inNullPointerException.
      • They are unmodifiable. Calls toEntry.setValue() on a returnedEntry result inUnsupportedOperationException.
      • They are not serializable.
      • They arevalue-based. Programmers should treat instances that areequal 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 serializableEntry, seeAbstractMap.SimpleEntry orAbstractMap.SimpleImmutableEntry.
      Type Parameters:
      K - the key's type
      V - the value's type
      Parameters:
      k - the key
      v - the value
      Returns:
      anEntry containing the specified key and value
      Throws:
      NullPointerException - if the key or value isnull
      Since:
      9
      See Also:

    • copyOf

      static <K,V> Map<K,V> copyOf(Map<? extends K, ? extends V> map)
      Returns anunmodifiable 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 anunmodifiable Map, calling copyOf will generally not create a copy.
      Type Parameters:
      K - theMap's key type
      V - theMap's value type
      Parameters:
      map - aMap from which entries are drawn, must be non-null
      Returns:
      aMap containing the entries of the givenMap
      Throws:
      NullPointerException - if map is null, or if it contains any null keys or values
      Since:
      10