Module java.base
Package java.util

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>, 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 interface Map<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 the Dictionary class, which was a totally abstract class rather than an interface.

The Map interface provides three collection views, which allow a map's contents to be viewed as a set of keys, collection of values, or set of key-value mappings. The order of a map is defined as the order in which the iterators on the map's collection views return their elements. Some map implementations, like the TreeMap class, make specific guarantees as to their order; others, like the HashMap class, do not.

Note: great care must be exercised if mutable objects are used as map keys. The behavior of a map is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is a key in the map. A special case of this prohibition is that it is not permissible for a map to contain itself as a key. While it is permissible for a map to contain itself as a value, extreme caution is advised: the equals and hashCode methods are no longer well defined on such a map.

All general-purpose map implementation classes should provide two "standard" constructors: a void (no arguments) constructor which creates an empty map, and a constructor with a single argument of type Map, which creates a new map with the same key-value mappings as its argument. In effect, the latter constructor allows the user to copy any map, producing an equivalent map of the desired class. There is no way to enforce this recommendation (as interfaces cannot contain constructors) but all of the general-purpose map implementations in the JDK comply.

The "destructive" methods contained in this interface, that is, the methods that modify the map on which they operate, are specified to throw UnsupportedOperationException if this map does not support the operation. If this is the case, these methods may, but are not required to, throw an UnsupportedOperationException if the invocation would have no effect on the map. For example, invoking the putAll(Map) method on an unmodifiable map may, but is not required to, throw the exception if the map whose mappings are to be "superimposed" is empty.

Some map implementations have restrictions on the keys and values they may contain. For example, some implementations prohibit null keys and values, and some have restrictions on the types of their keys. Attempting to insert an ineligible key or value throws an unchecked exception, typically NullPointerException or ClassCastException. Attempting to query the presence of an ineligible key or value may throw an exception, or it may simply return false; some implementations will exhibit the former behavior and some will exhibit the latter. More generally, attempting an operation on an ineligible key or value whose completion would not result in the insertion of an ineligible element into the map may throw an exception or it may succeed, at the option of the implementation. Such exceptions are marked as "optional" in the specification for this interface.

Many methods in Collections Framework interfaces are defined in terms of the equals method. For example, the specification for the containsKey(Object key) method says: "returns true if and only if this map contains a mapping for a key k such that (key==null ? k==null : key.equals(k))." This specification should not be construed to imply that invoking Map.containsKey with a non-null argument key will cause key.equals(k) to be invoked for any key k. Implementations are free to implement optimizations whereby the equals invocation is avoided, for example, by first comparing the hash codes of the two keys. (The Object.hashCode() specification guarantees that two objects with unequal hash codes cannot be equal.) More generally, implementations of the various Collections Framework interfaces are free to take advantage of the specified behavior of underlying Object methods wherever the implementor deems it appropriate.

Some map operations which perform recursive traversal of the map may fail with an exception for self-referential instances where the map directly or indirectly contains itself. This includes the clone(), equals(), hashCode() and toString() methods. Implementations may optionally handle the self-referential scenario, however most current implementations do not do so.

Unmodifiable Maps

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

  • They are unmodifiable. Keys and values cannot be added, removed, or updated. Calling any mutator method on the Map will always cause UnsupportedOperationException to be thrown. However, if the contained keys or values are themselves mutable, this may cause the Map to behave inconsistently or its contents to appear to change.
  • They disallow null keys and values. Attempts to create them with null keys or values result in NullPointerException.
  • They are serializable if all keys and values are serializable.
  • They reject duplicate keys at creation time. Duplicate keys passed to a static factory method result in IllegalArgumentException.
  • The iteration order of mappings is unspecified and is subject to change.
  • They are value-based. Programmers should treat instances that are equal as interchangeable and should not use them for synchronization, or unpredictable behavior may occur. For example, in a future release, synchronization may fail. Callers should make no assumptions about the identity of the returned instances. Factories are free to create new instances or reuse existing ones.
  • They are serialized as specified on the Serialized Form page.

This interface is a member of the Java Collections Framework.

Since:
1.2
See Also:

  • Nested Class Summary

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

  • Method Summary

    Modifier and Type
    Method
    Description
    void
    clear()
    Removes all of the mappings from this map (optional operation).
    default V
    compute(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)
    Attempts to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping).
    default V
    computeIfAbsent(K key, Function<? super K,? extends V> mappingFunction)
    If the specified key is not already associated with a value (or is mapped to null), attempts to compute its value using the given mapping function and enters it into this map unless null.
    default V
    computeIfPresent(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)
    If the value for the specified key is present and non-null, attempts to compute a new mapping given the key and its current mapped value.
    boolean
    containsKey(Object key)
    Returns true if this map contains a mapping for the specified key.
    boolean
    containsValue(Object value)
    Returns true if this map maps one or more keys to the specified value.
    static <K, V> Map<K,V>
    copyOf(Map<? extends K,? extends V> map)
    Returns an unmodifiable Map containing the entries of the given Map.
    static <K, V> Map.Entry<K,V>
    entry(K k, V v)
    Returns an unmodifiable Map.Entry containing the given key and value.
    Set<Map.Entry<K,V>>
    entrySet()
    Returns a Set view of the mappings contained in this map.
    boolean
    equals(Object o)
    Compares the specified object with this map for equality.
    default void
    forEach(BiConsumer<? super K,? super V> action)
    Performs the given action for each entry in this map until all entries have been processed or the action throws an exception.
    V
    get(Object key)
    Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.
    default V
    getOrDefault(Object key, V defaultValue)
    Returns the value to which the specified key is mapped, or defaultValue if this map contains no mapping for the key.
    int
    hashCode()
    Returns the hash code value for this map.
    boolean
    isEmpty()
    Returns true if this map contains no key-value mappings.
    Set<K>
    keySet()
    Returns a Set view of the keys contained in this map.
    default V
    merge(K key, V value, BiFunction<? super V,? super V,? extends V> remappingFunction)
    If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value.
    static <K, V> Map<K,V>
    of()
    Returns an 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.
    V
    put(K key, V value)
    Associates the specified value with the specified key in this map (optional operation).
    void
    putAll(Map<? extends K,? extends V> m)
    Copies all of the mappings from the specified map to this map (optional operation).
    default V
    putIfAbsent(K key, V value)
    If the specified key is not already associated with a value (or is mapped to null) associates it with the given value and returns null, else returns the current value.
    V
    remove(Object key)
    Removes the mapping for a key from this map if it is present (optional operation).
    default boolean
    remove(Object key, Object value)
    Removes the entry for the specified key only if it is currently mapped to the specified value.
    default V
    replace(K key, V value)
    Replaces the entry for the specified key only if it is currently mapped to some value.
    default boolean
    replace(K key, V oldValue, V newValue)
    Replaces the entry for the specified key only if currently mapped to the specified value.
    default void
    replaceAll(BiFunction<? super K,? super V,? extends V> function)
    Replaces each entry's value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception.
    int
    size()
    Returns the number of key-value mappings in this map.
    Collection<V>
    values()
    Returns a Collection view of the values contained in this map.

  • Method Details

    • size

      int size()
      Returns the number of key-value mappings in this map. If the map contains more than Integer.MAX_VALUE elements, returns Integer.MAX_VALUE.
      Returns:
      the number of key-value mappings in this map

    • isEmpty

      boolean isEmpty()
      Returns true if this map contains no key-value mappings.
      Returns:
      true if this map contains no key-value mappings

    • containsKey

      boolean containsKey(Object key)
      Returns true if this map contains a mapping for the specified key. More formally, returns true if and only if this map contains a mapping for a key k such that Objects.equals(key, k). (There can be at most one such mapping.)
      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)
      Returns true if this map maps one or more keys to the specified value. More formally, returns true if and only if this map contains at least one mapping to a value v such that Objects.equals(value, v). This operation will probably require time linear in the map size for most implementations of the Map interface.
      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, or null if this map contains no mapping for the key.

      More formally, if this map contains a mapping from a key k to a value v such that Objects.equals(key, k), then this method returns v; otherwise it returns null. (There can be at most one such mapping.)

      If this map permits null values, then a return value of null does not necessarily indicate that the map contains no mapping for the key; it's also possible that the map explicitly maps the key to null. The containsKey operation may be used to distinguish these two cases.

      Parameters:
      key - the key whose associated value is to be returned
      Returns:
      the value to which the specified key is mapped, or null if this map contains no mapping for the key
      Throws:
      ClassCastException - if the key is of an inappropriate type for this map (optional)
      NullPointerException - if the specified key is null and this map does not permit null keys (optional)

    • put

      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 map m is said to contain a mapping for a key k if and only if m.containsKey(k) would return true.)
      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 key, or null if there was no mapping for key. (A null return can also indicate that the map previously associated null with key, if the implementation supports null values.)
      Throws:
      UnsupportedOperationException - if the put 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 key k to value v such that Objects.equals(key, k), that mapping is removed. (The map can contain at most one such mapping.)

      Returns the value to which this map previously associated the key, or null if the map contained no mapping for the key.

      If this map permits null values, then a return value of null does not necessarily indicate that the map contained no mapping for the key; it's also possible that the map explicitly mapped the key to null.

      The map will not contain a mapping for the specified key once the call returns.

      Parameters:
      key - key whose mapping is to be removed from the map
      Returns:
      the previous value associated with key, or null if there was no mapping for key.
      Throws:
      UnsupportedOperationException - if the remove 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<? extends K,? extends V> 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 calling put(k, v) on this map once for each mapping from key k to value v in the specified map. The behavior of this operation is undefined if the specified map is modified while the operation is in progress.
      Parameters:
      m - mappings to be stored in this map
      Throws:
      UnsupportedOperationException - if the putAll 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 the clear operation is not supported by this map

    • keySet

      Set<K> keySet()
      Returns a Set view of the keys contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress (except through the iterator's own remove operation), the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Set.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations.
      Returns:
      a set view of the keys contained in this map

    • values

      Collection<V> values()
      Returns a Collection view of the values contained in this map. The collection is backed by the map, so changes to the map are reflected in the collection, and vice-versa. If the map is modified while an iteration over the collection is in progress (except through the iterator's own remove operation), the results of the iteration are undefined. The collection supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Collection.remove, removeAll, retainAll and clear operations. It does not support the add or addAll operations.
      Returns:
      a collection view of the values contained in this map

    • entrySet

      Set<Map.Entry<K,V>> entrySet()
      Returns a Set view of the mappings contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress (except through the iterator's own remove operation, or through the setValue operation on a map entry returned by the iterator) the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Set.remove, removeAll, retainAll and clear operations. It does not support the add or addAll operations.
      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. Returns true if the given object is also a map and the two maps represent the same mappings. More formally, two maps m1 and m2 represent the same mappings if m1.entrySet().equals(m2.entrySet()). This ensures that the equals method works properly across different implementations of the Map interface.
      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's entrySet() view. This ensures that m1.equals(m2) implies that m1.hashCode()==m2.hashCode() for any two maps m1 and m2, as required by the general contract of Object.hashCode().
      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, or defaultValue 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, or defaultValue if this map contains no mapping for the key
      Throws:
      ClassCastException - if the key is of an inappropriate type for this map (optional)
      NullPointerException - if the specified key is null and this map does not permit null keys (optional)
      Since:
      1.8

    • forEach

      default void forEach(BiConsumer<? super K,? super V> action)
      Performs the given action for each entry in this map until all entries have been processed or the action throws an exception. Unless otherwise specified by the implementing class, actions are performed in the order of entry set iteration (if an iteration order is specified.) Exceptions thrown by the action are relayed to the caller.
      Implementation Requirements:
      The default implementation is equivalent to, for this map: 
       
 for (Map.Entry<K, V> entry : map.entrySet())
     action.accept(entry.getKey(), entry.getValue());
      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<? super K,? super V,? extends V> function)
      Replaces each entry's value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception. Exceptions thrown by the function are relayed to the caller.
      Implementation Requirements:

      The default implementation is equivalent to, for this map: 

       
 for (Map.Entry<K, V> entry : map.entrySet())
     entry.setValue(function.apply(entry.getKey(), entry.getValue()));

      The default implementation makes no guarantees about synchronization or atomicity properties of this method. Any implementation providing atomicity guarantees must override this method and document its concurrency properties.

      Parameters:
      function - the function to apply to each entry
      Throws:
      UnsupportedOperationException - if the set operation is not supported by this map's entry set iterator.
      ClassCastException - if the class of a replacement value prevents it from being stored in this 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 to null) associates it with the given value and returns null, else returns the current value.
      Implementation Requirements:
      The default implementation is equivalent to, for this map: 
       
 V v = map.get(key);
 if (v == null)
     v = map.put(key, value);

 return v;

      The default implementation makes no guarantees about synchronization or atomicity properties of this method. Any implementation providing atomicity guarantees must override this method and document its concurrency properties.

      Parameters:
      key - key with which the specified value is to be associated
      value - value to be associated with the specified key
      Returns:
      the previous value associated with the specified key, or null if there was no mapping for the key. (A null return can also indicate that the map previously associated null with the key, if the implementation supports null values.)
      Throws:
      UnsupportedOperationException - if the put operation is not supported by this map (optional)
      ClassCastException - if the key or value is of an inappropriate type for this map (optional)
      NullPointerException - if the specified key or value is null, and this map does not permit null keys or values (optional)
      IllegalArgumentException - if some property of the specified key or value prevents it from being stored in this map (optional)
      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 this map: 
       
 if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
     map.remove(key);
     return true;
 } else
     return false;

      The default implementation makes no guarantees about synchronization or atomicity properties of this method. Any implementation providing atomicity guarantees must override this method and document its concurrency properties.

      Parameters:
      key - key with which the specified value is associated
      value - value expected to be associated with the specified key
      Returns:
      true if the value was removed
      Throws:
      UnsupportedOperationException - if the remove operation is not supported by this map (optional)
      ClassCastException - if the key or value is of an inappropriate type for this map (optional)
      NullPointerException - if the specified key or value is null, and this map does not permit null keys or values (optional)
      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 this map: 
       
 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 the put operation is not supported by this map (optional)
      ClassCastException - if the class of a specified key or value prevents it from being stored in this 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 this map: 
       
 if (map.containsKey(key)) {
     return map.put(key, value);
 } else
     return null;

      The default implementation makes no guarantees about synchronization or atomicity properties of this method. Any implementation providing atomicity guarantees must override this method and document its concurrency properties.

      Parameters:
      key - key with which the specified value is associated
      value - value to be associated with the specified key
      Returns:
      the previous value associated with the specified key, or null if there was no mapping for the key. (A null return can also indicate that the map previously associated null with the key, if the implementation supports null values.)
      Throws:
      UnsupportedOperationException - if the put operation is not supported by this map (optional)
      ClassCastException - if the class of the specified key or value prevents it from being stored in this map (optional)
      NullPointerException - if the specified key or value is null, and this map does not permit null keys or 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<? super K,? extends V> mappingFunction)
      If the specified key is not already associated with a value (or is mapped to null), attempts to compute its value using the given mapping function and enters it into this map unless null.

      If the mapping function returns null, no mapping is recorded. If the mapping function itself throws an (unchecked) exception, the exception is rethrown, and no mapping is recorded. The most common usage is to construct a new object serving as an initial mapped value or memoized result, as in: 

       
 map.computeIfAbsent(key, k -> new Value(f(k)));

      Or to implement a multi-value map, Map<K,Collection<V>>, supporting multiple values per key: 

       
 map.computeIfAbsent(key, k -> new HashSet<V>()).add(v);

      The mapping function should not modify this map during computation.

      Implementation Requirements:
      The default implementation is equivalent to the following steps for this map, then returning the current value or null if now absent: 
       
 if (map.get(key) == null) {
     V newValue = mappingFunction.apply(key);
     if (newValue != null)
         map.put(key, newValue);
 }

      The default implementation makes no guarantees about detecting if the mapping function modifies this map during computation and, if appropriate, reporting an error. Non-concurrent implementations should override this method and, on a best-effort basis, throw a ConcurrentModificationException if it is detected that the mapping function modifies this map during computation. Concurrent implementations should override this method and, on a best-effort basis, throw an IllegalStateException if it is detected that the mapping function modifies this map during computation and as a result computation would never complete.

      The default implementation makes no guarantees about synchronization or atomicity properties of this method. Any implementation providing atomicity guarantees must override this method and document its concurrency properties. In particular, all implementations of subinterface ConcurrentMap must document whether the mapping function is applied once atomically only if the value is not present.

      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 the put operation is not supported by this map (optional)
      ClassCastException - if the class of the specified key or value prevents it from being stored in this map (optional)
      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<? super K,? super V,? extends V> remappingFunction)
      If the value for the specified key is present and non-null, attempts to compute a new mapping given the key and its current mapped value.

      If the remapping function returns null, the mapping is removed. If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.

      The remapping function should not modify this map during computation.

      Implementation Requirements:
      The default implementation is equivalent to performing the following steps for this map, then returning the current value or null if now absent: 
       
 if (map.get(key) != null) {
     V oldValue = map.get(key);
     V newValue = remappingFunction.apply(key, oldValue);
     if (newValue != null)
         map.put(key, newValue);
     else
         map.remove(key);
 }

      The default implementation makes no guarantees about detecting if the remapping function modifies this map during computation and, if appropriate, reporting an error. Non-concurrent implementations should override this method and, on a best-effort basis, throw a ConcurrentModificationException if it is detected that the remapping function modifies this map during computation. Concurrent implementations should override this method and, on a best-effort basis, throw an IllegalStateException if it is detected that the remapping function modifies this map during computation and as a result computation would never complete.

      The default implementation makes no guarantees about synchronization or atomicity properties of this method. Any implementation providing atomicity guarantees must override this method and document its concurrency properties. In particular, all implementations of subinterface ConcurrentMap must document whether the remapping function is applied once atomically only if the value is not present.

      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 the put operation is not supported by this map (optional)
      ClassCastException - if the class of the specified key or value prevents it from being stored in this map (optional)
      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<? super K,? super V,? extends V> remappingFunction)
      Attempts to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping). For example, to either create or append a String msg to a value mapping: 
       
 map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))
      (Method merge() is often simpler to use for such purposes.)

      If the remapping function returns null, the mapping is removed (or remains absent if initially absent). If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.

      The remapping function should not modify this map during computation.

      Implementation Requirements:
      The default implementation is equivalent to performing the following steps for this map: 
       
 V oldValue = map.get(key);
 V newValue = remappingFunction.apply(key, oldValue);
 if (newValue != null) {
     map.put(key, newValue);
 } else if (oldValue != null || map.containsKey(key)) {
     map.remove(key);
 }
 return newValue;

      The default implementation makes no guarantees about detecting if the remapping function modifies this map during computation and, if appropriate, reporting an error. Non-concurrent implementations should override this method and, on a best-effort basis, throw a ConcurrentModificationException if it is detected that the remapping function modifies this map during computation. Concurrent implementations should override this method and, on a best-effort basis, throw an IllegalStateException if it is detected that the remapping function modifies this map during computation and as a result computation would never complete.

      The default implementation makes no guarantees about synchronization or atomicity properties of this method. Any implementation providing atomicity guarantees must override this method and document its concurrency properties. In particular, all implementations of subinterface ConcurrentMap must document whether the remapping function is applied once atomically only if the value is not present.

      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 the put operation is not supported by this map (optional)
      ClassCastException - if the class of the specified key or value prevents it from being stored in this map (optional)
      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<? super V,? super V,? extends V> remappingFunction)
      If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value. Otherwise, replaces the associated value with the results of the given remapping function, or removes if the result is null. This method may be of use when combining multiple mapped values for a key. For example, to either create or append a String msg to a value mapping: 
       
 map.merge(key, msg, String::concat)

      If the remapping function returns null, the mapping is removed. If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.

      The remapping function should not modify this map during computation.

      Implementation Requirements:
      The default implementation is equivalent to performing the following steps for this map, then returning the current value or null if absent: 
       
 V oldValue = map.get(key);
 V newValue = (oldValue == null) ? value :
              remappingFunction.apply(oldValue, value);
 if (newValue == null)
     map.remove(key);
 else
     map.put(key, newValue);

      The default implementation makes no guarantees about detecting if the remapping function modifies this map during computation and, if appropriate, reporting an error. Non-concurrent implementations should override this method and, on a best-effort basis, throw a ConcurrentModificationException if it is detected that the remapping function modifies this map during computation. Concurrent implementations should override this method and, on a best-effort basis, throw an IllegalStateException if it is detected that the remapping function modifies this map during computation and as a result computation would never complete.

      The default implementation makes no guarantees about synchronization or atomicity properties of this method. Any implementation providing atomicity guarantees must override this method and document its concurrency properties. In particular, all implementations of subinterface ConcurrentMap must document whether the remapping function is applied once atomically only if the value is not present.

      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 the put operation is not supported by this map (optional)
      ClassCastException - if the class of the specified key or value prevents it from being stored in this map (optional)
      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. See Unmodifiable Maps for details.
      Type Parameters:
      K - the Map's key type
      V - the Map's value type
      Returns:
      an empty Map
      Since:
      9

    • of

      static <K, V> Map<K,V> of(K k1, V v1)
      Returns an unmodifiable map containing a single mapping. See Unmodifiable Maps for details.
      Type Parameters:
      K - the Map's key type
      V - the Map's value type
      Parameters:
      k1 - the mapping's key
      v1 - the mapping's value
      Returns:
      a Map containing the specified mapping
      Throws:
      NullPointerException - if the key or the value is null
      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. See Unmodifiable Maps for details.
      Type Parameters:
      K - the Map's key type
      V - the Map'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:
      a Map containing the specified mappings
      Throws:
      IllegalArgumentException - if the keys are duplicates
      NullPointerException - if any key or value is null
      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. See Unmodifiable Maps for details.
      Type Parameters:
      K - the Map's key type
      V - the Map'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:
      a Map containing the specified mappings
      Throws:
      IllegalArgumentException - if there are any duplicate keys
      NullPointerException - if any key or value is null
      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. See Unmodifiable Maps for details.
      Type Parameters:
      K - the Map's key type
      V - the Map'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:
      a Map containing the specified mappings
      Throws:
      IllegalArgumentException - if there are any duplicate keys
      NullPointerException - if any key or value is null
      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. See Unmodifiable Maps for details.
      Type Parameters:
      K - the Map's key type
      V - the Map'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:
      a Map containing the specified mappings
      Throws:
      IllegalArgumentException - if there are any duplicate keys
      NullPointerException - if any key or value is null
      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. See Unmodifiable Maps for details.
      Type Parameters:
      K - the Map's key type
      V - the Map'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:
      a Map containing the specified mappings
      Throws:
      IllegalArgumentException - if there are any duplicate keys
      NullPointerException - if any key or value is null
      Since:
      9

    • of

      static <K, V> Map<K,V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7)
      Returns an unmodifiable map containing seven mappings. See Unmodifiable Maps for details.
      Type Parameters:
      K - the Map's key type
      V - the Map'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:
      a Map containing the specified mappings
      Throws:
      IllegalArgumentException - if there are any duplicate keys
      NullPointerException - if any key or value is null
      Since:
      9

    • of

      static <K, V> Map<K,V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7, K k8, V v8)
      Returns an unmodifiable map containing eight mappings. See Unmodifiable Maps for details.
      Type Parameters:
      K - the Map's key type
      V - the Map'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:
      a Map containing the specified mappings
      Throws:
      IllegalArgumentException - if there are any duplicate keys
      NullPointerException - if any key or value is null
      Since:
      9

    • of

      static <K, V> Map<K,V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7, K k8, V v8, K k9, V v9)
      Returns an unmodifiable map containing nine mappings. See Unmodifiable Maps for details.
      Type Parameters:
      K - the Map's key type
      V - the Map'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:
      a Map containing the specified mappings
      Throws:
      IllegalArgumentException - if there are any duplicate keys
      NullPointerException - if any key or value is null
      Since:
      9

    • of

      static <K, V> Map<K,V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7, K k8, V v8, K k9, V v9, K k10, V v10)
      Returns an unmodifiable map containing ten mappings. See Unmodifiable Maps for details.
      Type Parameters:
      K - the Map's key type
      V - the Map'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:
      a Map containing the specified mappings
      Throws:
      IllegalArgumentException - if there are any duplicate keys
      NullPointerException - if any key or value is null
      Since:
      9

    • ofEntries

      @SafeVarargs 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. The entries themselves are not stored in the map. See Unmodifiable Maps for details.
      API Note:
      It is convenient to create the map entries using the Map.entry() method. For example, 
      
     import static java.util.Map.entry;

     Map<Integer,String> map = Map.ofEntries(
         entry(1, "a"),
         entry(2, "b"),
         entry(3, "c"),
         ...
         entry(26, "z"));
      Type Parameters:
      K - the Map's key type
      V - the Map's value type
      Parameters:
      entries - Map.Entrys containing the keys and values from which the map is populated
      Returns:
      a Map containing the specified mappings
      Throws:
      IllegalArgumentException - if there are any duplicate keys
      NullPointerException - if any entry, key, or value is null, or if the entries array is null
      Since:
      9
      See Also:

    • entry

      static <K, V> Map.Entry<K,V> entry(K k, V v)
      Returns an unmodifiable Map.Entry containing the given key and value. These entries are suitable for populating Map instances using the Map.ofEntries() method. The Entry instances created by this method have the following characteristics:
      • They disallow null keys and values. Attempts to create them using a null key or value result in NullPointerException.
      • They are unmodifiable. Calls to Entry.setValue() on a returned Entry result in UnsupportedOperationException.
      • They are not serializable.
      • They are value-based. Programmers should treat instances that are equal as interchangeable and should not use them for synchronization, or unpredictable behavior may occur. For example, in a future release, synchronization may fail. Callers should make no assumptions about the identity of the returned instances. This method is free to create new instances or reuse existing ones.
      API Note:
      For a serializable Entry, see AbstractMap.SimpleEntry or AbstractMap.SimpleImmutableEntry.
      Type Parameters:
      K - the key's type
      V - the value's type
      Parameters:
      k - the key
      v - the value
      Returns:
      an Entry containing the specified key and value
      Throws:
      NullPointerException - if the key or value is null
      Since:
      9
      See Also:

    • copyOf

      static <K, V> Map<K,V> copyOf(Map<? extends K,? extends V> map)
      Returns an unmodifiable Map containing the entries of the given Map. The given Map must not be null, and it must not contain any null keys or values. If the given Map is subsequently modified, the returned Map will not reflect such modifications.
      Implementation Note:
      If the given Map is an unmodifiable Map, calling copyOf will generally not create a copy.
      Type Parameters:
      K - the Map's key type
      V - the Map's value type
      Parameters:
      map - a Map from which entries are drawn, must be non-null
      Returns:
      a Map containing the entries of the given Map
      Throws:
      NullPointerException - if map is null, or if it contains any null keys or values
      Since:
      10