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public interface Map
``

Known indirect subclasses

AbstractMap<K, V>, ArrayMap<K, V>, Attributes, AuthProvider, ConcurrentHashMap<K, V>, ConcurrentMap<K, V>, ConcurrentNavigableMap<K, V>, ConcurrentSkipListMap<K, V>, EnumMap<K extends Enum, V>, HashMap<K, V>, Hashtable<K, V>, IdentityHashMap<K, V>, LinkedHashMap<K, V>, NavigableMap<K, V>, Properties, Provider, SequencedMap<K, V>, SortedMap<K, V>, TreeMap<K, V>, WeakHashMap<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 encounter order; others, like theHashMap class, do not. Maps with a definedencounter order are generally subtypes of the [SequencedMap](/reference/java/util/SequencedMap) interface.

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

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

The "destructive" methods contained in this interface, that is, the methods that modify the map on which they operate, are specified to throwUnsupportedOperationException 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)](/reference/java/util/Map#putAll%28java.util.Map<?%20extends%20K,?%20extends%20V>%29) 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](/reference/java/lang/Object#equals%28java.lang.Object%29) method. For example, the specification for the [containsKey(Object key)](/reference/java/util/Map#containsKey%28java.lang.Object%29) 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()](/reference/java/lang/Object#hashCode%28%29) 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](/reference/java/lang/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](/reference/java/util/Map#of%28%29),[Map.ofEntries](/reference/java/util/Map#ofEntries%28java.util.Map.Entry<?%20extends%20K,?%20extends%20V>[]%29), and[Map.copyOf](/reference/java/util/Map#copyOf%28java.util.Map<?%20extends%20K,?%20extends%20V>%29) static factory methods provide a convenient way to create unmodifiable maps. The Map instances created by these methods have the following characteristics:

• They are unmodifiable. Keys and values cannot be added, removed, or updated. Calling any mutator method on the Map will always cause UnsupportedOperationException to be thrown. However, if the contained keys or values are themselves mutable, this may cause the Map to behave inconsistently or its contents to appear to change.
• They disallow null keys and values. Attempts to create them withnull keys or values result 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 theSerialized Form page.

This interface is a member of the Java Collections Framework.

Summary

Public methods

clear

public abstract void clear ()

Removes all of the mappings from this map (optional operation). The map will be empty after this call returns.

compute

public 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 (optional operation). 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()](/reference/java/util/Map#merge%28K,%20V,%20java.util.function.BiFunction<?%20super%20V,?%20super%20V,?%20extends%20V>%29) 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 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 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](/reference/java/util/concurrent/ConcurrentMap) must document whether the remapping function is applied once atomically only if the value is not present.

computeIfAbsent

public 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 (optional operation).

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 thismap, 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 aConcurrentModificationExceptionif 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 anIllegalStateExceptionif 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.

computeIfPresent

public 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 (optional operation).

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

The remapping function should not modify this map during computation.

Implementation Requirements:

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

}
The default implementation makes no guarantees about detecting if the remapping function modifies this map during computation and, if appropriate, reporting an error. Non-concurrent implementations should override this method and, on a best-effort basis, throw aConcurrentModificationExceptionif 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 anIllegalStateExceptionif 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.

copyOf

public static 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.

entry

public static Entry<K, V> entry (K k, V v)

Returns an unmodifiable [Entry](/reference/java/util/Map.Entry) containing the given key and value. These entries are suitable for populating Map instances using the[Map.ofEntries()](/reference/java/util/Map#ofEntries%28java.util.Map.Entry<?%20extends%20K,?%20extends%20V>[]%29) 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()](/reference/java/util/Map.Entry#setValue%28V%29) 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.

entrySet

public abstract Set<Entry<K, V>> entrySet ()

Returns a [Set](/reference/java/util/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 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 the Iterator.remove,Set.remove, removeAll, retainAll andclear operations. It does not support theadd or addAll operations.

equals

public abstract 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 maps m1 andm2 represent the same mappings ifm1.entrySet().equals(m2.entrySet()). This ensures that theequals method works properly across different implementations of the Map interface.

forEach

public 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.

get

public abstract 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 keyk to a value v such thatObjects.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 ofnull 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](/reference/java/util/Map#containsKey%28java.lang.Object%29) operation may be used to distinguish these two cases.

getOrDefault

public 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.

hashCode

public abstract 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 that m1.equals(m2) implies that m1.hashCode()==m2.hashCode() for any two mapsm1 and m2, as required by the general contract of[Object.hashCode](/reference/java/lang/Object#hashCode%28%29).

isEmpty

public abstract boolean isEmpty ()

Returns true if this map contains no key-value mappings.

keySet

public abstract Set keySet ()

Returns a [Set](/reference/java/util/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 theIterator.remove, Set.remove,removeAll, retainAll, and clear operations. It does not support the add or addAll operations.

merge

public 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 (optional operation). 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 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 aConcurrentModificationExceptionif 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 anIllegalStateExceptionif 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.

of

public static 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.

of

public static 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.

of

public static Map<K, V> of (K k1, V v1)

Returns an unmodifiable map containing a single mapping. See Unmodifiable Maps for details.

of

public static 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.

of

public static Map<K, V> of (K k1, V v1, K k2, V v2)

Returns an unmodifiable map containing two mappings. See Unmodifiable Maps for details.

of

public static 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.

of

public static Map<K, V> of ()

Returns an unmodifiable map containing zero mappings. See Unmodifiable Maps for details.

of

public static 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.

of

public static 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.

of

public static 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.

of

public static 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.

ofEntries

public static Map<K, V> ofEntries (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()](/reference/java/util/Map#entry%28K,%20V%29) 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"));  
   

put

public abstract 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 key k if and only if [m.containsKey(k)](/reference/java/util/Map#containsKey%28java.lang.Object%29) would returntrue.)

putAll

public abstract 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)](/reference/java/util/Map#put%28K,%20V%29) 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. If the specified map has a definedencounter order, processing of its mappings generally occurs in that order.

remove

public abstract 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 thatObjects.equals(key, k), that mapping is removed. (The map can contain at most one such mapping.)

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

If this map permits null values, then a return value ofnull 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.

replace

public 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 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.

replace

public 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 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.

size

public abstract int size ()

Returns the number of key-value mappings in this map. If the map contains more than Integer.MAX_VALUE elements, returnsInteger.MAX_VALUE.

values

public abstract Collection values ()

Returns a [Collection](/reference/java/util/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.