public interface Map<K, V>

Known subtypesProperties, WeakHashMap, AbstractMap, HashMap, Hashtable, IdentityHashMap, LinkedHashMap, NavigableMap, SortedMap, TreeMap

A Map is a data structure consisting of a set of keys and values in which each key is mapped to a single value. The class of the objects used as keys is declared when the Map is declared, as is the class of the corresponding values.

A Map provides helper methods to iterate through all of the keys contained in it, as well as various methods to access and update the key/value pairs.

Nested types

interface Map.EntryMap.Entry is a key/value mapping contained in a Map.

Methods

public abstract void clear()Removes all elements from this Map, leaving it empty.
public abstract boolean containsKey(Object key)Returns whether this Map contains the specified key.
public abstract boolean containsValue(Object value)Returns whether this Map contains the specified value.
public abstract Set<Map.Entry<K, V>> entrySet()Returns a Set containing all of the mappings in this Map.
public abstract boolean equals(Object object)Compares the argument to the receiver, and returns true if the specified object is a Map and both Maps contain the same mappings.
public abstract V get(Object key)Returns the value of the mapping with the specified key.
public abstract int hashCode()Returns an integer hash code for the receiver.
public abstract boolean isEmpty()Returns whether this map is empty.
public abstract Set<K> keySet()Returns a set of the keys contained in this Map.
public abstract V put(K key, V value)Maps the specified key to the specified value.
public abstract void putAll(Map<? extends K, ? extends V> map)Copies every mapping in the specified Map to this Map.
public abstract V remove(Object key)Removes a mapping with the specified key from this Map.
public abstract int size()Returns the number of mappings in this Map.
public abstract Collection<V> values()Returns a Collection of the values contained in this Map.
public 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.
public 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.
public default boolean remove(Object key, Object value)Removes the entry for the specified key only if it is currently mapped to the specified value.
public default boolean replace(K key, V oldValue, V newValue)Replaces the entry for the specified key only if currently mapped to the specified value.
public default V replace(K key, V value)Replaces the entry for the specified key only if it is currently mapped to some value.
public default void forEach(BiConsumer<? super K, ? super V> action)Performs the given action for each entry in this map.
public 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.
public 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.
public 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.
public 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).
public 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.

Method details

clear

public abstract void clear()
Removes all elements from this Map, leaving it empty.

Throws

UnsupportedOperationException
if removing elements from this Map is not supported.

See also

containsKey

public abstract boolean containsKey(Object key)
Returns whether this Map contains the specified key.

Parameters

key Object
the key to search for.

Returns

true if this map contains the specified key, false otherwise.

containsValue

public abstract boolean containsValue(Object value)
Returns whether this Map contains the specified value.

Parameters

value Object
the value to search for.

Returns

true if this map contains the specified value, false otherwise.

entrySet

public abstract Set<Map.Entry<K, V>> entrySet()
Returns a Set containing all of the mappings in this Map. Each mapping is an instance of Map.Entry. As the Set is backed by this Map, changes in one will be reflected in the other.

Returns

a set of the mappings

equals

public abstract boolean equals(Object object)
Compares the argument to the receiver, and returns true if the specified object is a Map and both Maps contain the same mappings.

Parameters

object Object
the Object to compare with this Object.

Returns

boolean true if the Object is the same as this Object false if it is different from this Object.

See also

get

public abstract V get(Object key)
Returns the value of the mapping with the specified key.

Parameters

key Object
the key.

Returns

the value of the mapping with the specified key, or null if no mapping for the specified key is found.

hashCode

public abstract int hashCode()
Returns an integer hash code for the receiver. Objects which are equal return the same value for this method.

Returns

the receiver’s hash.

See also

isEmpty

public abstract boolean isEmpty()
Returns whether this map is empty.

Returns

true if this map has no elements, false otherwise.

See also

keySet

public abstract Set<K> keySet()
Returns a set of the keys contained in this Map. The Set is backed by this Map so changes to one are reflected by the other. The Set does not support adding.

Returns

a set of the keys.

put

public abstract V put(K key, V value)
Maps the specified key to the specified value.

Parameters

key K
the key.
value V
the value.

Returns

the value of any previous mapping with the specified key or null if there was no mapping.

Throws

UnsupportedOperationException
if adding to this Map is not supported.
ClassCastException
if the class of the key or value is inappropriate for this Map.
IllegalArgumentException
if the key or value cannot be added to this Map.
NullPointerException
if the key or value is null and this Map does not support null keys or values.

putAll

public abstract void putAll(Map<? extends K, ? extends V> map)
Copies every mapping in the specified Map to this Map.

Parameters

map Map<? extends K, ? extends V>
the Map to copy mappings from.

Throws

UnsupportedOperationException
if adding to this Map is not supported.
ClassCastException
if the class of a key or a value of the specified Map is inappropriate for this Map.
IllegalArgumentException
if a key or value cannot be added to this Map.
NullPointerException
if a key or value is null and this Map does not support null keys or values.

remove

public abstract V remove(Object key)
Removes a mapping with the specified key from this Map.

Parameters

key Object
the key of the mapping to remove.

Returns

the value of the removed mapping or null if no mapping for the specified key was found.

Throws

UnsupportedOperationException
if removing from this Map is not supported.

size

public abstract int size()
Returns the number of mappings in this Map.

Returns

the number of mappings in this Map.

values

public abstract Collection<V> values()

Returns a Collection of the values contained in this Map. The Collection is backed by this Map so changes to one are reflected by the other. The Collection supports Collection#remove, Collection#removeAll, Collection#retainAll, and Collection#clear operations, and it does not support Collection#add or Collection#addAll operations.

This method returns a Collection which is the subclass of AbstractCollection. The AbstractCollection#iterator method of this subclass returns a “wrapper object” over the iterator of this Map’s #entrySet(). The AbstractCollection#size method wraps this Map’s #size method and the AbstractCollection#contains method wraps this Map’s #containsValue method.

The collection is created when this method is called at first time and returned in response to all subsequent calls. This method may return different Collection when multiple calls to this method, since it has no synchronization performed.

Returns

a collection of the values contained in this map.

getOrDefault

public 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.

putIfAbsent

public 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.

remove

public default boolean remove(Object key, Object value)
Removes the entry for the specified key only if it is currently mapped to the specified value.

replace

public default boolean replace(K key, V oldValue, V newValue)
Replaces the entry for the specified key only if currently mapped to the specified value.

replace

public default V replace(K key, V value)
Replaces the entry for the specified key only if it is currently mapped to some value.

forEach

public default void forEach(BiConsumer<? super K, ? super V> action)
Performs the given action for each entry in this map.

replaceAll

public 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.

computeIfAbsent

public 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.

computeIfPresent

public 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.

compute

public 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).

merge

public 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 result of the given remapping function, or removes if the result is null.