See also:

• HashMap
• TreeMap
• Hashtable
• SortedMap
• Collection
• Set

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). 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))

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.

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.

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.

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.

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.

containsKey

public abstract 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.)

containsValue

public abstract 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.

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.

entry

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

Returns an unmodifiable 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.

entrySet

public abstract Set<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.

equals

public abstract 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.

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.

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 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.

getOrDefault

public 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.

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'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.

isEmpty

public abstract boolean isEmpty ()

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

keySet

public abstract 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.

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. 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.

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.

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 map m is said to contain a mapping for a key k if and only if m.containsKey(k) would return true.)

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) 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 defined encounter order, processing of its mappings generally occurs in that order.

putIfAbsent

public 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 boolean remove (Object key, 
                Object value)

Removes the entry for the specified key only if it is currently mapped to the specified value.

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 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.

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.

replace

public V replace (K key, 
                V value)

Replaces the entry for the specified key only if it is currently mapped to some value.

replaceAll

public 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.

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, returns Integer.MAX_VALUE.

values

public abstract 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.