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.

This interface is a member of the Java Collections Framework.

Methods

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 (key==null ? k==null : key.equals(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 (value==null ? v==null : value.equals(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)

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)

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:

Object.hashCode(), HashMap

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:

Map.Entry.hashCode(), Object.equals(Object), equals(Object)

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

NullPointerException - if the specified function is null, or the specified replacement value is null, and this map does not permit null values

ClassCastException - if a replacement value is of an inappropriate type for this map (optional)

NullPointerException - if function or a replacement value is null, and this map does not permit null keys or 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), value)) {
     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)

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

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

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 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 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 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)

Since:

1.8

computeIfPresent

default V computeIfPresent(K key,
                           BiFunction<? super K,? super V,? extends V> remappingFunction)

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 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 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 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)

Since:

1.8

compute

default V compute(K key,
                  BiFunction<? super K,? super V,? extends V> remappingFunction)

map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))

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 = remappingFunction.apply(key, oldValue);
 if (oldValue != null ) {
    if (newValue != null)
       map.put(key, newValue);
    else
       map.remove(key);
 } else {
    if (newValue != null)
       map.put(key, newValue);
    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. In particular, all implementations of subinterface ConcurrentMap must document whether the 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 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)

Since:

1.8

merge

default V merge(K key,
                V value,
                BiFunction<? super V,? super V,? extends V> remappingFunction)

map.merge(key, msg, String::concat)

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 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 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 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)

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