java.util

Interface Map<K,V>

Type Parameters:

K - the type of keys maintained by this map

V - the type of mapped values

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.

This interface is a member of the Java Collections Framework.

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.

Since:

1.2

See Also:

HashMap, TreeMap, Hashtable, SortedMap, Collection, Set

Nested Class Summary

Nested Classes

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

Method Summary

Modifier and Type	Method and Description
void	clear() Removes all of the mappings from this map (optional operation).
V	compute(K key, java.util.function.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).
V	computeIfAbsent(K key, java.util.function.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.
V	computeIfPresent(K key, java.util.function.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(java.lang.Object key) Returns true if this map contains a mapping for the specified key.
boolean	containsValue(java.lang.Object value) Returns true if this map maps one or more keys to the specified value.
java.util.Set<Map.Entry<K,V>>	entrySet() Returns a Set view of the mappings contained in this map.
boolean	equals(java.lang.Object o) Compares the specified object with this map for equality.
V	get(java.lang.Object key) Returns the value to which the specified key is mapped, or null 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.
java.util.Set<K>	keySet() Returns a Set view of the keys contained in this map.
V	merge(K key, V value, java.util.function.BiFunction<? super V,? super V,? extends V> remappingFunction) If the specified key is not already associated with a (non-null) value, associates it with the given value.
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).
V	putIfAbsent(K key, V value) If the specified key is not already associated with a value, associates it with the given value and returns null, else returns the current value.
V	remove(java.lang.Object key) Removes the mapping for a key from this map if it is present (optional operation).
boolean	remove(java.lang.Object key, java.lang.Object value) Removes the entry for the specified key only if it is currently mapped to the specified value.
V	replace(K key, V value) Replaces the entry for the specified key only if it is currently mapped to some value.
boolean	replace(K key, V oldValue, V newValue) Replaces the entry for the specified key only if currently mapped to the specified value.
int	size() Returns the number of key-value mappings in this map.
java.util.Collection<V>	values() Returns a Collection view of the values contained in this map.

Method Detail

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(java.lang.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:

java.lang.ClassCastException - if the key is of an inappropriate type for this map (optional)

java.lang.NullPointerException - if the specified key is null and this map does not permit null keys (optional)

containsValue

boolean containsValue(java.lang.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:

java.lang.ClassCastException - if the value is of an inappropriate type for this map (optional)

java.lang.NullPointerException - if the specified value is null and this map does not permit null values (optional)

get

V get(java.lang.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 (key==null ? k==null : key.equals(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:

java.lang.ClassCastException - if the key is of an inappropriate type for this map (optional)

java.lang.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:

java.lang.UnsupportedOperationException - if the put operation is not supported by this map

java.lang.ClassCastException - if the class of the specified key or value prevents it from being stored in this map

java.lang.NullPointerException - if the specified key or value is null and this map does not permit null keys or values

java.lang.IllegalArgumentException - if some property of the specified key or value prevents it from being stored in this map

remove

V remove(java.lang.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 (key==null ? k==null : key.equals(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:

java.lang.UnsupportedOperationException - if the remove operation is not supported by this map

java.lang.ClassCastException - if the key is of an inappropriate type for this map (optional)

java.lang.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:

java.lang.UnsupportedOperationException - if the putAll operation is not supported by this map

java.lang.ClassCastException - if the class of a key or value in the specified map prevents it from being stored in this map

java.lang.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

java.lang.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:

java.lang.UnsupportedOperationException - if the clear operation is not supported by this map

keySet

java.util.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

java.util.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

java.util.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(java.lang.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 java.lang.Object

Parameters:

o - object to be compared for equality with this map

Returns:

true if the specified object is equal to this map

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 java.lang.Object

Returns:

the hash code value for this map

See Also:

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

putIfAbsent

V putIfAbsent(K key,
            V value)

If the specified key is not already associated with a value, associates it with the given value and returns null, else returns the current value.

The default implementation is equivalent to, for this map:

 if (!map.containsKey(key))
   return map.put(key, value);
 else
   return map.get(key);

The default implementation makes no guarantees about synchronization or atomicity properties of this method. Any class overriding this method must specify its concurrency properties. In particular, all implementations of subinterface ConcurrentMap must ensure that this operation is performed atomically.

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:

java.lang.UnsupportedOperationException - if the put operation is not supported by this map

java.lang.ClassCastException - if the class of the specified key or value prevents it from being stored in this map

java.lang.NullPointerException - if the specified key or value is null, and this map does not permit null keys or values

java.lang.IllegalArgumentException - if some property of the specified key or value prevents it from being stored in this map

remove

boolean remove(java.lang.Object key,
             java.lang.Object value)

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

The default implementation is equivalent to, for this map:

 if (map.containsKey(key) && map.get(key).equals(value)) {
   map.remove(key);
   return true;
 } else
   return false;

The default implementation makes no guarantees about synchronization or atomicity properties of this method. Any class overriding this method must specify its concurrency properties. In particular, all implementations of subinterface ConcurrentMap must ensure that this operation is performed atomically.

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:

java.lang.UnsupportedOperationException - if the remove operation is not supported by this map

java.lang.ClassCastException - if the key or value is of an inappropriate type for this map (optional)

java.lang.NullPointerException - if the specified key or value is null, and this map does not permit null keys or values (optional)

replace

boolean replace(K key,
              V oldValue,
              V newValue)

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

The default implementation is equivalent to, for this map:

 if (map.containsKey(key) && map.get(key).equals(oldValue)) {
   map.put(key, newValue);
   return true;
 } else
   return false;

The default implementation makes no guarantees about synchronization or atomicity properties of this method. Any class overriding this method must specify its concurrency properties. In particular, all implementations of subinterface ConcurrentMap must ensure that this operation is performed atomically.

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:

java.lang.UnsupportedOperationException - if the put operation is not supported by this map

java.lang.ClassCastException - if the class of a specified key or value prevents it from being stored in this map

java.lang.NullPointerException - if a specified key or value is null, and this map does not permit null keys or values

java.lang.IllegalArgumentException - if some property of a specified key or value prevents it from being stored in this map

replace

V replace(K key,
        V value)

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

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 class overriding this method must specify its concurrency properties. In particular, all implementations of subinterface ConcurrentMap must ensure that this operation is performed atomically.

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:

java.lang.UnsupportedOperationException - if the put operation is not supported by this map

java.lang.ClassCastException - if the class of the specified key or value prevents it from being stored in this map

java.lang.NullPointerException - if the specified key or value is null, and this map does not permit null keys or values

java.lang.IllegalArgumentException - if some property of the specified key or value prevents it from being stored in this map

computeIfAbsent

V computeIfAbsent(K key,
                java.util.function.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.

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.putIfAbsent(key, newValue);
 }

If the function returns null no mapping is recorded. If the 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)));

The default implementation makes no guarantees about synchronization or atomicity properties of this method or the application of the mapping function. Any class overriding this method must specify 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. Any class that permits null values must document whether and how this method distinguishes absence from null mappings.

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:

java.lang.NullPointerException - if the specified key is null and this map does not support null keys, or the mappingFunction is null

java.lang.UnsupportedOperationException - if the put operation is not supported by this map

java.lang.ClassCastException - if the class of the specified key or value prevents it from being stored in this map

java.lang.RuntimeException - or Error if the mappingFunction does so, in which case the mapping is left unestablished

computeIfPresent

V computeIfPresent(K key,
                 java.util.function.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.

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.replace(key, oldValue, newValue);
   else
     map.remove(key, oldValue);
 }

In concurrent contexts, the default implementation may retry these steps when multiple threads attempt updates. If the function returns null, the mapping is removed (or remains absent if initially absent). If the function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.

The default implementation makes no guarantees about synchronization or atomicity properties of this method or the application of the remapping function. Any class overriding this method must specify its concurrency properties. In particular, all implementations of subinterface ConcurrentMap must document whether the function is applied once atomically only if the value is present. Any class that permits null values must document whether and how this method distinguishes absence from null mappings.

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:

java.lang.NullPointerException - if the specified key is null and this map does not support null keys, or the remappingFunction is null

java.lang.UnsupportedOperationException - if the put operation is not supported by this map

java.lang.ClassCastException - if the class of the specified key or value prevents it from being stored in this map

java.lang.RuntimeException - or Error if the remappingFunction does so, in which case the mapping is unchanged

compute

V compute(K key,
        java.util.function.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, msg) -> (v == null) ? msg : v.concat(msg))

(Method merge(K, V, java.util.function.BiFunction<? super V, ? super V, ? extends V>) is often simpler to use for such purposes.)

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 (newValue != null)
   map.replace(key, oldValue, newValue);
 else
   map.remove(key, oldValue);
 

In concurrent contexts, the default implementation may retry these steps when multiple threads attempt updates. If the function returns null, the mapping is removed (or remains absent if initially absent). If the function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.

The default implementation makes no guarantees about synchronization or atomicity properties of this method or the application of the remapping function. Any class overriding this method must specify its concurrency properties. In particular, all implementations of subinterface ConcurrentMap must document whether the function is applied exactly once atomically. Any class that permits null values must document whether and how this method distinguishes absence from null mappings.

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:

java.lang.NullPointerException - if the specified key is null and this map does not support null keys, or the remappingFunction is null

java.lang.UnsupportedOperationException - if the put operation is not supported by this map

java.lang.ClassCastException - if the class of the specified key or value prevents it from being stored in this map

java.lang.RuntimeException - or Error if the remappingFunction does so, in which case the mapping is unchanged

merge

V merge(K key,
      V value,
      java.util.function.BiFunction<? super V,? super V,? extends V> remappingFunction)

If the specified key is not already associated with a (non-null) value, associates it with the given value. Otherwise, replaces the value with the results of the given remapping function, or removes if 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)

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, oldValue);
 else if (oldValue == null)
   map.putIfAbsent(key, newValue);
 else
   map.replace(key, oldValue, newValue);
 

In concurrent contexts, the default implementation may retry these steps when multiple threads attempt updates. If the function returns null, the mapping is removed (or remains absent if initially absent). If the function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.

The default implementation makes no guarantees about synchronization or atomicity properties of this method or the application of the remapping function. Any class overriding this method must specify its concurrency properties. In particular, all implementations of subinterface ConcurrentMap must document whether the function is applied exactly once atomically. Any class that permits null values must document whether and how this method distinguishes absence from null mappings.

Parameters:

key - key with which the specified value is to be associated

value - the value to use if absent

remappingFunction - the function to recompute a value if present

Returns:

the new value associated with the specified key, or null if none

Throws:

java.lang.NullPointerException - if the specified key is null and this map does not support null keys, or the remappingFunction is null

java.lang.RuntimeException - or Error if the remappingFunction does so, in which case the mapping is unchanged