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 a 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 SDK 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 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.

Since: 1.2

Version: 1.43, 06/22/03

Author: Josh Bloch

Nested Class Summary
static interface	Map.Entry<K,V> A map entry (key-value pair).

Method Summary
void	clear() Removes all mappings from this map (optional operation).
boolean	containsKey(Object key) Returns `true` if this map contains a mapping for the specified key.
boolean	containsValue(Object value) Returns `true` if this map maps one or more keys to the specified value.
Set<Entry<K,V>>	entrySet() Returns a set view of the mappings contained in this map.
boolean	equals(Object o) Compares the specified object with this map for equality.
V	get(Object key) Returns the value to which this map maps the specified key.
int	hashCode() Returns the hash code value for this map.
boolean	isEmpty() Returns `true` if this map contains no key-value mappings.
Set<K>	keySet() Returns a set view of the keys contained in this map.
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> t) Copies all of the mappings from the specified map to this map (optional operation).
V	remove(Object key) Removes the mapping for this key from this map if it is present (optional operation).
int	size() Returns the number of key-value mappings in this map.
Collection<V>	values() Returns a collection view of the values contained in this map.

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 value is null and this map does not not permit null values (optional).

entrySet

equals

get

Returns the value to which this map maps the specified key. Returns null if the map contains no mapping for this key. 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.

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.)

Parameters: key key whose associated value is to be returned.

Returns: the value to which this map maps the specified key, or null if the map contains no mapping for this key.

Throws: ClassCastException if the key is of an inappropriate type for this map (optional). NullPointerException key is null and this map does not not permit null keys (optional).

Interface Map<K,V>