Collections, Maps and Iterators - Java - page 864

Display 16.9 Method Headings in the Map<K,V> Interface (part 2 of 3)
METHODS
boolean isEmpty( )
Returns true if the calling object is empty; otherwise returns false .
public boolean containsValue(Object value)
Returns true if the calling object contains at least one or more keys that map to an instance of
value .
public boolean containsKey(Object key)
Returns true if the calling object contains key as one of its keys.
public boolean equals(Object other)
This is the equals of the map, not the equals of the elements in the map. Overrides the inher-
ited method equals .
public int size( )
Returns the number of (key, value) mappings in the calling object.
public int hashCode( )
Returns the hash code value for the calling object.
public Set<Map.Entry<K,V>> entrySet( )
Returns a set view consisting of (key, value) mappings for all entries in the map. Changes to the
map are reflected in the set and vice-versa.
public Collection<V> values( )
Returns a collection view consisting of all values in the map. Changes to the map are reflected in
the collection and vice-versa.
public V get(Object key)
Returns the value to which the calling object maps key . If key is not in the map, then null is
returned. Note that this does not always mean that the key is not in the map since it is possible to
map a key to null . The containsKey method can be used to distinguish the two cases.
OPTIONAL METHODS
The following methods are optional, which means they still must be implemented, but the imple-
mentation can simply throw an UnsupportedOperationException if, for some reason, you do
not want to give the methods a “real” implementation. An UnsupportedOperationException is
a RunTimeException and so is not required to be caught or declared in a throws clause.
public V put(K key, V value) (Optional)
Associates key to value in the map. If key was associated with an existing value then the old
value is overwritten and returned. Otherwise null is returned.