java.util
Class AbstractMap<K,V>

java.lang.Object
  java.util.AbstractMap<K,V>

Type Parameters:

K - the type of keys maintained by this map

V - the type of mapped values

All Implemented Interfaces:

Map <K,V>

Direct Known Subclasses:

ConcurrentHashMap , ConcurrentSkipListMap , EnumMap , HashMap , IdentityHashMap , TreeMap , WeakHashMap

public abstract class AbstractMap<K,V>
extends Object
implements Map<K,V>

This class provides a skeletal implementation of the Map interface, to minimize the effort required to implement this interface.

To implement an unmodifiable map, the programmer needs only to extend this class and provide an implementation for the entrySet method, which returns a set-view of the map's mappings. Typically, the returned set will, in turn, be implemented atop AbstractSet . This set should not support the add or remove methods, and its iterator should not support the remove method.

To implement a modifiable map, the programmer must additionally override this class's put method (which otherwise throws an UnsupportedOperationException ), and the iterator returned by entrySet().iterator() must additionally implement its remove method.

The programmer should generally provide a void (no argument) and map constructor, as per the recommendation in the Map interface specification.

The documentation for each non-abstract methods in this class describes its implementation in detail. Each of these methods may be overridden if the map being implemented admits a more efficient implementation.

This class is a member of the Java Collections Framework .

Since:

1.2

See Also:

Map , Collection

Nested Class Summary
static class	AbstractMap.SimpleEntry < K , V           An Entry maintaining a key and a value.
static class	AbstractMap.SimpleImmutableEntry < K , V           An Entry maintaining an immutable key and value.

 

Nested classes/interfaces inherited from interface java.util. Map
Map.Entry < K , V >

 

Constructor Summary
protected	AbstractMap ()           Sole constructor.

 

Method Summary
void	clear ()           Removes all of the mappings from this map (optional operation).
protected   Object	clone ()           Returns a shallow copy of this AbstractMap instance: the keys and values themselves are not cloned.
boolean	containsKey ( Object  key)           Returns true if this map contains a mapping for the specified key.
boolean	containsValue ( Object           Returns true if this map maps one or more keys to the specified this value.
abstract   Set < Map.Entry < K , V	entrySet ()           Returns a Set 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           Returns the value to which the specified key is mapped, or null if this map contains no mapping for 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 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           Copies all of the mappings from the specified map to this map (optional operation).
V	remove ( Object           Removes the mapping for a this key from this map if it is present (optional operation).
int	size ()           Returns the number of key-value mappings in this map.
String	toString ()           Returns a string representation of this map.
Collection < V	values ()           Returns a Collection collection view of the values contained in this map.

 

Methods inherited from class java.lang. Object
finalize , getClass , notify , notifyAll , wait , wait , wait

 

Constructor Detail

AbstractMap

protected AbstractMap()

Sole constructor. (For invocation by subclass constructors, typically implicit.)

size

public 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 .

This implementation returns entrySet().size() .

Specified by:

size in interface Map < K , V >

Returns:

the number of key-value mappings in this map map.

isEmpty

public boolean isEmpty()

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

This implementation returns size() == 0 .

Specified by:

isEmpty in interface Map < K , V >

Returns:

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

containsValue

public boolean containsValue(Object value)

Returns true if this map maps one or more keys to the specified this 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. map.

This implementation iterates over entrySet() searching for an entry with the specified value. If such an entry is found, true is returned. If the iteration terminates without finding such an entry, false is returned. Note that this implementation requires linear time in the size of the map.

Specified by:

containsValue in interface Map < K , V >

Parameters:

value - value whose presence in this map is to be tested 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) if this map maps one or more keys to this value.

containsKey

public 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.)

This implementation iterates over entrySet() searching for an entry with the specified key. If such an entry is found, true is returned. If the iteration terminates without finding such an entry, false is returned. Note that this implementation requires linear time in the size of the map; many implementations will override this method.

Specified by:

containsKey in interface Map < K , V >

Parameters:

key - key whose presence in this map is to be tested tested.

Returns:

true if this map contains a mapping for the specified key 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) keys.

get

public 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 (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 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. null . The containsKey containsKey operation may be used to distinguish these two cases.

This implementation iterates over entrySet() searching for an entry with the specified key. If such an entry is found, the entry's value is returned. If the iteration terminates without finding such an entry, null is returned. Note that this implementation requires linear time in the size of the map; many implementations will override this method.

Specified by:

get in interface Map < K , V >

Parameters:

key - the key whose associated value is to be returned returned.

Returns:

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

Throws:

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

See Also:

NullPointerException containsKey(Object) - if the specified key is null and this map does not permit null keys (optional)

put

public 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 .) If the map previously contained a mapping for this key, the old value is replaced.

This implementation always throws an UnsupportedOperationException .

Specified by:

put in interface Map < K , V >

Parameters:

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

value - value to be associated with the specified key key.

Returns:

the previous value associated with specified key, or key , or null if there was no mapping for key . key. (A null return can also indicate that the map previously associated null with with the specified key, if the implementation supports key , if the implementation supports null values.)

Throws:

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

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

NullPointerException IllegalArgumentException - if the specified key or value is null and this map does not permit null keys or values - if some aspect of this key or value * prevents it from being stored in this map.

IllegalArgumentException NullPointerException - if some property of the specified key or value prevents it from being stored in this map - if this map does not permit null keys or values, and the specified key or value is null .

remove

public 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 (key==null ? k==null : key.equals(k)), that mapping is removed. (The map can contain at most one such mapping.) Removes the mapping for this key from this map if present (optional operation).

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.

This implementation iterates over entrySet() searching for an entry with the specified key. If such an entry is found, its value is obtained with its getValue operation, the entry is removed from the collection Collection (and the backing map) with the iterator's remove operation, and the saved value is returned. If the iteration terminates without finding such an entry, null is returned. Note that this implementation requires linear time in the size of the map; many implementations will override this method.

Note that this implementation throws an UnsupportedOperationException if the entrySet iterator does not support the remove method and this map contains a mapping for the specified key.

Specified by:

remove in interface Map < K , V >

Parameters:

key - key whose mapping is to be removed from the map map.

Returns:

the previous value associated with specified key, or key null , or if there was no entry for key. (A null if there was no mapping for return can also indicate that the map previously associated key null . with the specified key, if the implementation supports null values.)

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) operation is not supported by this map.

putAll

public void putAll(Map<? extends K,? extends V> m) > t) 

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. These mappings will replace any mappings that this map had for any of the keys currently in the specified map.

This implementation iterates over the specified map's entrySet() collection, and calls this map's put operation once for each entry returned by the iteration.

Note that this implementation throws an UnsupportedOperationException if this map does not support the put operation and the specified map is nonempty.

Specified by:

putAll in interface Map < K , V >

Parameters:

m t - mappings to be stored in this map map.

Throws:

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

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

NullPointerException IllegalArgumentException - 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 - if some aspect of a key or value in the specified map prevents it from being stored in this map.

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

clear

public void clear()

Removes all of the mappings from this map (optional operation). The map will be empty after this call returns.

This implementation calls entrySet().clear() .

Note that this implementation throws an UnsupportedOperationException if the entrySet does not support the clear operation.

Specified by:

clear in interface Map < K , V >

Throws:

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

keySet

public Set<K> keySet()

Returns a Set Set view of the keys contained in this map. The set Set is backed by the map, so changes to the map are reflected in the set, Set, and vice-versa. If (If the map is modified while an iteration over the set Set is in progress (except through progress, the iterator's own remove operation), the results of the iteration are undefined. undefined.) The set Set supports element removal, which removes the corresponding mapping entry from the map, via the Iterator.remove , Set.remove , Iterator.remove, Set.remove, removeAll , retainAll , retainAll, and clear operations. It does not support the add or addAll operations.

This implementation returns a set Set that subclasses AbstractSet . AbstractSet. The subclass's iterator method returns a "wrapper object" over this map's entrySet() iterator. The size method delegates to this map's size method and the contains method delegates to this map's containsKey method.

The set Set is created the first time this method is called, and returned in response to all subsequent calls. No synchronization is performed, so there is a slight chance that multiple calls to this method will not all return the same set. Set.

Specified by:

keySet in interface Map < K , V >

Returns:

a set Set view of the keys contained in this map map.

values

public Collection<V> values()

Returns a Collection 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 (If the map is modified while an iteration over the collection is in progress, the results of the iteration are undefined.) The collection supports element removal, which removes the corresponding entry from the map, via the 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.

This implementation returns a collection that subclasses AbstractCollection . abstract collection. The subclass's iterator method returns a "wrapper object" over this map's entrySet() iterator. The size method delegates to this map's size method and the contains method delegates to this map's containsValue method.

The collection is created the first time this method is called, and returned in response to all subsequent calls. No synchronization is performed, so there is a slight chance that multiple calls to this method will not all return the same collection. Collection.

Specified by:

values in interface Map < K , V >

Returns:

a collection view of the values contained in this map map.

entrySet

public abstract Set<Map.Entry<K,V>> entrySet()

Description copied from interface: Map

Returns a Set view of the mappings contained in this map. Returns a set view of the mappings contained in this map. Each element in this set is a Map.Entry. 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 (If the map is modified while an iteration over the set is in progress, the results of the iteration are undefined.) The set supports element removal, which removes the corresponding entry from the map, via the 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.

Specified by:

entrySet in interface Map < K , V >

Returns:

a set view of the mappings contained in this map map.

equals

public 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 t1 and m2 t2 represent the same mappings if m1.entrySet().equals(m2.entrySet()) t1.keySet().equals(t2.keySet()) and for every key k in t1.keySet() , (t1.get(k)==null ? t2.get(k)==null : t1.get(k).equals(t2.get(k))) . This ensures that the equals method works properly across different implementations of the Map map interface.

This implementation first checks if the specified object is this map; if so it returns true . Then, it checks if the specified object is a map whose size is identical to the size of this map; set; if not, it returns false . If so, it iterates over this map's entrySet collection, and checks that the specified map contains each mapping that this map contains. If the specified map fails to contain such a mapping, false is returned. If the iteration completes, true is returned.

Specified by:

equals in interface Map < K , V >

Overrides:

equals in class Object

Parameters:

o - object to be compared for equality with this map map.

Returns:

true if the specified object is equal to this map map.

See Also:

Object.hashCode() , Hashtable

hashCode

public 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) t1.equals(t2) implies that m1.hashCode()==m2.hashCode() t1.hashCode()==t2.hashCode() for any two maps m1 t1 and m2 t2 , as required by the general contract of Object.hashCode() . Object.hashCode.

This implementation iterates over entrySet() , calling hashCode() hashCode on each element (entry) in the set, Collection, and adding up the results.

Specified by:

hashCode in interface Map < K , V >

Overrides:

hashCode in class Object

Returns:

the hash code value for this map map.

See Also:

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

toString

public String toString()

Returns a string representation of this map. The string representation consists of a list of key-value mappings in the order returned by the map's entrySet view's iterator, enclosed in braces ( "{}" ). Adjacent mappings are separated by the characters ", " (comma and space). Each key-value mapping is rendered as the key followed by an equals sign ( "=" ) followed by the associated value. Keys and values are converted to strings as by String.valueOf(Object) String.valueOf(Object) .

This implementation creates an empty string buffer, appends a left brace, and iterates over the map's entrySet view, appending the string representation of each map.entry in turn. After appending each entry except the last, the string ", " is appended. Finally a right brace is appended. A string is obtained from the stringbuffer, and returned.

Overrides:

toString in class Object

Returns:

a string String representation of this map map.

clone

protected Object clone()
                throws CloneNotSupportedException

Returns a shallow copy of this AbstractMap instance: the keys and values themselves are not cloned.

Overrides:

clone in class Object

Returns:

a shallow copy of this map map.

Throws:

CloneNotSupportedException - if the object's class does not support the Cloneable interface. Subclasses that override the clone method can also throw this exception to indicate that an instance cannot be cloned.

See Also:

Cloneable