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AbstractMap

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

java.lang.Object
   ↳ java.util.AbstractMap<K, V>
Known Direct Subclasses
Known Indirect Subclasses


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 method 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.

See also:

Summary

Nested classes

class AbstractMap.SimpleEntry<K, V>

An Entry maintaining a key and a value. 

class AbstractMap.SimpleImmutableEntry<K, V>

An Entry maintaining an immutable key and value. 

Protected constructors

AbstractMap()

Sole constructor.

Public methods

void clear()

Removes all of the mappings from this map (optional operation).

This implementation calls entrySet().clear().

boolean containsKey(Object key)

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

This implementation iterates over entrySet() searching for an entry with the specified key.

boolean containsValue(Object value)

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

This implementation iterates over entrySet() searching for an entry with the specified value.

abstract 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 the specified key is mapped, or null if this map contains no mapping for the key.

This implementation iterates over entrySet() searching for an entry with 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.

This implementation returns size() == 0.

Set<K> keySet()

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

This implementation returns a set that subclasses AbstractSet.

V put(K key, V value)

Associates the specified value with the specified key in this map (optional operation).

This implementation always throws an UnsupportedOperationException.

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

Copies all of the mappings from the specified map to this map (optional operation).

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.

V remove(Object key)

Removes the mapping for a key from this map if it is present (optional operation).

This implementation iterates over entrySet() searching for an entry with the specified key.

int size()

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

This implementation returns entrySet().size().

String toString()

Returns a string representation of this map.

Collection<V> values()

Returns a Collection view of the values contained in this map.

This implementation returns a collection that subclasses AbstractCollection.

Protected methods

Object clone()

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

Inherited methods

From class java.lang.Object
From interface java.util.Map

Protected constructors

AbstractMap

Added in API level 1
AbstractMap ()

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

Public methods

clear

Added in API level 1
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.

Throws
UnsupportedOperationException

containsKey

Added in API level 1
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 Objects.equals(key, 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.

Parameters
key Object: key whose presence in this map is to be tested
Returns
boolean true if this map contains a mapping for the specified key
Throws
ClassCastException
NullPointerException

containsValue

Added in API level 1
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 Objects.equals(value, v). This operation will probably require time linear in the map size for most implementations of the Map interface.

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.

Parameters
value Object: value whose presence in this map is to be tested
Returns
boolean true if this map maps one or more keys to the specified value
Throws
ClassCastException
NullPointerException

entrySet

Added in API level 1
Set<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
Set<Entry<K, V>> a set view of the mappings contained in this map

equals

Added in API level 1
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.

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; 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.

Parameters
o Object: object to be compared for equality with this map
Returns
boolean true if the specified object is equal to this map

get

Added in API level 1
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 Objects.equals(key, 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.

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.

Parameters
key Object: the key whose associated value is to be returned
Returns
V the value to which the specified key is mapped, or null if this map contains no mapping for the key
Throws
ClassCastException
NullPointerException

hashCode

Added in API level 1
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 hashCode().

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

Returns
int the hash code value for this map

See also:

isEmpty

Added in API level 1
boolean isEmpty ()

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

This implementation returns size() == 0.

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

keySet

Added in API level 1
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.

This implementation returns a set that subclasses 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 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.

Returns
Set<K> a set view of the keys contained in this map

put

Added in API level 1
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.)

This implementation always throws an UnsupportedOperationException.

Parameters
key K: key with which the specified value is to be associated
value V: value to be associated with the specified key
Returns
V 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
ClassCastException
NullPointerException
IllegalArgumentException

putAll

Added in API level 1
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.

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.

Parameters
m Map: mappings to be stored in this map
Throws
UnsupportedOperationException
ClassCastException
NullPointerException
IllegalArgumentException

remove

Added in API level 1
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 Objects.equals(key, 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.

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 (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.

Parameters
key Object: key whose mapping is to be removed from the map
Returns
V the previous value associated with key, or null if there was no mapping for key.
Throws
UnsupportedOperationException
ClassCastException
NullPointerException

size

Added in API level 1
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().

Returns
int the number of key-value mappings in this map

toString

Added in API level 1
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 valueOf(Object).

Returns
String a string representation of this map

values

Added in API level 1
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.

This implementation returns a collection that subclasses AbstractCollection. 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.

Returns
Collection<V> a collection view of the values contained in this map

Protected methods

clone

Added in API level 1
Object clone ()

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

Returns
Object a shallow copy of this map
Throws
CloneNotSupportedException
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