AbstractMap (Java Platform SE 6) (original) (raw)

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 anUnsupportedOperationException), and the iterator returned byentrySet().iterator() must additionally implement itsremove 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.

Since:

1.2

See Also:

Map, Collection

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, returnsInteger.MAX_VALUE.

This implementation returns entrySet().size().

Specified by:

[size](../../java/util/Map.html#size%28%29) in interface [Map](../../java/util/Map.html "interface in java.util")<[K](../../java/util/AbstractMap.html "type parameter in AbstractMap"),[V](../../java/util/AbstractMap.html "type parameter in AbstractMap")>

Returns:

the number of key-value mappings in this map

isEmpty

public boolean isEmpty()

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

This implementation returns size() == 0.

Specified by:

[isEmpty](../../java/util/Map.html#isEmpty%28%29) in interface [Map](../../java/util/Map.html "interface in java.util")<[K](../../java/util/AbstractMap.html "type parameter in AbstractMap"),[V](../../java/util/AbstractMap.html "type parameter in AbstractMap")>

Returns:

true if this map contains no key-value mappings

containsValue

public 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(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.

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](../../java/util/Map.html#containsValue%28java.lang.Object%29) in interface [Map](../../java/util/Map.html "interface in java.util")<[K](../../java/util/AbstractMap.html "type parameter in AbstractMap"),[V](../../java/util/AbstractMap.html "type parameter in AbstractMap")>

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](../../java/lang/ClassCastException.html "class in java.lang") - if the value is of an inappropriate type for this map (optional)

[NullPointerException](../../java/lang/NullPointerException.html "class in java.lang") - if the specified value is null and this map does not permit null values (optional)

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](../../java/util/Map.html#containsKey%28java.lang.Object%29) in interface [Map](../../java/util/Map.html "interface in java.util")<[K](../../java/util/AbstractMap.html "type parameter in AbstractMap"),[V](../../java/util/AbstractMap.html "type parameter in AbstractMap")>

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:

[ClassCastException](../../java/lang/ClassCastException.html "class in java.lang") - if the key is of an inappropriate type for this map (optional)

[NullPointerException](../../java/lang/NullPointerException.html "class in java.lang") - if the specified key is null and this map does not permit null keys (optional)

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 keyk 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 ofnull 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.

Specified by:

[get](../../java/util/Map.html#get%28java.lang.Object%29) in interface [Map](../../java/util/Map.html "interface in java.util")<[K](../../java/util/AbstractMap.html "type parameter in AbstractMap"),[V](../../java/util/AbstractMap.html "type parameter in AbstractMap")>

Parameters:

key - the key whose associated value is to be returned

Returns:

the value to which the specified key is mapped, ornull if this map contains no mapping for the key

Throws:

[ClassCastException](../../java/lang/ClassCastException.html "class in java.lang") - if the key is of an inappropriate type for this map (optional)

[NullPointerException](../../java/lang/NullPointerException.html "class in java.lang") - 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 mapm is said to contain a mapping for a key k if and only if m.containsKey(k) would returntrue.)

This implementation always throws anUnsupportedOperationException.

Specified by:

[put](../../java/util/Map.html#put%28K, V%29) in interface [Map](../../java/util/Map.html "interface in java.util")<[K](../../java/util/AbstractMap.html "type parameter in AbstractMap"),[V](../../java/util/AbstractMap.html "type parameter in AbstractMap")>

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, ornull 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](../../java/lang/UnsupportedOperationException.html "class in java.lang") - if the put operation is not supported by this map

[ClassCastException](../../java/lang/ClassCastException.html "class in java.lang") - if the class of the specified key or value prevents it from being stored in this map

[NullPointerException](../../java/lang/NullPointerException.html "class in java.lang") - if the specified key or value is null and this map does not permit null keys or values

[IllegalArgumentException](../../java/lang/IllegalArgumentException.html "class in java.lang") - if some property of the specified key or value prevents it from being stored in this map

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.)

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 ofnull 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'sremove 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 anUnsupportedOperationException if the entrySet iterator does not support the remove method and this map contains a mapping for the specified key.

Specified by:

[remove](../../java/util/Map.html#remove%28java.lang.Object%29) in interface [Map](../../java/util/Map.html "interface in java.util")<[K](../../java/util/AbstractMap.html "type parameter in AbstractMap"),[V](../../java/util/AbstractMap.html "type parameter in AbstractMap")>

Parameters:

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

Returns:

the previous value associated with key, ornull if there was no mapping for key.

Throws:

[UnsupportedOperationException](../../java/lang/UnsupportedOperationException.html "class in java.lang") - if the remove operation is not supported by this map

[ClassCastException](../../java/lang/ClassCastException.html "class in java.lang") - if the key is of an inappropriate type for this map (optional)

[NullPointerException](../../java/lang/NullPointerException.html "class in java.lang") - if the specified key is null and this map does not permit null keys (optional)

putAll

public 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)](../../java/util/Map.html#put%28K, V%29) 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'sentrySet() collection, and calls this map's put operation once for each entry returned by the iteration.

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

Specified by:

[putAll](../../java/util/Map.html#putAll%28java.util.Map%29) in interface [Map](../../java/util/Map.html "interface in java.util")<[K](../../java/util/AbstractMap.html "type parameter in AbstractMap"),[V](../../java/util/AbstractMap.html "type parameter in AbstractMap")>

Parameters:

m - mappings to be stored in this map

Throws:

[UnsupportedOperationException](../../java/lang/UnsupportedOperationException.html "class in java.lang") - if the putAll operation is not supported by this map

[ClassCastException](../../java/lang/ClassCastException.html "class in java.lang") - if the class of a key or value in the specified map prevents it from being stored in this map

[NullPointerException](../../java/lang/NullPointerException.html "class in java.lang") - 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

[IllegalArgumentException](../../java/lang/IllegalArgumentException.html "class in java.lang") - if some property of a key or value in the specified map prevents it from being stored in this map

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 anUnsupportedOperationException if the entrySet does not support the clear operation.

Specified by:

[clear](../../java/util/Map.html#clear%28%29) in interface [Map](../../java/util/Map.html "interface in java.util")<[K](../../java/util/AbstractMap.html "type parameter in AbstractMap"),[V](../../java/util/AbstractMap.html "type parameter in AbstractMap")>

Throws:

[UnsupportedOperationException](../../java/lang/UnsupportedOperationException.html "class in java.lang") - if the clear operation is not supported by this map

keySet

public 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 theIterator.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 thecontains method delegates to this map'scontainsKey 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.

Specified by:

[keySet](../../java/util/Map.html#keySet%28%29) in interface [Map](../../java/util/Map.html "interface in java.util")<[K](../../java/util/AbstractMap.html "type parameter in AbstractMap"),[V](../../java/util/AbstractMap.html "type parameter in AbstractMap")>

Returns:

a set view of the keys contained in this map

values

public 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'scontainsValue 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.

Specified by:

[values](../../java/util/Map.html#values%28%29) in interface [Map](../../java/util/Map.html "interface in java.util")<[K](../../java/util/AbstractMap.html "type parameter in AbstractMap"),[V](../../java/util/AbstractMap.html "type parameter in AbstractMap")>

Returns:

a collection view of the values contained in this map

entrySet

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

Description copied from interface: [Map](../../java/util/Map.html#entrySet%28%29)

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 thesetValue 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 andclear operations. It does not support theadd or addAll operations.

Specified by:

[entrySet](../../java/util/Map.html#entrySet%28%29) in interface [Map](../../java/util/Map.html "interface in java.util")<[K](../../java/util/AbstractMap.html "type parameter in AbstractMap"),[V](../../java/util/AbstractMap.html "type parameter in AbstractMap")>

Returns:

a set view of the mappings contained in this map

equals

public boolean equals(Object o)

Compares the specified object with this map for equality. Returnstrue if the given object is also a map and the two maps represent the same mappings. More formally, two maps m1 andm2 represent the same mappings ifm1.entrySet().equals(m2.entrySet()). This ensures that theequals 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'sentrySet 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](../../java/util/Map.html#equals%28java.lang.Object%29) in interface [Map](../../java/util/Map.html "interface in java.util")<[K](../../java/util/AbstractMap.html "type parameter in AbstractMap"),[V](../../java/util/AbstractMap.html "type parameter in AbstractMap")>

Overrides:

[equals](../../java/lang/Object.html#equals%28java.lang.Object%29) in class [Object](../../java/lang/Object.html "class in java.lang")

Parameters:

o - object to be compared for equality with this map

Returns:

true if the specified object is equal to this 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'sentrySet() view. This ensures that m1.equals(m2) implies that m1.hashCode()==m2.hashCode() for any two mapsm1 and m2, as required by the general contract ofObject.hashCode().

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

Specified by:

[hashCode](../../java/util/Map.html#hashCode%28%29) in interface [Map](../../java/util/Map.html "interface in java.util")<[K](../../java/util/AbstractMap.html "type parameter in AbstractMap"),[V](../../java/util/AbstractMap.html "type parameter in AbstractMap")>

Overrides:

[hashCode](../../java/lang/Object.html#hashCode%28%29) in class [Object](../../java/lang/Object.html "class in java.lang")

Returns:

the hash code value for this map

See Also:

Map.Entry.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 byString.valueOf(Object).

Overrides:

[toString](../../java/lang/Object.html#toString%28%29) in class [Object](../../java/lang/Object.html "class in java.lang")

Returns:

a string representation of this 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](../../java/lang/Object.html#clone%28%29) in class [Object](../../java/lang/Object.html "class in java.lang")

Returns:

a shallow copy of this map

Throws:

[CloneNotSupportedException](../../java/lang/CloneNotSupportedException.html "class in java.lang") - 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

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