K - the type of keys maintained by this mapV - the type of mapped valuespublic abstract class AbstractMap<K,V> extends Object implements Map<K,V>
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
Map,
Collection| Modifier and Type | Class and Description |
|---|---|
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.
|
| Modifier | Constructor and Description |
|---|---|
protected |
AbstractMap()
Sole constructor.
|
| Modifier and Type | Method and Description |
|---|---|
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 value)
Returns
true if this map maps one or more keys to the specified value. |
abstract Set<Map.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. |
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> m)
Copies all of the mappings from the specified map to this map (optional operation).
|
V |
remove(Object key)
Removes the mapping for a 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 view of the values contained in this map. |
protected AbstractMap()
public void clear()
This implementation calls entrySet().clear().
Note that this implementation throws an UnsupportedOperationException if the
entrySet does not support the clear operation.
clear in interface Map<K,V>UnsupportedOperationException - if the clear operation is not supported by this mapprotected Object clone() throws CloneNotSupportedException
AbstractMap instance: the keys and values themselves are
not cloned.clone in class ObjectCloneNotSupportedException - 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.Cloneablepublic boolean containsKey(Object key)
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.
containsKey in interface Map<K,V>key - key whose presence in this map is to be testedtrue if this map contains a mapping for the specified keyClassCastException - 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)public boolean containsValue(Object value)
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.
containsValue in interface Map<K,V>value - value whose presence in this map is to be testedtrue if this map maps one or more keys to the specified valueClassCastException - 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)public abstract Set<Map.Entry<K,V>> entrySet()
MapSet 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.public boolean equals(@Nullable Object o)
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.
@Nullable public V get(Object key)
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 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.
get in interface Map<K,V>key - the key whose associated value is to be returnednull if this map contains no
mapping for the keyClassCastException - 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)public int hashCode()
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 Object.hashCode().
This implementation iterates over entrySet(), calling hashCode() on each element (entry) in the set, and adding up the results.
hashCode in interface Map<K,V>hashCode in class ObjectMap.Entry.hashCode(),
Object.equals(Object),
Set.equals(Object)public boolean isEmpty()
true if this map contains no key-value mappings.
This implementation returns size() == 0.
public Set<K> keySet()
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.
@Nullable public V put(K key, V value)
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.
put in interface Map<K,V>key - key with which the specified value is to be associatedvalue - value to be associated with the specified keykey, 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.)UnsupportedOperationException - if the put operation is not supported by this mapClassCastException - if the class of the specified key or value prevents it from being stored in this mapNullPointerException - if the specified key or value is null and this map does not permit null keys or valuesIllegalArgumentException - if some property of the specified key or value prevents it from being stored in this mappublic void putAll(Map<? extends K,? extends V> m)
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.
putAll in interface Map<K,V>m - mappings to be stored in this mapUnsupportedOperationException - if the putAll operation is not supported by this mapClassCastException - if the class of a key or value in the specified map prevents it from being stored in this
mapNullPointerException - 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 valuesIllegalArgumentException - if some property of a key or value in the specified map prevents it from being stored in
this map@Nullable public V remove(Object 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 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.
remove in interface Map<K,V>key - key whose mapping is to be removed from the mapkey, or null if there was no mapping
for key.UnsupportedOperationException - if the remove operation is not supported by this mapClassCastException - 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)public int size()
Integer.MAX_VALUE elements, returns Integer.MAX_VALUE.
This implementation returns entrySet().size().
public String toString()
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).public Collection<V> values()
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.