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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
method
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
Java Collections Framework
.
-
Since:
-
1.2
-
See Also:
-
Map
,
Collection
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
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. |
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
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
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
in interface
Map
<
K
,
V
>
-
-
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
- 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)
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
-
Returns:
-
true
if this map contains a mapping for the specified 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)
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
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
in interface
Map
<
K
,
V
>
-
-
Parameters:
-
key - the key whose associated value is to be returned
-
Returns:
-
the value to which the specified key is mapped, or null if this map contains no mapping for the 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)
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
.)
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
-
value - value to be associated with the specified key
-
Returns:
-
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
- if the
put
operation is not supported by this map
-
ClassCastException
- if the class of the specified key or value prevents it from being stored in this map
-
NullPointerException
- if the specified key or value is null and this map does not permit null keys or values
-
IllegalArgumentException
- 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 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.
-
-
Specified by:
-
remove
in interface
Map
<
K
,
V
>
-
-
Parameters:
-
key - key whose mapping is to be removed from the map
-
Returns:
-
the previous value associated with
key
, or
null
if there was no mapping for
key
.
-
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)
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)
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.
-
-
Specified by:
-
putAll
in interface
Map
<
K
,
V
>
-
-
Parameters:
-
m - mappings to be stored in this map
-
Throws:
-
UnsupportedOperationException
- if the
putAll
operation is not supported by this map
-
ClassCastException
- if the class of a key or value in the specified map prevents it from being stored in this map
-
NullPointerException
- 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
- 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 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
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 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.
-
-
Specified by:
-
keySet
in interface
Map
<
K
,
V
>
-
-
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'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.
-
-
Specified by:
-
values
in interface
Map
<
K
,
V
>
-
-
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
-
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.
-
-
Specified by:
-
entrySet
in interface
Map
<
K
,
V
>
-
-
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. 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.
-
-
Specified by:
-
equals
in interface
Map
<
K
,
V
>
-
Overrides:
-
equals
in class
Object
-
-
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'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
Object.hashCode()
.
This implementation iterates over
entrySet()
, calling
hashCode()
on each element (entry) in the set, 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
-
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 by
String.valueOf(Object)
.
-
-
Overrides:
-
toString
in class
Object
-
-
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
in class
Object
-
-
Returns:
-
a shallow copy of this 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