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java.util
Class
AbstractMap<K,V>
AbstractMap
java.lang.Object
java.util.AbstractMap
-
All Implemented Interfaces:
-
Map
-
Direct Known Subclasses:
-
ConcurrentHashMap
HashMap
,
EnumMap
,
HashMap
,
IdentityHashMap
,
TreeMap
,
WeakHashMap
-
public abstract class
AbstractMap<K,V>
AbstractMap
-
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
Constructor Summary
|
protected |
AbstractMap
()
Sole constructor. |
Method Summary
|
void |
clear
()
Removes all 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 this 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
Object
|
get
(
Object
Returns the value to which this map 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 view of the keys contained in this map. |
V
Object
|
put
put
(
K
Object
key,
V
Object
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
Object
|
remove
(
Object
Removes the mapping for this key from this map if 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
-
-
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
-
-
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 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 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
-
-
Parameters:
-
value - value whose presence in this map is to be tested.
-
Returns:
-
true
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.
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
-
-
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:
-
NullPointerException
- key is
null
and this map does not not permit
null
keys.
get
public VObjectget(Object key)
-
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
. 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
-
-
Parameters:
-
key - key whose associated value is to be returned.
-
Returns:
-
the value to which this map maps the specified key.
-
Throws:
-
NullPointerException
- if the key is
null
and this map does not not permit
null
keys.
-
See Also:
-
containsKey(Object)
put
public VObjectput( KObject key,
VObject value)
-
Associates the specified value with the specified key in this map (optional operation). If the map previously contained a mapping for this key, the old value is replaced.
This implementation always throws an
UnsupportedOperationException
.
-
-
Specified by:
-
put
put
in interface
Map
-
-
Parameters:
-
key - key with which the specified value is to be associated.
-
value - value to be associated with the specified key.
-
Returns:
-
previous value associated with specified key, or
null
if there was no mapping for key. (A
null
return can also indicate that the map previously associated
null
with the specified 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.
-
IllegalArgumentException
- if some aspect of this key or value * prevents it from being stored in this map.
-
NullPointerException
- this map does not permit
null
keys or values, and the specified key or value is
null
.
remove
public VObjectremove(Object key)
-
Removes the mapping for this key from this map if present (optional operation).
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 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
-
-
Parameters:
-
key - key whose mapping is to be removed from the map.
-
Returns:
-
previous value associated with specified key, or
null
if there was no entry for key. (A
null
return can also indicate that the map previously associated
null
with the specified key, if the implementation supports
null
values.)
-
Throws:
-
UnsupportedOperationException
- if the
remove
operation is not supported by this map.
putAll
public void putAll(Map<? extends K,? extends V> t) t)
-
Copies all of the mappings from the specified map to this map (optional operation). 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
-
-
Parameters:
-
t - 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.
-
IllegalArgumentException
- if some aspect of a key or value in the specified map prevents it from being stored in this map.
-
NullPointerException
- 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 mappings from this map (optional operation).
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
-
-
Throws:
-
UnsupportedOperationException
- clear 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, the results of the iteration are undefined.) The Set supports element removal, which removes the corresponding entry 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
-
-
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, the results of the iteration are undefined.) The collection supports element removal, which removes the corresponding entry 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 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.
-
-
Specified by:
-
values
in interface
Map
-
-
Returns:
-
a collection view of the values contained in this map.
entrySet
public abstract Set< Map.Entry< K, V>> entrySet()
-
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, the results of the iteration are undefined.) The set supports element removal, which removes the corresponding entry 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
-
-
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
t1
and
t2
represent the same mappings if
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 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 set; if not, it 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
-
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
t1.equals(t2)
implies that
t1.hashCode()==t2.hashCode()
for any two maps
t1
and
t2
, as required by the general contract of Object.hashCode.
This implementation iterates over
entrySet()
, calling
hashCode
on each element (entry) in the Collection, and adding up the results.
-
-
Specified by:
-
hashCode
in interface
Map
-
Overrides:
-
hashCode
in class
Object
-
-
Returns:
-
the hash code value for this 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)
.
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 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