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java.util
Interface Map<K,V>
-
-
Type Parameters:
All Known Subinterfaces:
-
K - the type of keys maintained by this map
-
V - the type of mapped values
ConcurrentMap
<K,V>,
SortedMap
<K,V>
-
All Known
Subinterfaces:
Implementing Classes:
-
ConcurrentMap
AbstractMap
<K,V>,
ConcurrentNavigableMap
,
Attributes
<K,V>,
NavigableMap
,
AuthProvider
<K,V>,
SortedMap
,
ConcurrentHashMap
<K,V>
,
EnumMap
,
HashMap
,
Hashtable
,
IdentityHashMap
,
LinkedHashMap
,
PrinterStateReasons
,
Properties
,
Provider
,
RenderingHints
,
TabularDataSupport
,
TreeMap
,
UIDefaults
,
WeakHashMap
-
All Known Implementing Classes:
-
AbstractMap
,
Attributes
,
AuthProvider
,
ConcurrentHashMap
,
ConcurrentSkipListMap
,
EnumMap
,
HashMap
,
Hashtable
,
IdentityHashMap
,
LinkedHashMap
,
PrinterStateReasons
,
Properties
,
Provider
,
RenderingHints
,
TreeMap
,
UIDefaults
,
WeakHashMap
-
public interface Map<K,V>
An object that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value.
This interface takes the place of the
Dictionary
class, which was a totally abstract class rather than an interface.
The
Map
interface provides three
collection views
, which allow a map's contents to be viewed as a set of keys, collection of values, or set of key-value mappings. The
order
of a map is defined as the order in which the iterators on the map's collection views return their elements. Some map implementations, like the
TreeMap
class, make specific guarantees as to their order; others, like the
HashMap
class, do not.
Note: great care must be exercised if mutable objects are used as map keys. The behavior of a map is not specified if the value of an object is changed in a manner that affects
equals
comparisons while the object is a key in the map. A special case of this prohibition is that it is not permissible for a map to contain itself as a key. While it is permissible for a map to contain itself as a value, extreme caution is advised: the
equals
and
hashCode
methods are no longer well defined on
a
such a map.
All general-purpose map implementation classes should provide two "standard" constructors: a void (no arguments) constructor which creates an empty map, and a constructor with a single argument of type
Map
, which creates a new map with the same key-value mappings as its argument. In effect, the latter constructor allows the user to copy any map, producing an equivalent map of the desired class. There is no way to enforce this recommendation (as interfaces cannot contain constructors) but all of the general-purpose map implementations in the JDK comply.
The "destructive" methods contained in this interface, that is, the methods that modify the map on which they operate, are specified to throw
UnsupportedOperationException
if this map does not support the operation. If this is the case, these methods may, but are not required to, throw an
UnsupportedOperationException
if the invocation would have no effect on the map. For example, invoking the
putAll(Map)
method on an unmodifiable map may, but is not required to, throw the exception if the map whose mappings are to be "superimposed" is empty.
Some map implementations have restrictions on the keys and values they may contain. For example, some implementations prohibit null keys and values, and some have restrictions on the types of their keys. Attempting to insert an ineligible key or value throws an unchecked exception, typically
NullPointerException
or
ClassCastException
. Attempting to query the presence of an ineligible key or value may throw an exception, or it may simply return false; some implementations will exhibit the former behavior and some will exhibit the latter. More generally, attempting an operation on an ineligible key or value whose completion would not result in the insertion of an ineligible element into the map may throw an exception or it may succeed, at the option of the implementation. Such exceptions are marked as "optional" in the specification for this interface.
This interface is a member of the
Java Collections Framework
.
Many methods in Collections Framework interfaces are defined in terms of the
equals
method. For example, the specification for the
containsKey(Object key)
contains(Object key)
method says: "returns
true
if and only if this map
contains
contain
a mapping for a key
k
such that
(key==null ? k==null : key.equals(k))
." This specification should
not
be construed to imply that invoking
Map.containsKey
with a non-null argument
key
will cause
key.equals(k)
to be invoked for any key
k
. Implementations are free to implement optimizations whereby the
equals
invocation is avoided, for example, by first comparing the hash codes of the two keys. (The
Object.hashCode()
specification guarantees that two objects with unequal hash codes cannot be equal.) More generally, implementations of the various Collections Framework interfaces are free to take advantage of the specified behavior of underlying
Object
methods wherever the implementor deems it appropriate.
-
Since:
-
1.2
-
See Also:
-
HashMap
,
TreeMap
,
Hashtable
,
SortedMap
,
Collection
,
Set
Nested Class Summary
|
static interface |
Map.Entry
<
K
,
V
>
A map entry (key-value pair). |
Method Summary
|
void |
clear
()
Removes all
of the
mappings from this map (optional operation). |
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. |
Set
<
Map.Entry
<
K
,
V
|
entrySet
()
Returns a
Set
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
Returns the value to which
the specified key is mapped, or null if
this map
contains no mapping for
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
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
Copies all of the mappings from the specified map to this map (optional operation). |
V
|
remove
(
Object
Removes the mapping for
a
this
key from this map if it is present (optional operation). |
int |
size
()
Returns the number of key-value mappings in this map. |
Collection
<
V
|
values
()
Returns a
Collection
collection view of the values contained in this map.
|
size
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
.
-
-
Returns:
-
the number of key-value mappings in this
map
map.
isEmpty
boolean isEmpty()
-
Returns
true
if this map contains no key-value mappings.
-
-
Returns:
-
true
if this map contains no key-value
mappings
mappings.
containsKey
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.)
-
-
Parameters:
-
key - key whose presence in this map is to be
tested
tested.
-
Returns:
-
true
if this map contains a mapping for the specified
key
key.
-
Throws:
-
ClassCastException
- if the key is of an inappropriate type for this map
(optional)
(optional).
-
NullPointerException
- if the
specified
key is
null
and this map does not permit
null
keys
(optional)
(optional).
containsValue
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.
-
-
Parameters:
-
value - value whose presence in this map is to be
tested
tested.
-
Returns:
-
true
if this map maps one or more keys to the specified
value
value.
-
Throws:
-
ClassCastException
- if the value is of an inappropriate type for this map
(optional)
(optional).
-
NullPointerException
- if the
specified
value is
null
and this map does not permit
null
values
(optional)
(optional).
get
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
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.
null
.
The
containsKey
containsKey
operation may be used to distinguish these two cases.
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.)
-
-
Parameters:
-
key -
the
key whose associated value is to be
returned
returned.
-
Returns:
-
the value to which
this map maps
the specified
key is mapped,
key,
or
null
if
this
the
map contains no mapping for
the key
this key.
-
Throws:
-
ClassCastException
- if the key is of an inappropriate type for this map
(optional)
(optional).
-
NullPointerException
- if the
specified
key is
null
and this map does not permit
null
keys
(optional)
(optional).
-
See Also:
-
containsKey(Object)
put
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
this
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
.)
.))
-
-
Parameters:
-
key - key with which the specified value is to be
associated
associated.
-
value - value to be associated with the specified
key
key.
-
Returns:
-
the
previous value associated with
specified key, or
key
, or
null
if there was no mapping for
key. A
key
. (A
null
return can also indicate that the map previously associated
null
with
with the specified key, if the implementation supports
key
, if the implementation supports
null
values.)
values.
-
Throws:
-
UnsupportedOperationException
- if the
put
operation is not supported by this
map
map.
-
ClassCastException
- if the class of the specified key or value prevents it from being stored in this
map
map.
-
NullPointerException
IllegalArgumentException
- if the specified key or value is null and this map does not permit null keys or values
- if some aspect of this key or value prevents it from being stored in this map.
-
IllegalArgumentException
NullPointerException
- if some property of the specified key or value prevents it from being stored in this map
- if this map does not permit
null
keys or values, and the specified key or value is
null
.
remove
V remove(Object key)
-
Removes the mapping for
a
this
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
the
map previously associated the key, or
null
if the map contained no mapping for
the
this
key.
If this map permits null values, then a return value of
(A
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
return can also indicate that the map previously associated
null
.
with the specified key if the implementation supports
null
values.) The map will not contain a mapping for the specified key once the call returns.
The map will not contain a mapping for the specified key once the call returns.
-
-
Parameters:
-
key - key whose mapping is to be removed from the
map
map.
-
Returns:
-
the
previous value associated with
specified key, or
key
null
, or
if there was no mapping for key.
-
Throws:
-
ClassCastException
- if the key is of an inappropriate type for this map (optional).
-
NullPointerException
- if the key is
null
if there was no mapping for
and this map does not permit
key
null
.
-
Throws:
keys (optional).
-
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)
method is not supported by this map.
putAll
void putAll(Map<? extends K,? extends V> m) > t)
-
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
unspecified
if the specified map is modified while the operation is in progress.
-
-
Parameters:
-
m
t
-
mappings
Mappings
to be stored in this
map
map.
-
Throws:
-
UnsupportedOperationException
- if the
putAll
operation
method
is not supported by this
map
map.
-
ClassCastException
- if the class of a key or value in the specified map prevents it from being stored in this
map
map.
-
NullPointerException
IllegalArgumentException
- 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
- some aspect of a key or value in the specified map prevents it from being stored in this map.
-
IllegalArgumentException
NullPointerException
- if some property of a key or value in the specified map prevents it from being stored in this map
- 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.
clear
void clear()
-
Removes all
of the
mappings from this map (optional operation).
The map will be empty after this call returns.
-
-
Throws:
-
UnsupportedOperationException
-
if the
clear
operation
is not supported by this
map
map.
keySet
Set<K> keySet()
-
Returns a
Set
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
add
or
addAll
operations.
-
-
Returns:
-
a set view of the keys contained in this
map
map.
values
Collection<V> values()
-
Returns a
Collection
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
add
or
addAll
operations.
-
-
Returns:
-
a collection view of the values contained in this
map
map.
entrySet
Set<Map.Entry<K,V>> entrySet()
-
Returns a
Set
Returns a set view of the mappings contained in this map. Each element in the returned set is a
Map.Entry
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:
-
a set view of the mappings contained in this
map
map.
equals
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
Maps
represent the same mappings. More formally, two maps
m1
t1
and
m2
t2
represent the same mappings if
m1.entrySet().equals(m2.entrySet())
t1.entrySet().equals(t2.entrySet())
. This ensures that the
equals
method works properly across different implementations of the
Map
interface.
-
-
Overrides:
-
equals
in class
Object
-
-
Parameters:
-
o - object to be compared for equality with this
map
map.
-
Returns:
-
true
if the specified object is equal to this
map
map.
-
See Also:
-
Object.hashCode()
,
Hashtable
hashCode
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
hashCodes
of each entry in the map's
entrySet()
entrySet
view. This ensures that
m1.equals(m2)
t1.equals(t2)
implies that
m1.hashCode()==m2.hashCode()
t1.hashCode()==t2.hashCode()
for any two maps
m1
t1
and
m2
t2
, as required by the general contract of
Object.hashCode()
.
Object.hashCode.
-
-
Overrides:
-
hashCode
in class
Object
-
-
Returns:
-
the hash code value for this
map
map.
-
See Also:
-
Map.Entry.hashCode()
,
Object.hashCode()
,
Object.equals(Object)
,
equals(Object)