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 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
Nested Class Summary
Modifier and Type | Class | 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. |
Nested classes/interfaces declared in interface java.util.Map
Map.Entry<K,V>
Constructor Summary
Modifier | Constructor | Description |
---|---|---|
protected | AbstractMap() | Sole constructor. |
Method Summary
Modifier and Type | Method | Description |
---|---|---|
void | clear() | Removes all of the mappings from this map (optional operation). |
protected Object | clone() | Returns a shallow copy of this |
boolean | containsKey(Object key) | Returns |
boolean | containsValue(Object value) | Returns |
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 |
int | hashCode() | Returns the hash code value for this map. |
boolean | isEmpty() | Returns |
Set<K> | keySet() | Returns a |
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 |
Methods declared in class java.lang.Object
finalize, getClass, notify, notifyAll, wait, wait, wait
Methods declared in interface java.util.Map
compute, computeIfAbsent, computeIfPresent, entrySet, forEach, getOrDefault, merge, putIfAbsent, remove, replace, replace, replaceAll
Constructor Detail
AbstractMap
protected AbstractMap()
Sole constructor. (For invocation by subclass constructors, typically implicit.)
Method Detail
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
.
- Specified by:
-
size
in interfaceMap<K,V>
- Implementation Requirements:
- This implementation returns
entrySet().size()
. - Returns:
- the number of key-value mappings in this map
isEmpty
public boolean isEmpty()
Returns true
if this map contains no key-value mappings.
- Specified by:
-
isEmpty
in interfaceMap<K,V>
- Implementation Requirements:
- This implementation returns
size() == 0
. - 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 Objects.equals(value, v)
. This operation will probably require time linear in the map size for most implementations of the Map
interface.
- Specified by:
-
containsValue
in interfaceMap<K,V>
- Implementation Requirements:
- 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. - 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 Objects.equals(key, k)
. (There can be at most one such mapping.)
- Specified by:
-
containsKey
in interfaceMap<K,V>
- Implementation Requirements:
- 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. - 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 Objects.equals(key, 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.
- Specified by:
-
get
in interfaceMap<K,V>
- Implementation Requirements:
- 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. - 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
.)
- Specified by:
-
put
in interfaceMap<K,V>
- Implementation Requirements:
- This implementation always throws an
UnsupportedOperationException
. - 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 forkey
. (Anull
return can also indicate that the map previously associatednull
withkey
, if the implementation supportsnull
values.) - Throws:
-
UnsupportedOperationException
- if theput
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 Objects.equals(key, 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.
- Specified by:
-
remove
in interfaceMap<K,V>
- Implementation Requirements:
- This implementation iterates over
entrySet()
searching for an entry with the specified key. If such an entry is found, its value is obtained with itsgetValue
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 an
UnsupportedOperationException
if theentrySet
iterator does not support theremove
method and this map contains a mapping for the specified key. - 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 forkey
. - Throws:
-
UnsupportedOperationException
- if theremove
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.
- Specified by:
-
putAll
in interfaceMap<K,V>
- Implementation Requirements:
- This implementation iterates over the specified map's
entrySet()
collection, and calls this map'sput
operation once for each entry returned by the iteration.Note that this implementation throws an
UnsupportedOperationException
if this map does not support theput
operation and the specified map is nonempty. - Parameters:
-
m
- mappings to be stored in this map - Throws:
-
UnsupportedOperationException
- if theputAll
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.
- Specified by:
-
clear
in interfaceMap<K,V>
- Implementation Requirements:
- This implementation calls
entrySet().clear()
.Note that this implementation throws an
UnsupportedOperationException
if theentrySet
does not support theclear
operation. - Throws:
-
UnsupportedOperationException
- if theclear
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.
- Specified by:
-
keySet
in interfaceMap<K,V>
- Implementation Requirements:
- This implementation returns a set that subclasses
AbstractSet
. The subclass's iterator method returns a "wrapper object" over this map'sentrySet()
iterator. Thesize
method delegates to this map'ssize
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.
- 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.
- Specified by:
-
values
in interfaceMap<K,V>
- Implementation Requirements:
- This implementation returns a collection that subclasses
AbstractCollection
. The subclass's iterator method returns a "wrapper object" over this map'sentrySet()
iterator. Thesize
method delegates to this map'ssize
method and thecontains
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.
- Returns:
- a collection view of the values 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.
- Specified by:
-
equals
in interfaceMap<K,V>
- Overrides:
-
equals
in classObject
- Implementation Requirements:
- 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 returnsfalse
. 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. - 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()
,HashMap
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()
.
- Specified by:
-
hashCode
in interfaceMap<K,V>
- Overrides:
-
hashCode
in classObject
- Implementation Requirements:
- This implementation iterates over
entrySet()
, callinghashCode()
on each element (entry) in the set, and adding up the results. - 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)
.
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 classObject
- Returns:
- a shallow copy of this map
- Throws:
-
CloneNotSupportedException
- if the object's class does not support theCloneable
interface. Subclasses that override theclone
method can also throw this exception to indicate that an instance cannot be cloned. - See Also:
Cloneable