K
- the type of keys maintained by this mapV
- the type of mapped valuesMap<K,V>
ConcurrentHashMap
, ConcurrentSkipListMap
, EnumMap
, HashMap
, IdentityHashMap
, TreeMap
, WeakHashMap
public 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.
Modifier and Type | Class | Description |
---|---|---|
static class |
AbstractMap.SimpleEntry<K, |
An Entry maintaining a key and a value. |
static class |
AbstractMap.SimpleImmutableEntry<K, |
An unmodifiable Entry maintaining a key and a value. |
Modifier | Constructor | Description |
---|---|---|
protected |
Sole constructor. |
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 AbstractMap instance: the keys and values themselves are not cloned. |
boolean |
containsKey |
Returns true if this map contains a mapping for the specified key. |
boolean |
containsValue |
Returns true if this map maps one or more keys to the specified value. |
boolean |
equals |
Compares the specified object with this map for equality. |
V |
get |
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 |
keySet() |
Returns a Set view of the keys contained in this map. |
V |
put |
Associates the specified value with the specified key in this map (optional operation). |
void |
putAll |
Copies all of the mappings from the specified map to this map (optional operation). |
V |
remove |
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 |
values() |
Returns a Collection view of the values contained in this map. |
compute, computeIfAbsent, computeIfPresent, entrySet, forEach, getOrDefault, merge, putIfAbsent, remove, replace, replace, replaceAll
protected AbstractMap()
public int size()
Integer.MAX_VALUE
elements, returns Integer.MAX_VALUE
.public boolean isEmpty()
true
if this map contains no key-value mappings.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 Objects.equals(value, v)
. This operation will probably require time linear in the map size for most implementations of the Map
interface.containsValue
in interface Map<K,V>
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.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 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 Objects.equals(key, k)
. (There can be at most one such mapping.)containsKey
in interface Map<K,V>
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.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 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 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.
get
in interface Map<K,V>
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.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 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
.)put
in interface Map<K,V>
UnsupportedOperationException
.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 V remove(Object 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.
remove
in interface Map<K,V>
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.
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 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. If the specified map has a defined encounter order, processing of its mappings generally occurs in that order.putAll
in interface Map<K,V>
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.
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 mappublic void clear()
clear
in interface Map<K,V>
entrySet().clear()
. Note that this implementation throws an UnsupportedOperationException
if the entrySet
does not support the clear
operation.
UnsupportedOperationException
- if the clear
operation is not supported by this mappublic 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.keySet
in interface Map<K,V>
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.
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.values
in interface Map<K,V>
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.
public boolean equals(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.equals
in interface Map<K,V>
equals
in class Object
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.o
- object to be compared for equality with this maptrue
if the specified object is equal to this mappublic 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()
.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)
.protected Object clone() throws CloneNotSupportedException
AbstractMap
instance: the keys and values themselves are not cloned.clone
in class Object
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.
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Documentation extracted from Debian's OpenJDK Development Kit package.
Licensed under the GNU General Public License, version 2, with the Classpath Exception.
Various third party code in OpenJDK is licensed under different licenses (see Debian package).
Java and OpenJDK are trademarks or registered trademarks of Oracle and/or its affiliates.
https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/AbstractMap.html