K - the type of keys maintained by this map
V - the type of mapped values
Map<K,V>
ConcurrentHashMap, ConcurrentSkipListMap, EnumMap, HashMap, IdentityHashMap, TreeMap, WeakHashMap
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.
Nested Classes
static class
An Entry maintaining a key and a value.
static class
An unmodifiable Entry maintaining a key and a value.
Nested classes/interfaces declared in interface java.util.MapMap.Entry<K,V>Constructors
void
Removes all of the mappings from this map (optional operation).
Returns a shallow copy of this AbstractMap instance: the keys and values themselves are not cloned.
boolean
Returns true if this map contains a mapping for the specified key.
boolean
Returns true if this map maps one or more keys to the specified value.
boolean
Compares the specified object with this map for equality.
Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.
int
Returns the hash code value for this map.
boolean
Returns true if this map contains no key-value mappings.
Returns a
Set
view of the keys contained in this map.
Associates the specified value with the specified key in this map (optional operation).
void
Copies all of the mappings from the specified map to this map (optional operation).
Removes the mapping for a key from this map if it is present (optional operation).
int
Returns the number of key-value mappings in this map.
Returns a string representation of this map.
Returns a
Collection
view of the values contained in this map.
Methods declared in interface java.util.Mapcompute, computeIfAbsent, computeIfPresent, entrySet, forEach, getOrDefault, merge, putIfAbsent, remove, replace, replace, replaceAllprotected AbstractMap()
Sole constructor. (For invocation by subclass constructors, typically implicit.)
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.
public boolean isEmpty()
Returns true if this map contains no key-value mappings.
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.
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 tested
true if this map maps one or more keys to the specified value
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)
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.)
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 tested
true if this map contains a mapping for the specified key
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)
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.
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 returned
null if this map contains no mapping for the key
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)
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
.)
put in interface Map<K,V>
UnsupportedOperationException.
key - key with which the specified value is to be associated
value - value to be associated with the specified key
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.)
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
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.
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 map
key, or null if there was no mapping for key.
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)
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.
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 map
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
public void clear()
Removes all of the mappings from this map (optional operation). The map will be empty after this call returns.
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 map
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.
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.
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.
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.
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.
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 map
true if the specified object is equal to this map
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()
.
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)
.
Returns a shallow copy of this 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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