- java.lang.Object
-
- java.util.AbstractMap<K,V>
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- Type Parameters:
K
- the type of keys maintained by this mapV
- 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
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Nested Class Summary
Nested Classes Modifier and Type Class and 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.
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Constructor Summary
Constructors Modifier Constructor and Description protected
AbstractMap()
Sole constructor.
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Method Summary
Methods Modifier and Type Method and 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(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.abstract Set<Map.Entry<K,V>>
entrySet()
Returns aSet
view of the mappings contained in this map.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, ornull
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<K>
keySet()
Returns aSet
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> 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 aCollection
view of the values contained in this map.
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-
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Constructor Detail
-
AbstractMap
protected AbstractMap()
Sole constructor. (For invocation by subclass constructors, typically implicit.)
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Method Detail
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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().
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isEmpty
public boolean isEmpty()
Returns true if this map contains no key-value mappings.This implementation returns size() == 0.
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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 (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.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 interfaceMap<K,V>
- 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)
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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 (key==null ? k==null : key.equals(k)). (There can be at most one such mapping.)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 interfaceMap<K,V>
- 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)
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get
public V get(Object key)
Returns the value to which the specified key is mapped, ornull
if this map contains no mapping for the key.More formally, if this map contains a mapping from a key
k
to a valuev
such that(key==null ? k==null : key.equals(k))
, then this method returnsv
; otherwise it returnsnull
. (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 tonull
. ThecontainsKey
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 interfaceMap<K,V>
- 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)
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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 ifm.containsKey(k)
would return true.)This implementation always throws an UnsupportedOperationException.
- Specified by:
put
in interfaceMap<K,V>
- Parameters:
key
- key with which the specified value is to be associatedvalue
- value to be associated with the specified key- Returns:
- the previous value associated with 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.)
- Throws:
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 map
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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(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 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.
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 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 interfaceMap<K,V>
- Parameters:
key
- key whose mapping is to be removed from the map- Returns:
- the previous value associated with key, or null if there was no mapping for key.
- Throws:
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)
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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 callingput(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.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 interfaceMap<K,V>
- Parameters:
m
- mappings to be stored in this map- Throws:
UnsupportedOperationException
- 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 map
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clear
public void clear()
Removes all of the mappings from this map (optional operation). The map will be empty after this call returns.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 interfaceMap<K,V>
- Throws:
UnsupportedOperationException
- if the clear operation is not supported by this map
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keySet
public Set<K> keySet()
Returns aSet
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.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.
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values
public Collection<V> values()
Returns aCollection
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.This implementation returns a collection that subclasses
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.
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entrySet
public abstract Set<Map.Entry<K,V>> entrySet()
Description copied from interface:Map
Returns aSet
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.
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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.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 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.
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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 ofObject.hashCode()
.This implementation iterates over entrySet(), calling
hashCode()
on each element (entry) in the set, and adding up the results.- Specified by:
hashCode
in interfaceMap<K,V>
- Overrides:
hashCode
in classObject
- Returns:
- the hash code value for this map
- See Also:
Map.Entry.hashCode()
,Object.equals(Object)
,Set.equals(Object)
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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 byString.valueOf(Object)
.
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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
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Dokument erstellt 11/06/2005, zuletzt geändert 04/03/2020
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