Module java.base

Package java.util

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

Nested Classes

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 unmodifiable Entry maintaining a key and a value.

Nested classes/interfaces declared in interface java.util.Map

Map.Entry<K,V>

Constructor Summary

Constructors

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 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.
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 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<K>	keySet()	Returns a 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> 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 Collection view of the values contained in this map.

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 Details

AbstractMap

protected AbstractMap()

Sole constructor. (For invocation by subclass constructors, typically implicit.)

Method Details

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 interface Map<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 interface Map<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 interface Map<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 interface Map<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 interface Map<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 interface Map<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, 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 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 interface Map<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 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.

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 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. If the specified map has a defined encounter order, processing of its mappings generally occurs in that order.

Specified by:

putAll in interface Map<K,V>

Implementation Requirements:

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.

Parameters:

m - mappings to be stored in this map

Throws:

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

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 interface Map<K,V>

Implementation Requirements:

This implementation calls entrySet().clear().

Note that this implementation throws an UnsupportedOperationException if the entrySet does not support the clear operation.

Throws:

UnsupportedOperationException - if the clear 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 interface Map<K,V>

Implementation Requirements:

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.

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 interface Map<K,V>

Implementation Requirements:

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.

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 interface Map<K,V>

Overrides:

equals in class Object

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 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.

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 interface Map<K,V>

Overrides:

hashCode in class Object

Implementation Requirements:

This implementation iterates over entrySet(), calling hashCode() 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).

Overrides:

toString in class Object

Returns:

a string representation of this map

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 class Object

Returns:

a shallow copy of this map

Throws:

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.

See Also:

• Cloneable