AbstractMap

public abstract class AbstractMap extends Object
implements Map<K, V>

Known Direct Subclasses ConcurrentHashMap<K, V>, ConcurrentSkipListMap<K, V>, EnumMap<K extends Enum<K>, V>, HashMap<K, V>, IdentityHashMap<K, V>, TreeMap<K, V>, WeakHashMap<K, V> ConcurrentHashMap<K, V> A hash table supporting full concurrency of retrievals and high expected concurrency for updates.  ConcurrentSkipListMap<K, V> A scalable concurrent ConcurrentNavigableMap implementation.  EnumMap<K extends Enum<K>, V> A specialized Map implementation for use with enum type keys.  HashMap<K, V> Hash table based implementation of the Map interface.  IdentityHashMap<K, V> This class implements the Map interface with a hash table, using reference-equality in place of object-equality when comparing keys (and values).  TreeMap<K, V> A Red-Black tree based NavigableMap implementation.  WeakHashMap<K, V> Hash table based implementation of the Map interface, with weak keys.

Known Indirect Subclasses LinkedHashMap<K, V> LinkedHashMap<K, V> Hash table and linked list implementation of the Map interface, with predictable iteration order.

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.

See Also

• Map
• Collection

Nested Class Summary

class	AbstractMap.SimpleEntry<K, V>		An Entry maintaining a key and a value.
class	AbstractMap.SimpleImmutableEntry<K, V>		An Entry maintaining an immutable key and value.

Protected Constructor Summary

AbstractMap() Sole constructor.

Public Method Summary

void	clear() Removes all of the mappings from this map (optional operation).
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<Entry<K, V>>	entrySet() Returns a Set 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, 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.

Protected Method Summary

Object

clone() Returns a shallow copy of this AbstractMap instance: the keys and values themselves are not cloned.

Inherited Method Summary

From class java.lang.Object

Object	clone() Creates and returns a copy of this Object.
boolean	equals(Object obj) Compares this instance with the specified object and indicates if they are equal.
void	finalize() Invoked when the garbage collector has detected that this instance is no longer reachable.
final Class<?>	getClass() Returns the unique instance of Class that represents this object's class.
int	hashCode() Returns an integer hash code for this object.
final void	notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
final void	notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
String	toString() Returns a string containing a concise, human-readable description of this object.
final void	wait(long timeout, int nanos) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait(long timeout) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait() Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object.

From interface java.util.Map

abstract void	clear() Removes all of the mappings from this map (optional operation).
V	compute(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction) Attempts to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping).
V	computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction) If the specified key is not already associated with a value (or is mapped to null), attempts to compute its value using the given mapping function and enters it into this map unless null.
V	computeIfPresent(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction) If the value for the specified key is present and non-null, attempts to compute a new mapping given the key and its current mapped value.
abstract boolean	containsKey(Object key) Returns true if this map contains a mapping for the specified key.
abstract boolean	containsValue(Object value) Returns true if this map maps one or more keys to the specified value.
abstract Set<Entry<K, V>>	entrySet() Returns a Set view of the mappings contained in this map.
abstract boolean	equals(Object o) Compares the specified object with this map for equality.
void	forEach(BiConsumer<? super K, ? super V> action) Performs the given action for each entry in this map until all entries have been processed or the action throws an exception.
abstract 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.
V	getOrDefault(Object key, V defaultValue) Returns the value to which the specified key is mapped, or defaultValue if this map contains no mapping for the key.
abstract int	hashCode() Returns the hash code value for this map.
abstract boolean	isEmpty() Returns true if this map contains no key-value mappings.
abstract Set<K>	keySet() Returns a Set view of the keys contained in this map.
V	merge(K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction) If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value.
abstract V	put(K key, V value) Associates the specified value with the specified key in this map (optional operation).
abstract void	putAll(Map<? extends K, ? extends V> m) Copies all of the mappings from the specified map to this map (optional operation).
V	putIfAbsent(K key, V value) If the specified key is not already associated with a value (or is mapped to null) associates it with the given value and returns null, else returns the current value.
boolean	remove(Object key, Object value) Removes the entry for the specified key only if it is currently mapped to the specified value.
abstract V	remove(Object key) Removes the mapping for a key from this map if it is present (optional operation).
boolean	replace(K key, V oldValue, V newValue) Replaces the entry for the specified key only if currently mapped to the specified value.
V	replace(K key, V value) Replaces the entry for the specified key only if it is currently mapped to some value.
void	replaceAll(BiFunction<? super K, ? super V, ? extends V> function) Replaces each entry's value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception.
abstract int	size() Returns the number of key-value mappings in this map.
abstract Collection<V>	values() Returns a Collection view of the values contained in this map.

Protected Constructors

protected AbstractMap ()

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

Public Methods

public void clear ()

Removes all of the mappings from this map (optional operation). The map will be empty after this call returns.

Throws

UnsupportedOperationException

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.)

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
NullPointerException

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.

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
NullPointerException

public abstract Set<Entry<K, V>> entrySet ()

Returns a Set 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.

Returns

• a set view of the mappings contained in this map

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.

Parameters

o	object to be compared for equality with this map

Returns

• true if the specified object is equal to this map

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.

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
NullPointerException

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

• the hash code value for this map

See Also

• Map.Entry.hashCode()
• Object.equals(Object)
• Set.equals(Object)

public boolean isEmpty ()

Returns true if this map contains no key-value mappings.

Returns

• true if this map contains no key-value mappings

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.

Returns

• a set view of the keys contained in this map

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.)

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
ClassCastException
NullPointerException
IllegalArgumentException

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.

Parameters

m	mappings to be stored in this map

Throws

UnsupportedOperationException
ClassCastException
NullPointerException
IllegalArgumentException

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.

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
ClassCastException
NullPointerException

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.

Returns

• the number of key-value mappings in this map

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).

Returns

• a string representation of this map

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.

Returns

• a collection view of the values contained in this map

Protected Methods

protected Object clone ()

Returns a shallow copy of this AbstractMap instance: the keys and values themselves are not cloned.

Returns

• a shallow copy of this map

Throws

CloneNotSupportedException