Added in API level 1

Summary: Nested Classes | Protected Ctors | Methods | Protected Methods | Inherited Methods

AbstractMap

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

java.lang.Object
↳	java.util.AbstractMap<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 well-defined encounter order.

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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

Summary

Nested classes
class	AbstractMap.SimpleEntry<K, V> An Entry maintaining a key and a value.
class	AbstractMap.SimpleImmutableEntry<K, V> An unmodifiable Entry maintaining a key and a value.

Protected constructors
AbstractMap() Sole constructor.

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

Inherited methods
From class java.lang.Object Object clone() Creates and returns a copy of this object. boolean equals(Object obj) Indicates whether some other object is "equal to" this one. void finalize() Called by the garbage collector on an object when garbage collection determines that there are no more references to the object. final Class<?> getClass() Returns the runtime class of this Object. int hashCode() Returns a hash code value for the object. final void notify() Wakes up a single thread that is waiting on this object's monitor. final void notifyAll() Wakes up all threads that are waiting on this object's monitor. String toString() Returns a string representation of the object. final void wait(long timeoutMillis, int nanos) Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed. final void wait(long timeoutMillis) Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed. final void wait() Causes the current thread to wait until it is awakened, typically by being notified or interrupted.
From interface java.util.Map abstract void clear() Removes all of the mappings from this map (optional operation). default 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). default 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. default 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. static <K, V> Map<K, V> copyOf(Map<? extends K, ? extends V> map) Returns an unmodifiable Map containing the entries of the given Map. static <K, V> Entry<K, V> entry(K k, V v) Returns an unmodifiable Entry containing the given key and 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. default 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. default 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. default 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. static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5) Returns an unmodifiable map containing five mappings. static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4) Returns an unmodifiable map containing four mappings. static <K, V> Map<K, V> of(K k1, V v1) Returns an unmodifiable map containing a single mapping. static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7, K k8, V v8) Returns an unmodifiable map containing eight mappings. static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2) Returns an unmodifiable map containing two mappings. static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3) Returns an unmodifiable map containing three mappings. static <K, V> Map<K, V> of() Returns an unmodifiable map containing zero mappings. static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7, K k8, V v8, K k9, V v9) Returns an unmodifiable map containing nine mappings. static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6) Returns an unmodifiable map containing six mappings. static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7) Returns an unmodifiable map containing seven mappings. static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7, K k8, V v8, K k9, V v9, K k10, V v10) Returns an unmodifiable map containing ten mappings. static <K, V> Map<K, V> ofEntries(Entry...<? extends K, ? extends V> entries) Returns an unmodifiable map containing keys and values extracted from the given entries. 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). default 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. default 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). default boolean replace(K key, V oldValue, V newValue) Replaces the entry for the specified key only if currently mapped to the specified value. default V replace(K key, V value) Replaces the entry for the specified key only if it is currently mapped to some value. default 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

AbstractMap

Added in API level 1

protected AbstractMap ()

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

Public methods

clear

Added in API level 1

public void clear ()

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

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

containsKey

Added in API level 1

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.)

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	Object: key whose presence in this map is to be tested

Returns
boolean	true if this map contains a mapping for the specified key

Throws
ClassCastException
NullPointerException

containsValue

Added in API level 1

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.

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	Object: value whose presence in this map is to be tested

Returns
boolean	true if this map maps one or more keys to the specified value

Throws
ClassCastException
NullPointerException

entrySet

Added in API level 1

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
Set<Entry<K, V>>	a set view of the mappings contained in this map

equals

Added in API level 1

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.

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: object to be compared for equality with this map

Returns
boolean	true if the specified object is equal to this map

get

Added in API level 1

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.

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	Object: the key whose associated value is to be returned

Returns
V	the value to which the specified key is mapped, or null if this map contains no mapping for the key

Throws
ClassCastException
NullPointerException

hashCode

Added in API level 1

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.

Implementation Requirements:

• This implementation iterates over entrySet(), calling hashCode() on each element (entry) in the set, and adding up the results.

Returns
int	the hash code value for this map

See also:

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

isEmpty

Added in API level 1

public boolean isEmpty ()

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

Implementation Requirements:

• This implementation returns size() == 0.

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

keySet

Added in API level 1

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.

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
Set<K>	a set view of the keys contained in this map

put

Added in API level 1

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.)

Implementation Requirements:

• This implementation always throws an UnsupportedOperationException.

Parameters
key	K: key with which the specified value is to be associated
value	V: value to be associated with the specified key

Returns
V	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

putAll

Added in API level 1

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.

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	Map: mappings to be stored in this map

Throws
UnsupportedOperationException
ClassCastException
NullPointerException
IllegalArgumentException

remove

Added in API level 1

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.

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	Object: key whose mapping is to be removed from the map

Returns
V	the previous value associated with key, or null if there was no mapping for key.

Throws
UnsupportedOperationException
ClassCastException
NullPointerException

size

Added in API level 1

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.

Implementation Requirements:

• This implementation returns entrySet().size().

Returns
int	the number of key-value mappings in this map

toString

Added in API level 1

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
String	a string representation of this map

values

Added in API level 1

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.

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
Collection<V>	a collection view of the values contained in this map

Protected methods

clone

Added in API level 1

protected Object clone ()

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

Returns
Object	a shallow copy of this map

Throws
CloneNotSupportedException