java.util

Class AbstractMap<K,V>

java.lang.Object

java.util.AbstractMap<K,V>

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>

A base class for Map implementations.

Subclasses that permit new mappings to be added must override put(K, V).

The default implementations of many methods are inefficient for large maps. For example in the default implementation, each call to get(java.lang.Object) performs a linear iteration of the entry set. Subclasses should override such methods to improve their performance.

Since:

1.2

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
static class	AbstractMap.SimpleEntry<K,V> A key-value mapping with mutable values.
static class	AbstractMap.SimpleImmutableEntry<K,V> An immutable key-value mapping.

Nested classes/interfaces inherited from interface java.util.Map

Map.Entry<K,V>

Constructor Summary

Constructors

Modifier	Constructor and Description
protected	AbstractMap()

Method Summary

Modifier and Type	Method and Description
void	clear() Removes all of the mappings from this map (optional operation).
protected Object	clone() Creates and returns a copy of this Object.
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 a Set view of the mappings contained in this map.
boolean	equals(Object object) Compares this instance with the specified object and indicates if they are equal.
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 an integer hash code for this object.
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> map) 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 containing a concise, human-readable description of this object.
Collection<V>	values() Returns a Collection view of the values contained in this map.

Methods inherited from class java.lang.Object

finalize, getClass, notify, notifyAll, wait, wait, wait

Methods inherited from interface java.util.Map

compute, computeIfAbsent, computeIfPresent, copyOf, entry, forEach, getOrDefault, merge, of, of, of, of, of, of, of, of, of, of, of, ofEntries, putIfAbsent, remove, replace, replace, replaceAll

Constructor Detail

AbstractMap

protected AbstractMap()

Method Detail

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().

Specified by:

clear in interface Map<K,V>

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.)

This implementation iterates its key set, looking for a key that key equals.

Specified by:

containsKey in interface Map<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

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.

This implementation iterates its entry set, looking for an entry with a value that value equals.

Specified by:

containsValue in interface Map<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

entrySet

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

Description copied from interface: Map

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.

Specified by:

entrySet in interface Map<K,V>

Returns:

a set view of the mappings contained in this map

equals

public boolean equals(Object object)

Compares this instance with the specified object and indicates if they are equal. In order to be equal, o must represent the same object as this instance using a class-specific comparison. The general contract is that this comparison should be reflexive, symmetric, and transitive. Also, no object reference other than null is equal to null.

The default implementation returns true only if this == o. See Writing a correct equals method if you intend implementing your own equals method.

The general contract for the equals and Object.hashCode() methods is that if equals returns true for any two objects, then hashCode() must return the same value for these objects. This means that subclasses of Object usually override either both methods or neither of them.

This implementation first checks the structure of object. If it is not a map or of a different size, this returns false. Otherwise it iterates its own entry set, looking up each entry's key in object. If any value does not equal the other map's value for the same key, this returns false. Otherwise it returns true.

Specified by:

equals in interface Map<K,V>

Overrides:

equals in class Object

Parameters:

object - the object to compare this instance with.

Returns:

true if the specified object is equal to this Object; false otherwise.

See Also:

Object.hashCode()

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.

This implementation iterates its entry set, looking for an entry with a key that key equals.

Specified by:

get in interface Map<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

hashCode

public int hashCode()

Returns an integer hash code for this object. By contract, any two objects for which Object.equals(java.lang.Object) returns true must return the same hash code value. This means that subclasses of Object usually override both methods or neither method.

Note that hash values must not change over time unless information used in equals comparisons also changes.

See Writing a correct hashCode method if you intend implementing your own hashCode method.

This implementation iterates its entry set, summing the hashcodes of its entries.

Specified by:

hashCode in interface Map<K,V>

Overrides:

hashCode in class Object

Returns:

this object's hash code.

See Also:

Object.equals(java.lang.Object)

isEmpty

public boolean isEmpty()

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

This implementation compares size() to 0.

Specified by:

isEmpty in interface Map<K,V>

Returns:

true if this map contains no key-value mappings

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.

This implementation returns a view that calls through this to map. Its iterator transforms this map's entry set iterator to return keys.

Specified by:

keySet in interface Map<K,V>

Returns:

a set view of the keys contained in this map

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.)

This base implementation throws UnsupportedOperationException.

Specified by:

put in interface Map<K,V>

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.)

putAll

public void putAll(Map<? extends K,? extends V> map)

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.

This implementation iterates through map's entry set, calling put() for each.

Specified by:

putAll in interface Map<K,V>

Parameters:

map - mappings to be 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.

This implementation iterates its entry set, removing the entry with a key that key equals.

Specified by:

remove in interface Map<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.

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 its entry set's size.

Specified by:

size in interface Map<K,V>

Returns:

the number of key-value mappings in this map

toString

public String toString()

Returns a string containing a concise, human-readable description of this object. Subclasses are encouraged to override this method and provide an implementation that takes into account the object's type and data. The default implementation is equivalent to the following expression:

   getClass().getName() + '@' + Integer.toHexString(hashCode())

See Writing a useful toString method if you intend implementing your own toString method.

This implementation composes a string by iterating its entry set. If this map contains itself as a key or a value, the string "(this Map)" will appear in its place.

Overrides:

toString in class Object

Returns:

a printable representation of this object.

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.

This implementation returns a view that calls through this to map. Its iterator transforms this map's entry set iterator to return values.

Specified by:

values in interface Map<K,V>

Returns:

a collection view of the values contained in this map

clone

protected Object clone()
                throws CloneNotSupportedException

Description copied from class: Object

Creates and returns a copy of this Object. The default implementation returns a so-called "shallow" copy: It creates a new instance of the same class and then copies the field values (including object references) from this instance to the new instance. A "deep" copy, in contrast, would also recursively clone nested objects. A subclass that needs to implement this kind of cloning should call super.clone() to create the new instance and then create deep copies of the nested, mutable objects.

Overrides:

clone in class Object

Returns:

a copy of this object.

Throws:

CloneNotSupportedException - if this object's class does not implement the Cloneable interface.