java.util

Class AbstractMap<K,V>

java.lang.Object

java.util.AbstractMap<K,V>

All Implemented Interfaces:

Map<K,V>

Direct Known Subclasses:

HashMap, IdentityHashMap, TreeMap

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

An abstract implementation of Map to make it easier to create your own implementations. In order to create an unmodifiable Map, subclass AbstractMap and implement the entrySet (usually via an AbstractSet). To make it modifiable, also implement put, and have entrySet().iterator() support remove.

It is recommended that classes which extend this support at least the no-argument constructor, and a constructor which accepts another Map. Further methods in this class may be overridden if you have a more efficient implementation.

Since:

1.2

Author:

Original author unknown, Bryce McKinlay, Eric Blake (ebb9@email.byu.edu)

See Also:

Map, Collection, HashMap, LinkedHashMap, TreeMap, WeakHashMap, IdentityHashMap

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
static class	AbstractMap.SimpleEntry<K,V> A class which implements Map.Entry.
static class	AbstractMap.SimpleImmutableEntry<K,V> A class containing an immutable key and value.

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

Map.Entry<K,V>

Method Summary

Modifier and Type	Method and Description
void	clear() Remove all entries from this Map (optional operation).
boolean	containsKey(Object key) Returns true if this contains a mapping for the given key.
boolean	containsValue(Object value) Returns true if this contains at least one mapping with the given value.
abstract Set<Map.Entry<K,V>>	entrySet() Returns a set view of the mappings in this Map.
boolean	equals(Object o) Compares the specified object with this map for equality.
V	get(Object key) Returns the value mapped by the given key.
int	hashCode() Returns the hash code for this map.
boolean	isEmpty() Returns true if the map contains no mappings.
Set<K>	keySet() Returns a set view of this map's keys.
V	put(K key, V value) Associates the given key to the given value (optional operation).
void	putAll(Map<? extends K,? extends V> m) Copies all entries of the given map to this one (optional operation).
V	remove(Object key) Removes the mapping for this key if present (optional operation).
int	size() Returns the number of key-value mappings in the map.
String	toString() Returns a String representation of this map.
Collection<V>	values() Returns a collection or bag view of this map's values.

Methods inherited from class java.lang.Object

getClass, notify, notifyAll, wait, wait, wait

Method Detail

clear

public void clear()

Remove all entries from this Map (optional operation). This default implementation calls entrySet().clear(). NOTE: If the entry set does not permit clearing, then this will fail, too. Subclasses often override this for efficiency. Your implementation of entrySet() should not call AbstractMap.clear unless you want an infinite loop.

Specified by:

clear in interface Map<K,V>

Throws:

UnsupportedOperationException - if entrySet().clear() does not support clearing.

See Also:

Set.clear()

containsKey

public boolean containsKey(Object key)

Returns true if this contains a mapping for the given key. This implementation does a linear search, O(n), over the entrySet(), returning true if a match is found, false if the iteration ends. Many subclasses can implement this more efficiently.

Specified by:

containsKey in interface Map<K,V>

Parameters:

key - the key to search for

Returns:

true if the map contains the key

Throws:

NullPointerException - if key is null but the map does not permit null keys

See Also:

containsValue(Object)

containsValue

public boolean containsValue(Object value)

Returns true if this contains at least one mapping with the given value. This implementation does a linear search, O(n), over the entrySet(), returning true if a match is found, false if the iteration ends. A match is defined as a value, v, where (value == null ? v == null : value.equals(v)). Subclasses are unlikely to implement this more efficiently.

Specified by:

containsValue in interface Map<K,V>

Parameters:

value - the value to search for

Returns:

true if the map contains the value

See Also:

containsKey(Object)

entrySet

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

Returns a set view of the mappings in this Map. Each element in the set must be an implementation of Map.Entry. The set is backed by the map, so that changes in one show up in the other. Modifications made while an iterator is in progress cause undefined behavior. If the set supports removal, these methods must be valid: Iterator.remove, Set.remove, removeAll, retainAll, and clear. Element addition is not supported via this set.

Specified by:

entrySet in interface Map<K,V>

Returns:

the entry set

See Also:

Map.Entry

equals

public boolean equals(Object o)

Compares the specified object with this map for equality. Returns true if the other object is a Map with the same mappings, that is,
o instanceof Map && entrySet().equals(((Map) o).entrySet();

Specified by:

equals in interface Map<K,V>

Overrides:

equals in class Object

Parameters:

o - the object to be compared

Returns:

true if the object equals this map

See Also:

Set.equals(Object)

get

public V get(Object key)

Returns the value mapped by the given key. Returns null if there is no mapping. However, in Maps that accept null values, you must rely on containsKey to determine if a mapping exists. This iteration takes linear time, searching entrySet().iterator() of the key. Many implementations override this method.

Specified by:

get in interface Map<K,V>

Parameters:

key - the key to look up

Returns:

the value associated with the key, or null if key not in map

Throws:

NullPointerException - if this map does not accept null keys

See Also:

containsKey(Object)

hashCode

public int hashCode()

Returns the hash code for this map. As defined in Map, this is the sum of all hashcodes for each Map.Entry object in entrySet, or basically entrySet().hashCode().

Specified by:

hashCode in interface Map<K,V>

Overrides:

hashCode in class Object

Returns:

the hash code

See Also:

Map.Entry.hashCode(), Set.hashCode()

isEmpty

public boolean isEmpty()

Returns true if the map contains no mappings. This is implemented by size() == 0.

Specified by:

isEmpty in interface Map<K,V>

Returns:

true if the map is empty

See Also:

size()

keySet

public Set<K> keySet()

Returns a set view of this map's keys. The set is backed by the map, so changes in one show up in the other. Modifications while an iteration is in progress produce undefined behavior. The set supports removal if entrySet() does, but does not support element addition.

This implementation creates an AbstractSet, where the iterator wraps the entrySet iterator, size defers to the Map's size, and contains defers to the Map's containsKey. The set is created on first use, and returned on subsequent uses, although since no synchronization occurs, there is a slight possibility of creating two sets.

Specified by:

keySet in interface Map<K,V>

Returns:

a Set view of the keys

See Also:

Set.iterator(), size(), containsKey(Object), values()

put

public V put(K key,
             V value)

Associates the given key to the given value (optional operation). If the map already contains the key, its value is replaced. This implementation simply throws an UnsupportedOperationException. Be aware that in a map that permits null values, a null return does not always imply that the mapping was created.

Specified by:

put in interface Map<K,V>

Parameters:

key - the key to map

value - the value to be mapped

Returns:

the previous value of the key, or null if there was no mapping

Throws:

UnsupportedOperationException - if the operation is not supported

ClassCastException - if the key or value is of the wrong type

IllegalArgumentException - if something about this key or value prevents it from existing in this map

NullPointerException - if the map forbids null keys or values

See Also:

containsKey(Object)

putAll

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

Copies all entries of the given map to this one (optional operation). If the map already contains a key, its value is replaced. This implementation simply iterates over the map's entrySet(), calling put, so it is not supported if puts are not.

Specified by:

putAll in interface Map<K,V>

Parameters:

m - the mapping to load into this map

Throws:

UnsupportedOperationException - if the operation is not supported by this map.

ClassCastException - if a key or value is of the wrong type for adding to this map.

IllegalArgumentException - if something about a key or value prevents it from existing in this map.

NullPointerException - if the map forbids null keys or values.

NullPointerException - if m is null.

See Also:

put(Object, Object)

remove

public V remove(Object key)

Removes the mapping for this key if present (optional operation). This implementation iterates over the entrySet searching for a matching key, at which point it calls the iterator's remove method. It returns the result of getValue() on the entry, if found, or null if no entry is found. Note that maps which permit null values may also return null if the key was removed. If the entrySet does not support removal, this will also fail. This is O(n), so many implementations override it for efficiency.

Specified by:

remove in interface Map<K,V>

Parameters:

key - the key to remove

Returns:

the value the key mapped to, or null if not present. Null may also be returned if null values are allowed in the map and the value of this mapping is null.

Throws:

UnsupportedOperationException - if deletion is unsupported

See Also:

Iterator.remove()

size

public int size()

Returns the number of key-value mappings in the map. If there are more than Integer.MAX_VALUE mappings, return Integer.MAX_VALUE. This is implemented as entrySet().size().

Specified by:

size in interface Map<K,V>

Returns:

the number of mappings

See Also:

Set.size()

toString

public String toString()

Returns a String representation of this map. This is a listing of the map entries (which are specified in Map.Entry as being getKey() + "=" + getValue()), separated by a comma and space (", "), and surrounded by braces ('{' and '}'). This implementation uses a StringBuffer and iterates over the entrySet to build the String. Note that this can fail with an exception if underlying keys or values complete abruptly in toString().

Overrides:

toString in class Object

Returns:

a String representation

See Also:

Map.Entry#toString()

values

public Collection<V> values()

Returns a collection or bag view of this map's values. The collection is backed by the map, so changes in one show up in the other. Modifications while an iteration is in progress produce undefined behavior. The collection supports removal if entrySet() does, but does not support element addition.

This implementation creates an AbstractCollection, where the iterator wraps the entrySet iterator, size defers to the Map's size, and contains defers to the Map's containsValue. The collection is created on first use, and returned on subsequent uses, although since no synchronization occurs, there is a slight possibility of creating two collections.

Specified by:

values in interface Map<K,V>

Returns:

a Collection view of the values

See Also:

Collection.iterator(), size(), containsValue(Object), keySet()