java.util
Class Collections.EmptyMap

java.lang.Object
  java.util.AbstractMap
      java.util.Collections.EmptyMap

All Implemented Interfaces:

Map, Serializable

Enclosing class:

Collections

private static class Collections.EmptyMap

extends AbstractMap

implements Serializable

Nested Class Summary

Nested classes inherited from class java.util.AbstractMap

AbstractMap.SimpleEntry

Nested classes inherited from class java.util.Map

Map.Entry

Field Summary

private static long

serialVersionUID

Fields inherited from class java.util.AbstractMap

keySet, values

Constructor Summary

private

Collections.EmptyMap()

Method Summary

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 this value.
Set	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.
Object	get(Object key) Returns the value to which this map maps the specified key.
int	hashCode() Returns the hash code value for this map.
boolean	isEmpty() Returns true if this map contains no key-value mappings.
Set	keySet() Returns a Set view of the keys contained in this map.
private Object	readResolve()
int	size() Returns the number of key-value mappings in this map.
Collection	values() Returns a collection view of the values contained in this map.

Methods inherited from class java.util.AbstractMap

clear, clone, put, putAll, remove, toString

Methods inherited from class java.lang.Object

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

Field Detail

serialVersionUID

private static final long serialVersionUID

See Also:

Constant Field Values

Constructor Detail

Collections.EmptyMap

private Collections.EmptyMap()

Method Detail

size

public int size()

Description copied from class: AbstractMap

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 entrySet().size().

Specified by:

size in interface Map

Overrides:

size in class AbstractMap

Returns:

the number of key-value mappings in this map.

isEmpty

public boolean isEmpty()

Description copied from class: AbstractMap

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

This implementation returns size() == 0.

Specified by:

isEmpty in interface Map

Overrides:

isEmpty in class AbstractMap

Returns:

true if this map contains no key-value mappings.

containsKey

public boolean containsKey(Object key)

Description copied from class: AbstractMap

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

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.

Specified by:

containsKey in interface Map

Overrides:

containsKey in class AbstractMap

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)

Description copied from class: AbstractMap

Returns true if this map maps one or more keys to this value. More formally, returns true if and only if this map contains at least one mapping to a value v such that (value==null ? v==null : value.equals(v)). This operation will probably require time linear in the map size for most implementations of map.

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.

Specified by:

containsValue in interface Map

Overrides:

containsValue in class AbstractMap

Parameters:

value - value whose presence in this map is to be tested.

Returns:

true if this map maps one or more keys to this value.

get

public Object get(Object key)

Description copied from class: AbstractMap

Returns the value to which this map maps the specified key. Returns null if the map contains no mapping for this key. 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 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.

Specified by:

get in interface Map

Overrides:

get in class AbstractMap

Parameters:

key - key whose associated value is to be returned.

Returns:

the value to which this map maps the specified key.

See Also:

AbstractMap.containsKey(Object)

keySet

public Set keySet()

Description copied from class: AbstractMap

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, the results of the iteration are undefined.) The Set supports element removal, which removes the corresponding entry 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 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.

Specified by:

keySet in interface Map

Overrides:

keySet in class AbstractMap

Returns:

a Set view of the keys contained in this map.

values

public Collection values()

Description copied from class: AbstractMap

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, the results of the iteration are undefined.) The collection supports element removal, which removes the corresponding entry 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 collection that subclasses abstract collection. 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.

Specified by:

values in interface Map

Overrides:

values in class AbstractMap

Returns:

a collection view of the values contained in this map.

entrySet

public Set entrySet()

Description copied from class: AbstractMap

Returns a set view of the mappings contained in this map. Each element in this set is a Map.Entry. 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, the results of the iteration are undefined.) The set supports element removal, which removes the corresponding entry 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

Specified by:

entrySet in class AbstractMap

Returns:

a set view of the mappings contained in this map.

equals

public boolean equals(Object o)

Description copied from class: AbstractMap

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 t1 and t2 represent the same mappings if t1.keySet().equals(t2.keySet()) and for every key k in t1.keySet(), (t1.get(k)==null ? t2.get(k)==null : t1.get(k).equals(t2.get(k))) . This ensures that the equals method works properly across different implementations of the map interface.

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 set; if not, it 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.

Specified by:

equals in interface Map

Overrides:

equals in class AbstractMap

Parameters:

o - object to be compared for equality with this map.

Returns:

true if the specified object is equal to this map.

hashCode

public int hashCode()

Description copied from class: AbstractMap

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 t1.equals(t2) implies that t1.hashCode()==t2.hashCode() for any two maps t1 and t2, as required by the general contract of Object.hashCode.

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

Specified by:

hashCode in interface Map

Overrides:

hashCode in class AbstractMap

Returns:

the hash code value for this map.

See Also:

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

readResolve

private Object readResolve()