Class EnumMap<K extends Enum<K>,​V>

All Implemented Interfaces:
Serializable, Cloneable, Map<K,​V> 
public class EnumMap<K extends Enum<K>,V>
extends AbstractMap<K,​V>
implements Serializable, Cloneable

A specialized Map implementation for use with enum type keys. All of the keys in an enum map must come from a single enum type that is specified, explicitly or implicitly, when the map is created. Enum maps are represented internally as arrays. This representation is extremely compact and efficient.

Enum maps are maintained in the natural order of their keys (the order in which the enum constants are declared). This is reflected in the iterators returned by the collections views (keySet(), entrySet(), and values()).

Iterators returned by the collection views are weakly consistent: they will never throw ConcurrentModificationException and they may or may not show the effects of any modifications to the map that occur while the iteration is in progress.

Null keys are not permitted. Attempts to insert a null key will throw NullPointerException. Attempts to test for the presence of a null key or to remove one will, however, function properly. Null values are permitted.

Like most collection implementations EnumMap is not synchronized. If multiple threads access an enum map concurrently, and at least one of the threads modifies the map, it should be synchronized externally. This is typically accomplished by synchronizing on some object that naturally encapsulates the enum map. If no such object exists, the map should be "wrapped" using the Collections.synchronizedMap(java.util.Map<K, V>) method. This is best done at creation time, to prevent accidental unsynchronized access: 

Map<EnumKey, V> m
         = Collections.synchronizedMap(new EnumMap<EnumKey, V>(...));

Implementation note: All basic operations execute in constant time. They are likely (though not guaranteed) to be faster than their HashMap counterparts.

This class is a member of the Java Collections Framework.

Since:
1.5
See Also:
EnumSet, Serialized Form

Nested Classes

Nested classes/interfaces declared in class java.util.AbstractMap

AbstractMap.SimpleEntry<K,​V>, AbstractMap.SimpleImmutableEntry<K,​V>

Nested classes/interfaces declared in interface java.util.Map

Map.Entry<K,​V>

Constructors

Constructor Description
EnumMap​(Class<K> keyType)

Creates an empty enum map with the specified key type.
EnumMap​(EnumMap<K,​? extends V> m)

Creates an enum map with the same key type as the specified enum map, initially containing the same mappings (if any).
EnumMap​(Map<K,​? extends V> m)

Creates an enum map initialized from the specified map.

Methods

Modifier and Type Method Description
void clear()

Removes all mappings from this map.
EnumMap<K,​V> clone()

Returns a shallow copy of this enum map.
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.
Set<Map.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.
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.
void putAll​(Map<? extends K,​? extends V> m)

Copies all of the mappings from the specified map to this map.
V remove​(Object key)

Removes the mapping for this key from this map if present.
int size()

Returns the number of key-value mappings in this map.
Collection<V> values()

Returns a Collection view of the values contained in this map.

Methods declared in class java.util.AbstractMap

isEmpty, toString

Methods declared in class java.lang.Object

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

Methods declared in interface java.util.Map

compute, computeIfAbsent, computeIfPresent, forEach, getOrDefault, merge, putIfAbsent, remove, replace, replace, replaceAll

Constructors

EnumMap

public EnumMap(Class<K> keyType)

Creates an empty enum map with the specified key type.

Parameters:
keyType - the class object of the key type for this enum map
Throws:
NullPointerException - if keyType is null

EnumMap

public EnumMap(EnumMap<K,​? extends V> m)

Creates an enum map with the same key type as the specified enum map, initially containing the same mappings (if any).

Parameters:
m - the enum map from which to initialize this enum map
Throws:
NullPointerException - if m is null

EnumMap

public EnumMap(Map<K,​? extends V> m)

Creates an enum map initialized from the specified map. If the specified map is an EnumMap instance, this constructor behaves identically to EnumMap(EnumMap). Otherwise, the specified map must contain at least one mapping (in order to determine the new enum map's key type).

Parameters:
m - the map from which to initialize this enum map
Throws:
IllegalArgumentException - if m is not an EnumMap instance and contains no mappings
NullPointerException - if m is null

Methods

size

public int size()

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

Specified by:
size in interface Map<K extends Enum<K>,​V>
Overrides:
size in class AbstractMap<K extends Enum<K>,​V>
Returns:
the number of key-value mappings in this map

containsValue

public boolean containsValue(Object value)

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

Specified by:
containsValue in interface Map<K extends Enum<K>,​V>
Overrides:
containsValue in class AbstractMap<K extends Enum<K>,​V>
Parameters:
value - the value whose presence in this map is to be tested
Returns:
true if this map maps one or more keys to this value

containsKey

public boolean containsKey(Object key)

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

Specified by:
containsKey in interface Map<K extends Enum<K>,​V>
Overrides:
containsKey in class AbstractMap<K extends Enum<K>,​V>
Parameters:
key - the key whose presence in this map is to be tested
Returns:
true if this map contains a mapping for the specified key

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 (key == k), then this method returns v; otherwise it returns null. (There can be at most one such mapping.)

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.

Specified by:
get in interface Map<K extends Enum<K>,​V>
Overrides:
get in class AbstractMap<K extends Enum<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

put

public V put(K key,
             V value)

Associates the specified value with the specified key in this map. If the map previously contained a mapping for this key, the old value is replaced.

Specified by:
put in interface Map<K extends Enum<K>,​V>
Overrides:
put in class AbstractMap<K extends Enum<K>,​V>
Parameters:
key - the key with which the specified value is to be associated
value - the value to be associated with the specified key
Returns:
the previous value associated with specified key, or null if there was no mapping for key. (A null return can also indicate that the map previously associated null with the specified key.)
Throws:
NullPointerException - if the specified key is null

remove

public V remove(Object key)

Removes the mapping for this key from this map if present.

Specified by:
remove in interface Map<K extends Enum<K>,​V>
Overrides:
remove in class AbstractMap<K extends Enum<K>,​V>
Parameters:
key - the key whose mapping is to be removed from the map
Returns:
the previous value associated with specified key, or null if there was no entry for key. (A null return can also indicate that the map previously associated null with the specified key.)

putAll

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

Copies all of the mappings from the specified map to this map. These mappings will replace any mappings that this map had for any of the keys currently in the specified map.

Specified by:
putAll in interface Map<K extends Enum<K>,​V>
Overrides:
putAll in class AbstractMap<K extends Enum<K>,​V>
Parameters:
m - the mappings to be stored in this map
Throws:
NullPointerException - the specified map is null, or if one or more keys in the specified map are null

clear

public void clear()

Removes all mappings from this map.

Specified by:
clear in interface Map<K extends Enum<K>,​V>
Overrides:
clear in class AbstractMap<K extends Enum<K>,​V>

keySet

public Set<K> keySet()

Returns a Set view of the keys contained in this map. The returned set obeys the general contract outlined in Map.keySet(). The set's iterator will return the keys in their natural order (the order in which the enum constants are declared).

Specified by:
keySet in interface Map<K extends Enum<K>,​V>
Overrides:
keySet in class AbstractMap<K extends Enum<K>,​V>
Returns:
a set view of the keys contained in this enum map

values

public Collection<V> values()

Returns a Collection view of the values contained in this map. The returned collection obeys the general contract outlined in Map.values(). The collection's iterator will return the values in the order their corresponding keys appear in map, which is their natural order (the order in which the enum constants are declared).

Specified by:
values in interface Map<K extends Enum<K>,​V>
Overrides:
values in class AbstractMap<K extends Enum<K>,​V>
Returns:
a collection view of the values contained in this map

entrySet

public Set<Map.Entry<K,V>> entrySet()

Returns a Set view of the mappings contained in this map. The returned set obeys the general contract outlined in Map.keySet(). The set's iterator will return the mappings in the order their keys appear in map, which is their natural order (the order in which the enum constants are declared).

Specified by:
entrySet in interface Map<K extends Enum<K>,​V>
Returns:
a set view of the mappings contained in this enum map

equals

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, as specified in the Map.equals(Object) contract.

Specified by:
equals in interface Map<K extends Enum<K>,​V>
Overrides:
equals in class AbstractMap<K extends Enum<K>,​V>
Parameters:
o - the object to be compared for equality with this map
Returns:
true if the specified object is equal to this map
See Also:
Object.hashCode(), HashMap

hashCode

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.

Specified by:
hashCode in interface Map<K extends Enum<K>,​V>
Overrides:
hashCode in class AbstractMap<K extends Enum<K>,​V>
Returns:
the hash code value for this map
See Also:
Map.Entry.hashCode(), Object.equals(Object), Set.equals(Object)

clone

public EnumMap<K,V> clone()

Returns a shallow copy of this enum map. The values themselves are not cloned.

Overrides:
clone in class AbstractMap<K extends Enum<K>,​V>
Returns:
a shallow copy of this enum map
See Also:
Cloneable

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https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/EnumMap.html