java.util
class EnumMap

java.lang.Object java.util.AbstractMap java.util.EnumMap

All Implemented Interfaces:

Serializable, Cloneable, Map

public class EnumMap
extends AbstractMap
implements Serializable, Cloneable

A specialized {@link 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 ({@link #keySet()}, {@link #entrySet()}, and {@link #values()}).

Iterators returned by the collection views are weakly consistent: they will never throw {@link 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 {@link 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 {@link Collections#synchronizedMap} 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 {@link HashMap} counterparts.

This class is a member of the Java Collections Framework.

EnumMap

public EnumMap(Class keyType)

Creates an empty enum map with the specified key type.

Parameters:

keyType - the class object of the key type for this enum map

EnumMap

public EnumMap(EnumMap 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

EnumMap

public EnumMap(Map m)

Creates an enum map initialized from the specified map. If the specified map is an EnumMap instance, this constructor behaves identically to {@link #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

clear

public void clear()

Removes all mappings from this map.

Overrides:

clear in class AbstractMap

clone

public EnumMap clone()

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

Overrides:

clone in class AbstractMap

Returns:

a shallow copy of this enum map

containsKey

public boolean containsKey(Object key)

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

Overrides:

containsKey in class AbstractMap

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

containsValue

public boolean containsValue(Object value)

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

Overrides:

containsValue in class AbstractMap

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

entrySet

public Set entrySet()

Returns a {@link Set} view of the mappings contained in this map. The returned set obeys the general contract outlined in {@link 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).

Overrides:

entrySet in class AbstractMap

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 {@link Map#equals(Object)} contract.

Overrides:

equals in class AbstractMap

Parameters:

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

Returns:

true if the specified object is equal to this map

get

public Object get(Object key)

Returns the value to which the specified key is mapped, or {@code null} if this map contains no mapping for the key.

More formally, if this map contains a mapping from a key {@code k} to a value {@code v} such that {@code (key == k)}, then this method returns {@code v}; otherwise it returns {@code null}. (There can be at most one such mapping.)

A return value of {@code 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 {@code null}. The {@link #containsKey containsKey} operation may be used to distinguish these two cases.

Overrides:

get in class AbstractMap

Parameters:

key

keySet

public Set keySet()

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

Overrides:

keySet in class AbstractMap

Returns:

a set view of the keys contained in this enum map

put

public Object put(Enum key,
                  Object 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.

Overrides:

put in class AbstractMap

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.)

putAll

public void putAll(Map 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.

Overrides:

putAll in class AbstractMap

Parameters:

m - the mappings to be stored in this map

remove

public Object remove(Object key)

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

Overrides:

remove in class AbstractMap

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.)

size

public int size()

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

Overrides:

size in class AbstractMap

Returns:

the number of key-value mappings in this map

values

public Collection values()

Returns a {@link Collection} view of the values contained in this map. The returned collection obeys the general contract outlined in {@link 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).

Overrides:

values in class AbstractMap

Returns:

a collection view of the values contained in this map