Module java.base
Package java.util

Class EnumMap<K extendsEnum<K>,V>

java.lang.Object
java.util.AbstractMap<K,V>
java.util.EnumMap<K,V>
All Implemented Interfaces:
Serializable,Cloneable,Map<K,V>
public classEnumMap<K extendsEnum<K>,V>extendsAbstractMap<K,V>implementsSerializable,Cloneable
A specializedMap 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 thenatural 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(), andvalues()).

Iterators returned by the collection views areweakly consistent: they will never throwConcurrentModificationException 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 throwNullPointerException. 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 implementationsEnumMap 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 theCollections.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 theirHashMap counterparts.

This class is a member of the Java Collections Framework.

Since:
1.5
See Also:

  • Constructor Details

    • 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 - ifkeyType is null

    • EnumMap

      public EnumMap(EnumMap<K,? extendsV> 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 - ifm is null

    • EnumMap

      public EnumMap(Map<K,? extendsV> m)
      Creates an enum map initialized from the specified map. If the specified map is anEnumMap instance, this constructor behaves identically toEnumMap(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 - ifm is not anEnumMap instance and contains no mappings
      NullPointerException - ifm is null

  • Method Details

    • size

      public int size()
      Returns the number of key-value mappings in this map.
      Specified by:
      size in interface Map<K extendsEnum<K>,V>
      Overrides:
      size in class AbstractMap<K extendsEnum<K>,V>
      Returns:
      the number of key-value mappings in this map

    • containsValue

      public boolean containsValue(Object value)
      Returnstrue if this map maps one or more keys to the specified value.
      Specified by:
      containsValue in interface Map<K extendsEnum<K>,V>
      Overrides:
      containsValue in class AbstractMap<K extendsEnum<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)
      Returnstrue if this map contains a mapping for the specified key.
      Specified by:
      containsKey in interface Map<K extendsEnum<K>,V>
      Overrides:
      containsKey in class AbstractMap<K extendsEnum<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, ornull if this map contains no mapping for the key.

      More formally, if this map contains a mapping from a keyk to a valuev such that(key == k), then this method returnsv; otherwise it returnsnull. (There can be at most one such mapping.)

      A return value ofnull does notnecessarily indicate that the map contains no mapping for the key; it's also possible that the map explicitly maps the key tonull. ThecontainsKey operation may be used to distinguish these two cases.

      Specified by:
      get in interface Map<K extendsEnum<K>,V>
      Overrides:
      get in class AbstractMap<K extendsEnum<K>,V>
      Parameters:
      key - the key whose associated value is to be returned
      Returns:
      the value to which the specified key is mapped, ornull 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 extendsEnum<K>,V>
      Overrides:
      put in class AbstractMap<K extendsEnum<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, ornull if there was no mapping for key. (Anull return can also indicate that the map previously associatednull 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 extendsEnum<K>,V>
      Overrides:
      remove in class AbstractMap<K extendsEnum<K>,V>
      Parameters:
      key - the key whose mapping is to be removed from the map
      Returns:
      the previous value associated with specified key, ornull if there was no entry for key. (Anull return can also indicate that the map previously associatednull with the specified key.)

    • putAll

      public void putAll(Map<? extendsK,? extendsV> 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 extendsEnum<K>,V>
      Overrides:
      putAll in class AbstractMap<K extendsEnum<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 extendsEnum<K>,V>
      Overrides:
      clear in class AbstractMap<K extendsEnum<K>,V>

    • keySet

      public Set<K> keySet()
      Returns aSet view of the keys contained in this map. The returned set obeys the general contract outlined inMap.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 extendsEnum<K>,V>
      Overrides:
      keySet in class AbstractMap<K extendsEnum<K>,V>
      Returns:
      a set view of the keys contained in this enum map

    • values

      public Collection<V> values()
      Returns aCollection view of the values contained in this map. The returned collection obeys the general contract outlined inMap.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 extendsEnum<K>,V>
      Overrides:
      values in class AbstractMap<K extendsEnum<K>,V>
      Returns:
      a collection view of the values contained in this map

    • entrySet

      public Set<Map.Entry<K,V>> entrySet()
      Returns aSet view of the mappings contained in this map. The returned set obeys the general contract outlined inMap.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 extendsEnum<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. Returnstrue if the given object is also a map and the two maps represent the same mappings, as specified in theMap.equals(Object) contract.
      Specified by:
      equals in interface Map<K extendsEnum<K>,V>
      Overrides:
      equals in class AbstractMap<K extendsEnum<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:

    • 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 extendsEnum<K>,V>
      Overrides:
      hashCode in class AbstractMap<K extendsEnum<K>,V>
      Returns:
      the hash code value for this map
      See Also:

    • 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 extendsEnum<K>,V>
      Returns:
      a shallow copy of this enum map
      See Also: