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EnumMap.javaAPI DocJava SE 5 API22993Fri Aug 26 14:57:22 BST 2005java.util

EnumMap

public class EnumMap extends AbstractMap implements Cloneable, Serializable
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(...));

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.

author
Josh Bloch
version
1.10, 07/15/04
see
EnumSet
since
1.5

Fields Summary
private final Class
keyType
The Class object for the enum type of all the keys of this map.
private transient K[]
keyUniverse
All of the values comprising K. (Cached for performance.)
private transient Object[]
vals
Array representation of this map. The ith element is the value to which universe[i] is currently mapped, or null if it isn't mapped to anything, or NULL if it's mapped to null.
private transient int
size
The number of mappings in this map.
private static final Object
NULL
Distinguished non-null value for representing null values.
private static Enum[]
ZERO_LENGTH_ENUM_ARRAY
private transient Set
entrySet
This field is initialized to contain an instance of the entry set view the first time this view is requested. The view is stateless, so there's no reason to create more than one.
private static final long
serialVersionUID
Constructors Summary
public EnumMap(Class keyType)
Creates an empty enum map with the specified key type.

param
keyType the class object of the key type for this enum map
throws
NullPointerException if keyType is null


                                      
       
        this.keyType = keyType;
        keyUniverse = keyType.getEnumConstants();
        vals = new Object[keyUniverse.length];
    
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).

param
m the enum map from which to initialize this enum map
throws
NullPointerException if m is null

        keyType = m.keyType;
        keyUniverse = m.keyUniverse;
        vals = (Object[]) m.vals.clone();
        size = m.size;
    
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).

param
m the map from which to initialize this enum map
throws
IllegalArgumentException if m is not an EnumMap instance and contains no mappings
throws
NullPointerException if m is null

        if (m instanceof EnumMap) {
            EnumMap<K, ? extends V> em = (EnumMap<K, ? extends V>) m;
            keyType = em.keyType;
            keyUniverse = em.keyUniverse;
            vals = (Object[]) em.vals.clone();
            size = em.size;
        } else {
            if (m.isEmpty())
                throw new IllegalArgumentException("Specified map is empty");
            keyType = m.keySet().iterator().next().getDeclaringClass();
            keyUniverse = keyType.getEnumConstants();
            vals = new Object[keyUniverse.length];
            putAll(m);
        }
    
Methods Summary
public voidclear()
Removes all mappings from this map.

        Arrays.fill(vals, null);
        size = 0;
    
public java.util.EnumMapclone()
Returns a shallow copy of this enum map. (The values themselves are not cloned.

return
a shallow copy of this enum map

        EnumMap<K, V> result = null;
        try {
            result = (EnumMap<K, V>) super.clone();
        } catch(CloneNotSupportedException e) {
            throw new AssertionError();
        }
        result.vals = (Object[]) result.vals.clone();
        return result;
    
public booleancontainsKey(java.lang.Object key)
Returns true if this map contains a mapping for the specified key.

param
key the key whose presence in this map is to be tested
return
true if this map contains a mapping for the specified key

        return isValidKey(key) && vals[((Enum)key).ordinal()] != null;
    
private booleancontainsMapping(java.lang.Object key, java.lang.Object value)

        return isValidKey(key) &&
            maskNull(value).equals(vals[((Enum)key).ordinal()]);
    
public booleancontainsValue(java.lang.Object value)
Returns true if this map maps one or more keys to the specified value.

param
value the value whose presence in this map is to be tested
return
true if this map maps one or more keys to this value

        value = maskNull(value);

        for (Object val : vals)
            if (value.equals(val))
                return true;

        return false;
    
public java.util.SetentrySet()
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).

return
a set view of the mappings contained in this enum map

        Set<Map.Entry<K,V>> es = entrySet;
        if (es != null)
            return es;
        else
            return entrySet = new EntrySet();
    
public booleanequals(java.lang.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.

param
o the object to be compared for equality with this map
return
true if the specified object is equal to this map

        if (!(o instanceof EnumMap))
            return super.equals(o);

        EnumMap em = (EnumMap)o;
        if (em.keyType != keyType)
            return size == 0 && em.size == 0;

        // Key types match, compare each value
        for (int i = 0; i < keyUniverse.length; i++) {
            Object ourValue =    vals[i];
            Object hisValue = em.vals[i];
            if (hisValue != ourValue &&
                (hisValue == null || !hisValue.equals(ourValue)))
                return false;
        }
        return true;
    
public Vget(java.lang.Object key)
Returns the value to which this map maps the specified key, or null if this map contains no mapping for the specified key.

param
key the key whose associated value is to be returned
return
the value to which this map maps the specified key, or null if this map contains no mapping for the specified key

        return (isValidKey(key) ?
                unmaskNull(vals[((Enum)key).ordinal()]) : null);
    
private booleanisValidKey(java.lang.Object key)
Returns true if key is of the proper type to be a key in this enum map.

        if (key == null)
            return false;

        // Cheaper than instanceof Enum followed by getDeclaringClass
        Class keyClass = key.getClass();
        return keyClass == keyType || keyClass.getSuperclass() == keyType;
    
public java.util.SetkeySet()
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).

return
a set view of the keys contained in this enum map


                                                                 
       
        Set<K> ks = keySet;
        if (ks != null)
            return ks;
        else
            return keySet = new KeySet();
    
private java.lang.ObjectmaskNull(java.lang.Object value)


        
        return (value == null ? NULL : value);
    
public Vput(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.

param
key the key with which the specified value is to be associated
param
value the value to be associated with the specified key
return
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

        typeCheck(key);

        int index = ((Enum)key).ordinal();
        Object oldValue = vals[index];
        vals[index] = maskNull(value);
        if (oldValue == null)
            size++;
        return unmaskNull(oldValue);
    
public voidputAll(java.util.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.

param
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

        if (m instanceof EnumMap) {
            EnumMap<? extends K, ? extends V> em =
                (EnumMap<? extends K, ? extends V>)m;
            if (em.keyType != keyType) {
                if (em.isEmpty())
                    return;
                throw new ClassCastException(em.keyType + " != " + keyType);
            }

            for (int i = 0; i < keyUniverse.length; i++) {
                Object emValue = em.vals[i];
                if (emValue != null) {
                    if (vals[i] == null)
                        size++;
                    vals[i] = emValue;
                }
            }
        } else {
            super.putAll(m);
        }
    
private voidreadObject(java.io.ObjectInputStream s)
Reconstitute the EnumMap instance from a stream (i.e., deserialize it).

        // Read in the key type and any hidden stuff
        s.defaultReadObject();

        keyUniverse = keyType.getEnumConstants();
        vals = new Object[keyUniverse.length];

        // Read in size (number of Mappings)
        int size = s.readInt();

        // Read the keys and values, and put the mappings in the HashMap
        for (int i = 0; i < size; i++) {
            K key = (K) s.readObject();
            V value = (V) s.readObject();
            put(key, value);
        }
    
public Vremove(java.lang.Object key)
Removes the mapping for this key from this map if present.

param
key the key whose mapping is to be removed from the map
return
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.)

        if (!isValidKey(key))
            return null;
        int index = ((Enum)key).ordinal();
        Object oldValue = vals[index];
        vals[index] = null;
        if (oldValue != null)
            size--;
        return unmaskNull(oldValue);
    
private booleanremoveMapping(java.lang.Object key, java.lang.Object value)

        if (!isValidKey(key))
            return false;
        int index = ((Enum)key).ordinal();
        if (maskNull(value).equals(vals[index])) {
            vals[index] = null;
            size--;
            return true;
        }
        return false;
    
public intsize()
Returns the number of key-value mappings in this map.

return
the number of key-value mappings in this map

        return size;
    
private voidtypeCheck(K key)
Throws an exception if e is not of the correct type for this enum set.

        Class keyClass = key.getClass();
        if (keyClass != keyType && keyClass.getSuperclass() != keyType)
            throw new ClassCastException(keyClass + " != " + keyType);
    
private VunmaskNull(java.lang.Object value)

        return (V) (value == NULL ? null : value);
    
public java.util.Collectionvalues()
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).

return
a collection view of the values contained in this map

        Collection<V> vs = values;
        if (vs != null)
            return vs;
        else
            return values = new Values();
    
private voidwriteObject(java.io.ObjectOutputStream s)
Save the state of the EnumMap instance to a stream (i.e., serialize it).

serialData
The size of the enum map (the number of key-value mappings) is emitted (int), followed by the key (Object) and value (Object) for each key-value mapping represented by the enum map.


                                                                                          
       
         
    
        // Write out the key type and any hidden stuff
        s.defaultWriteObject();

        // Write out size (number of Mappings)
        s.writeInt(size);

        // Write out keys and values (alternating)
        for (Map.Entry<K,V> e :  entrySet()) {
            s.writeObject(e.getKey());
            s.writeObject(e.getValue());
        }