@version $Revision: 1.5 $ $ $Date: 2008/04/11 11:11:59 $
This example will return RED, GREEN, BLUE, YELLOW from the List and iterator methods in that order. The RED, GREEN and BLUE instances will be the same (==) as those from the superclass ColorEnum. Note that YELLOW is declared as a ColorEnum and not an ExtraColorEnum.
The enums can have functionality by defining subclasses and overriding the getEnumClass()
method:
public static final OperationEnum PLUS = new PlusOperation(); private static final class PlusOperation extends OperationEnum { private PlusOperation() { super("Plus"); } public int eval(int a, int b) { return a + b; } } public static final OperationEnum MINUS = new MinusOperation(); private static final class MinusOperation extends OperationEnum { private MinusOperation() { super("Minus"); } public int eval(int a, int b) { return a - b; } } private OperationEnum(String color) { super(color); } public final Class getEnumClass() { // NOTE: new method! return OperationEnum.class; } public abstract double eval(double a, double b); public static OperationEnum getEnum(String name) { return (OperationEnum) getEnum(OperationEnum.class, name); } public static Map getEnumMap() { return getEnumMap(OperationEnum.class); } public static List getEnumList() { return getEnumList(OperationEnum.class); } public static Iterator iterator() { return iterator(OperationEnum.class); } }
The code above will work on JDK 1.2. If JDK1.3 and later is used, the subclasses may be defined as anonymous.
Care must be taken with class loading when defining a static nested class for enums. The static nested class can be loaded without the surrounding outer class being loaded. This can result in an empty list/map/iterator being returned. One solution is to define a static block that references the outer class where the constants are defined. For example:
public final class Outer { public static final BWEnum BLACK = new BWEnum("Black"); public static final BWEnum WHITE = new BWEnum("White"); // static nested enum class public static final class BWEnum extends Enum { static { // explicitly reference BWEnum class to force constants to load Object obj = Outer.BLACK; } // ... other methods omitted } }
Although the above solves the problem, it is not recommended. The best solution is to define the constants in the enum class, and hold references in the outer class:
public final class Outer { public static final BWEnum BLACK = BWEnum.BLACK; public static final BWEnum WHITE = BWEnum.WHITE; // static nested enum class public static final class BWEnum extends Enum { // only define constants in enum classes - private if desired private static final BWEnum BLACK = new BWEnum("Black"); private static final BWEnum WHITE = new BWEnum("White"); // ... other methods omitted } }
For more details, see the 'Nested' test cases. @author Apache Avalon project @author Stephen Colebourne @author Chris Webb @author Mike Bowler @author Matthias Eichel @since 2.1 (class existed in enum package from v1.0) @version $Id: Enum.java 161243 2005-04-14 04:30:28Z ggregory $
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