/*______________________________________________________________________________ * * net.innig.util.EnumeratedType * *______________________________________________________________________________ * * Copyright 2001-2002 Kendall Helmstetter Gelner, Paul Cantrell * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions * are met: * * (1) Redistributions of source code must retain the above copyright * notice, this list of conditions and the following disclaimer. * * (2) Redistributions in binary form must reproduce the above copyright * notice, this list of conditions and the following disclaimer in * the documentation and/or other materials provided with the * distribution. * * (3) The name of the author may not be used to endorse or promote * products derived from this software without specific prior * written permission. * * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED * OF THE POSSIBILITY OF SUCH DAMAGE. *_______________________________________________________________________________ */ package net.innig.util; import java.util.*; /** An alternative to using hand-assigned integer constants for enumerated types. Subclass to create your own enumerated type (a.k.a. "typesafe enum"): 

public final class ColorType extends EnumeratedType
    *    {
    *    static public RED   = new ColorType("red");
    *    static public GREEN = new ColorType("green");
    *    static public BLUE  = new ColorType("blue");
    *    private ColorType( String type ) { super( type ); }
    *    }

There are three important features of an enumerated type class:

Because they have private constructors, you can use object identity (==) to differentiate enumerated types: 

void kermitCheck(ColorType color)
    *    {
    *    if(color == ColorType.GREEN)
    *        System.out.println("It's not easy!");
    *    }

Voila! Compile-time type safety! See Effective Java #21 for a superb exposition of enumerated types and their advantages, including examples with more conventional brace formatting.

This implementation of EnumeratedType keeps an internal pool of the types of a different class. For the class above, the call

EnumeratedType.allTypes(ColorType.class)
would return the set [ColorType.RED, ColorType.GREEN, ColorType.BLUE]. This internal pooling allows you to look up enumerated types by name:
EnumeratedType.resolveFromName(ColorType.class, "blue")
would return ColorType.BLUE.

EnumeratedType serializes correctly, preserving that all-important object identity on the receiving side -- you can use == on enumerated type objects sent over the wire.

Maturity: The serialization/resolve/allTypes support is relatively new. It's succeeded in unit tests, but not real-world tests. The basic enumerated type pattern is thoroughly established and well-worn.
Plans: Perhaps add JDO support.
@author Kendall Helmstetter Gelner, Paul Cantrell @version [Development version] */ public abstract class EnumeratedType implements java.io.Serializable { /** Returns the set of valid types for the given enumerated type class. This does * not include subclasses. * @throws IllegalArgumentException if the given class is not a subclass of EnumeratedType. */ public static Set/**/ allTypes(Class enumTypeClass) { checkEnumerated(enumTypeClass); return Collections.unmodifiableSet(new HashSet(getValuePool(enumTypeClass).values())); } /** Returns the set of valid type names for the given enumerated type class. This does * not include subclasses. * @throws IllegalArgumentException if the given class is not a subclass of EnumeratedType. */ public static Set/**/ allTypeNames(Class enumTypeClass) { checkEnumerated(enumTypeClass); return Collections.unmodifiableSet(getValuePool(enumTypeClass).keySet()); } /** Returns the enumerated type object of a given class with a given name, or null if there * is no type object with that name. * @throws IllegalArgumentException if the given class is not a subclass of EnumeratedType. */ public static EnumeratedType resolveFromName(Class enumTypeClass, String name) { checkEnumerated(enumTypeClass); return (EnumeratedType) getValuePool(enumTypeClass).get(name); } private static void checkEnumerated(Class enumTypeClass) { if(!EnumeratedType.class.isAssignableFrom(enumTypeClass)) throw new IllegalArgumentException( enumTypeClass.getName() + " is not a subclass of " + EnumeratedType.class.getName()); } private static Map/**/ getValuePool(Class c) { synchronized(valuePoolPool) { Map valuePool = (Map) valuePoolPool.get(c); if(valuePool == null) valuePoolPool.put(c, valuePool = new TreeMap()); return valuePool; } } private static Map/*>*/ valuePoolPool = new HashMap(); //----------------------------------------------------------------------------------------- /** Creates a new enumerated type with a name unique to the type's class. * @throws IllegalStateException if a type with the given name is already registered * for this class. */ protected EnumeratedType(String name) { this.name = name; synchronized(valuePoolPool) { Map pool = getValuePool(getClass()); if(pool.get(name) != null) throw new IllegalStateException( "Duplicate enumerated type name for " + getClass() + ": \"" + name + '"'); pool.put(name, this); } } /** Returns the name of this enumerated type. */ public final String getName() { return name; } /** Supports serialization of enumerated types. */ protected final Object readResolve() { return resolveFromName(getClass(), getName()); } public String toString() { return getName(); } /** Enumerated types always use object identity. */ public final boolean equals(Object that) { return this == that; } public final int hashCode() { return super.hashCode(); } private final String name; }