Re: Alternative to if...else for keyword based actions

Mark Space <>
Mon, 04 Aug 2008 16:24:53 -0700
<vdMlk.17898$> wrote:

Thanks Everyone!
Ram's suggestion for directly manipulating the classes would work best
for me in this instance since further on in the program the classes
and methods will be constructed at runtime from external files. For

Yes, if you're going to make tokens dynamically, you'll need a Map or
some close equivalent. Enums are only for finite lists who's values are
known at compile time. That's the whole point of them being enumerations.

I worked out a slightly cleaner example of Enums, using a constructor
and a separate Action object for the evaluation part. The Action
objects are function objects (I think) and using the pattern suggested
by Bloch for function objects cleans up the code nicely.

This is cut and paste parts, sorry if I miss something which causes the
code not to compile:

public class Main {

     public static void main(String[] args) {
         String token = "println";
         String param = "Hello World!";
// Tokens t = Tokens.valueOf( token.toUpperCase() );
// t.eval( param );
         Tokens2 t2 = Tokens2.valueOf( token.toUpperCase() );
         t2.eval( param );


enum Tokens2
     ADD( Dummy.EVAL ),
     DIVIDE( Dummy.EVAL ),
     MULTIPLY( Dummy.EVAL ),
     PRINTLN( PrintLn.EVAL ),
     SUBTRACT( Dummy.EVAL );

     final private Action action;

     Tokens2( Action a ){
         action = a;
     public Object eval( Object ... parmList ) {
         return action.doIt(parmList);

interface Action {
     Object doIt( Object ... paramList );

final class PrintLn implements Action
     public static final PrintLn EVAL = new PrintLn();
     private PrintLn() {};
     public Object doIt(Object... paramList) {
         for( Object o : paramList )
             System.out.print( o );
         return 1;

final class Dummy implements Action
     public static final Dummy EVAL = new Dummy();
     private Dummy() {};
     public Object doIt(Object... paramList) {
         throw new UnsupportedOperationException("Not supported yet.");

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