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From: markspace <markspace@nospam.nospam>
Newsgroups: comp.lang.java.programmer
Subject: Re: use case for extending enum, but this is not possible in java
Date: Sat, 14 Jun 2014 18:58:21 -0700
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On 6/14/2014 4:24 PM, Laura Schmidt wrote:

>   public void onListCommand (ListCommand cmd)
>   {
>    AppListCommand c = (AppListCommand) cmd;
>
>    switch (c)
>    {
>     ...
>    }


I have to agree with Marcel Muller here.  Enums, the way you use them, 
are not polymorphic.  And using a switch to emulate polymorphism is 
almost always a bad idea.  See Replace Conditional with Polymorphism.

http://www.refactoring.com/catalog/replaceConditionalWithPolymorphism.html

However you can fake it a bit in Java.  Java enum are sorta-kinda 
polymorphic.  (I think that's the technical term.)


interface MyCommand {
   Object eval();
}

void onListCommand( MyCommand cmd ) {
   System.out.println( cmd.eval() );
}

public enum ListCommand implements MyCommand
{
  OPEN,
  EDIT,
  DELETE;
  public Object eval() { return this.ordinal(); }
}

public enum AppListCommand implements MyCommand
{
  OPEN,
  CLOSE,
  SOMETHINGSPECIAL() {
    public Object eval() { return "Hi";}};
  public Object eval() { return this.ordinal(); }
}

Not compiled or tested, although part was:

    public static void main( String[] args )
    {
       onListCommand( AppListCommand.OPEN, AppListCommand.CLOSE,
               AppListCommand.SOMETHINGSPECIAL );
    }
    private static void onListCommand( MyCommand... cmd )
    {
       for( MyCommand x : cmd ) {
          System.out.println( x.eval() );
       }
    }
run:
0
1
Hi
BUILD SUCCESSFUL (total time: 1 second)