Groups > comp.lang.java.programmer > #28741

Re: use case for extending enum, but this is not possible in java

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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)

Thread

use case for extending enum, but this is not possible in java Laura Schmidt <ls@mailinator.com> - 2014-06-15 01:24 +0200
  Re: use case for extending enum, but this is not possible in java Steven Simpson <ss@domain.invalid> - 2014-06-15 01:04 +0100
  Re: use case for extending enum, but this is not possible in java Marcel Müller <news.5.maazl@spamgourmet.org> - 2014-06-15 02:53 +0200
  Re: use case for extending enum, but this is not possible in java markspace <markspace@nospam.nospam> - 2014-06-14 18:58 -0700
  Re: use case for extending enum, but this is not possible in java Robert Klemme <shortcutter@googlemail.com> - 2014-06-15 12:20 +0200
  Re: use case for extending enum, but this is not possible in java Robert Klemme <shortcutter@googlemail.com> - 2014-06-15 12:21 +0200
  Re: use case for extending enum, but this is not possible in java "Chris Uppal" <chris.uppal@metagnostic.REMOVE-THIS.org> - 2014-06-15 11:58 +0100
  Re: use case for extending enum, but this is not possible in java Wanja Gayk <brixomatic@yahoo.com> - 2014-06-16 01:08 +0200
  Re: use case for extending enum, but this is not possible in java Roedy Green <see_website@mindprod.com.invalid> - 2014-06-15 22:20 -0700

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