Re: I don't understand enums

From:

Robert Klemme <shortcutter@googlemail.com>

Newsgroups:

comp.lang.java.programmer

Date:

Sat, 26 Jan 2008 23:53:37 +0100

Message-ID:

<601rviF1nq3t6U1@mid.individual.net>

On 26.01.2008 23:45, Paul Tomblin wrote:

I have some existing code that uses the classic old

class Playlist
{
  public static final int PLAYLISTTYPE_FEATURE = 1;
  public static final int PLAYLISTTYPE_TRAILER = 2;
...

But now that we're using Java 1.5, I've got new code where I decided to
use a Java 1.5 enum:

public enum PlaylistType
{
    FEATURE(1, "Feature"),
    TRAILER(2, "Trailer"),
...

    final int id;
    final String name;

    PlaylistType(final int id, final String name)
    {
        this.id = id;
        this.name = name;
    }

    public int getID()
    {
        return id;
    }

    public String toString()
    {
        return name;
    }
}

In order to make sure the new code and the old code work together, I
changed the definition of the old ints to
  public static final int PLAYLISTTYPE_FEATURE = PlaylistType.FEATURE.getID();
  public static final int PLAYLISTTYPE_TRAILER = PlaylistType.TRAILER.getID();

Note that they're still "public static final".

But now case statements that worked before the change are breaking with
Eclipse complaining that "case expressions must be constant expressions".
Huh? They sure look constant to me. How could it think that
PLAYLISTTYPE_FEATURE could change values when it's declared final?

They are constant for one run of the program. But they might not
totally be constant. Eclipse cannot know that the return value of
getID() will never change. That's why. You probably should do yourself
a favour and do a global search and replace from the old constants to
the new constants. OR you drop the old constants and reuse the names.

Btw, id is not necessary, there is Enum.ordinal(). And if you do not
insist on the case of the name then you also do not need the name parameter.

Here's what I'd probably do

// untested
class PlayList
   public static enum Type {
     Feature,
     Trailer,
     // more to come
   }

   // other members
end

Note that you can use enum values directly in switch statements. There
is usually no need for access to the ordinal.

Kind regards

    robert

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