How to achieve abstract variable functionality in Java

Question

I am trying to create a custom class that extends another class (JFrame) but forces the assignment of a certain variable upon implementation (I want each JFrame in my application to have a "screen ID"). Java, however, does not have abstract variables. Nor am I able to figure out how to make an interface that extends JFrame. This one's really got my head spinning =D

The class code would look similar to this:

public interface CustomFrame extends javax.swing.JFrame {
    public abstract int screen_id;
}

And the implementation of CustomFrame would look something like this:

public class NewFrame implements CustomFrame {
    public int screen_id = 5;
    NewFrame() {
        setVisible(true);
        // etc...
    }
}

Does this problem even make sense to anyone?? I know what my objective is I am just lost trying to work it out in my brain....

1) Why would you even want to extend JFrame? 2) It's as if your program will have multiple JFrames, which it really shouldn't. I think that you might want to re-think your overall program design. — Hovercraft Full Of Eels, Aug 23 '13 at 22:22
@HovercraftFullOfEels, thanks for the suggestion. You can read my $.02 about that here: http://stackoverflow.com/questions/9554636/the-use-of-multiple-jframes-good-bad-practice/17961122#17961122. — ryvantage, Aug 23 '13 at 22:27
Make the frame abstract, require implementations to implement an abstract method — MadProgrammer, Aug 23 '13 at 22:43
interface is the contract you can't extends an interface with concrete implementation... i don't know what you want to achieve, i didnt understand but i think with a `Map` you get what you want.. — nachokk, Aug 23 '13 at 23:29
why do you need to id the _view_ (vs. the backing data model)? — kleopatra, Aug 24 '13 at 10:18

Luca Basso Ricci · Answer 1 · 2013-08-23T22:27:56.070

0

Create an external FrameID generator that generate a new ID at each call (just a simple example implemented as singleton: no synch or generic return type etc):

class FrameIDGenerator {
  private static int nextID = 0;
  private FrameIDGenerator(){}
  private static FrameIDGenerator instance = null;

  public final static FrameIDGenerator getInstance() {
    if(null == instance) instance = new FrameIDGenerator();
    return instance;
  }

  public int getNextID() {
    return ++nextID;
  }
}

edited Aug 23 '13 at 22:27

answered Aug 23 '13 at 22:22

Luca Basso Ricci

17,829
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this doesn't work for my solution because my screen_ids are written to a database. So, the screen_id really needs to be a serial # that only and always refers to one `JFrame` and that `JFrame`'s screen_id never changes, ever. – ryvantage Aug 24 '13 at 02:41

score 0 · Answer 2 · answered Aug 23 '13 at 22:33

0

From what I am understanding, it is a 3rd party (not JFrame itself) who needs the number. I don't think it's a good idea to try to force JFrame (who already has a lot of concerns) into this burden.

Please consider the alternative of keeping an external IdentityHashMap<JFrame,Integer> to store this information.

answered Aug 23 '13 at 22:33

Mario Rossi

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that is currently what I'm already doing. But it leaves a lot of room for careless mistakes, especially when things change. What I was hoping for was some sort of "contract" based numbering, i.e., where every `JFrame` was contractually bound to contain a screen_id – ryvantage Aug 24 '13 at 02:38
Actually, I don't use an `IdentityHashMap` because identity is established on an equals (`==`) basis, whereas what I need is for classes (not instances) to be equal to each other. So what I use is a `DualLinkedHashBidiMap`. – ryvantage Aug 24 '13 at 03:29

How to achieve abstract variable functionality in Java

2 Answers2