swing - How to achieve abstract variable functionality in Java -

i trying create custom class extends class (jframe) forces assignment of variable upon implementation (i want each jframe in application have "screen id"). java, however, not have abstract variables. nor able figure out how make interface extends jframe. one's got head spinning =d

the class code similar this:

public interface customframe extends javax.swing.jframe {     public abstract int screen_id; } 

and implementation of customframe this:

public class newframe implements customframe {     public int screen_id = 5;     newframe() {         setvisible(true);         // etc...     } } 

does problem make sense anyone?? know objective lost trying work out in brain....

create external frameid generator generate new id @ each call (just simple example implemented 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;   } }