Some basic ideas:

Classes:

Heirarchy:

Names:

Larger the scope, longer the name.
 

Global variables:

 AVOID, AVOID, AVOID. If you must use, need to have VERY STRONG justification.
 

Position of open and close braces:

Bad positioning:

     private void actionPerformed(ActionEvent e)
     {
         String s = e.getActionCommand();
         if (s.equals("Left"))
         {
                display.setText("  <---");
         }
         else if (s.equals("Right"))
         {
                 display.setText("  --->");
         }
      }

Should be:

     private void actionPerformed(ActionEvent e){
         String s = e.getActionCommand();
         if (s.equals("Left")){
                display.setText("  <---");
         }
         else if (s.equals("Right")){
                 display.setText("  --->");
         }
      }

Basic idea is to get more into a screenful.  Dont overdo this; if you put the "}" on the previous line, you will make it inconvenient to comment out "//" that line; or worse, when you comment out that line, you will comment out the "}" unwittingly.

Indentation amount:

3 characters.  Use an editor like emacs that sets indentation properly as you type and also pretty prints the code when needed.  Basic idea is to use the 80 columns you have efficiently.  If indentation is too high, you will need to break lines into 2, or worse, go beyond 80 columns.