Interfaces and Polymorphism - Java Concepts - page 578

29 }
30 }
31
32 ActionListener listener = new
TimerListener();
33
34 final int DELAY = 100 ; // Milliseconds between timer
ticks
35 Timer t = new Timer(DELAY, listener);
36 t.start();
37 }
38
39 private static final int FRAME_WIDTH = 300 ;
40 private static final int FRAME_HEIGHT = 400 ;
41 }

420
421

S ELF C HECK

19. Why does a timer require a listener object?

20. What would happen if you omitted the call to repaint in the moveBy
method?

C OMMON E RROR 9.5: Forgetting to Repaint

You have to be careful when your event handlers change the data in a painted
component. When you make a change to the data, the component is not
automatically painted with the new data. You must tell the Swing framework that
the component needs to be repainted, by calling the repaint method either in the
event handler or in the component's mutator methods. Your component's
paintComponent method will then be invoked at an opportune moment, with
an appropriate Graphics object. Note that you should not call the
paintComponent method directly.

This is a concern only for your own painted components. When you make a
change to a standard Swing component such as a JLabel , the component is
automatically repainted.