Interfaces and Polymorphism - Java Concepts - page 576

14 box = new Rectangle(BOX_X, BOX_Y,
15 BOX_WIDTH, BOX_HEIGHT);
16 }
17
18 public void paintComponent(Graphics g)
19 {
20 super .paintComponent(g);
21 Graphics2D g2 = (Graphics2D) g;
22
23 g2.draw(box);
24 }
25
26 /**
27 Moves the rectangle by a given amount.
28 @param x the amount to move in the x-direction
29 @param y the amount to move in the y-direction
30 */
31 public void moveBy( int dx, int dy)
32 {
33 box.translate(dx, dy);
34 repaint();
35 }
36
37 private Rectangle box;
38
39 private static final int BOX_X = 100 ;
40 private static final int BOX_Y = 100 ;
41 private static final int BOX_WIDTH = 20 ;
42 private static final int BOX_HEIGHT = 30 ;
43 }

419
420

Note the call to repaint in the moveBy method. This call is necessary to ensure
that the component is repainted after the state of the rectangle object has been
changed. Keep in mind that the component object does not contain the pixels that
show the drawing. The component merely contains a Rectangle object, which
itself contains four coordinate values. Calling translate updates the rectangle
coordinate values. The call to repaint forces a call to the paintComponent
method. The paintComponent method redraws the component, causing the
rectangle to appear at the updated location.