GUI Components: Part 2 - Java: How to Program Early Objects - page 913

Thumb Tick mark

Fig. 22.1 | JSlider component with horizontal orientation.

Most Swing GUI components support mouse and keyboard interactions—e.g., if a
JSlider has the focus (i.e., it's the currently selected GUI component in the user inter-
face), pressing the left arrow key or right arrow key causes the JSlider 's thumb to decrease
or increase by 1, respectively. The down arrow key and up arrow key also cause the thumb
to decrease or increase by 1 tick, respectively. The PgDn (page down) key and PgUp (page
up) key cause the thumb to decrease or increase by block increments of one-tenth of the
range of values, respectively. The Home key moves the thumb to the minimum value of the
JSlider , and the End key moves the thumb to the maximum value of the JSlider .
JSlider s have either a horizontal or a vertical orientation. For a horizontal JSlider ,
the minimum value is at the left end and the maximum is at the right end. For a vertical
JSlider , the minimum value is at the bottom and the maximum is at the top. The min-
imum and maximum value positions on a JSlider can be reversed by invoking JSlider
method setInverted with boolean argument true . The relative position of the thumb
indicates the current value of the JSlider .
The program in Figs. 22.2-22.4 allows the user to size a circle drawn on a subclass of
JPanel called OvalPanel (Fig. 22.2). The user specifies the circle's diameter with a hori-
zontal JSlider . Class OvalPanel knows how to draw a circle on itself, using its own
instance variable diameter to determine the diameter of the circle—the diameter is used
as the width and height of the bounding box in which the circle is displayed. The diameter
value is set when the user interacts with the JSlider . The event handler calls method set-
Diameter in class OvalPanel to set the diameter and calls repaint to draw the new circle.
The repaint call results in a call to OvalPanel 's paintComponent method.

1 // Fig. 22.2: OvalPanel.java

2 // A customized JPanel class.

3 import java.awt.Graphics;

4 import java.awt.Dimension;

5 import javax.swing.JPanel;

6
7 public class OvalPanel extends JPanel

8 {

9 private int diameter = 10 ; // default diameter

10
11 // draw an oval of the specified diameter

12 @Override

13 public void paintComponent(Graphics g)

14 {

15 super .paintComponent(g);

16 g.fillOval( 10 , 10 , diameter, diameter);

17 }

18

Fig. 22.2 | JPanel subclass for drawing circles of a specified diameter. (Part 1 of 2.)