Java How to Program Fourth Edition - DCC

Chapter 13 Graphical User Interface Components: Part 2 733
Method Math.abs gets the absolute value of the subtractions x1 - x2 and y1 - y2 that
determine the width and height of the oval’s bounding rectangle, respectively. When the
calculations are complete, paintComponent draws the oval. The call to the superclass
version of paintComponent at the beginning of the method guarantees that the previous
oval displayed on the SelfContainedPanel is erased before the new one is displayed.
Look-and-Feel Observation 13.9
Most Swing GUI components can be transparent or opaque. If a Swing GUI component is
opaque, when its paintComponent method is called, its background will be cleared. Otherwise,
its background will not be cleared. Only opaque components can display a customized
background color. 13.9
Look-and-Feel Observation 13.10
JPanel objects are opaque by default. 13.10
When the user releases the mouse button, method mouseReleased (lines 34–39)
captures in variables x2 and y2 the final location of the mouse and invokes repaint to
draw the final version of the oval.
Class SelfContainedPanelTest’s constructor (lines 21–57 of Fig. 13.5) creates
an instance of class SelfContainedPanel (line 24) and sets the background color (line
25) of the SelfContainedPanel to yellow so that its area is visible against the background
of the application window.
1 // Fig. 13.5: SelfContainedPanelTest.java
2 // Creating a self-contained subclass of JPanel
3 // that processes its own mouse events.
4
5 // Java core packages
6 import java.awt.*;
7 import java.awt.event.*;
8
9 // Java extension packages
10 import javax.swing.*;
11
12 // Deitel packages
13 import com.deitel.jhtp4.ch13.SelfContainedPanel;
14
15 public class SelfContainedPanelTest extends JFrame {
16 private SelfContainedPanel myPanel;
17
18
19 // set up GUI and mouse motion event handlers for
20 // application window
21 public SelfContainedPanelTest()
22 {
23 // set up a SelfContainedPanel
24 myPanel = new SelfContainedPanel();
25 myPanel.setBackground( Color.yellow );
26
27 Container container = getContentPane();
Fig. Fig. 13.5 13.5 Capturing mouse events with a JPanel (part 1 of 3).