498 Chapter 15 Graphics

FIGURE 15.1 You can draw shapes using the drawing methods in the Graphics class.

15.2 Graphical Coordinate Systems

To paint, you need to specify where to paint. Each component has its own coordinate system with the origin (0, 0) at the upper-left corner. The x-coordinate increases to the right, and the y-coordinate increases downward. Note that the Java coordinate system differs from the conventional coordinate system, as shown in Figure 15.2.

y

(x, y)

Y axis

FIGURE 15.2 The Java coordinate system is measured in pixels, with (0, 0) at its upperleft corner.

The location of the upper-left corner of a component c1 (e.g., a button) inside its parent component c2 (e.g., a panel) can be located using c1.getX() and c1.getY(). As shown in Figure 15.3, (x1, y1) = (c1.getX(), c1.getY()), (x2, y2) = (c2.getX(),

FIGURE 15.3 Each GUI component has its own coordinate system.

15.3 The Graphics Class 499

15.3 The Graphics Class

The Graphics class provides the methods for drawing strings, lines, rectangles, ovals, arcs, polygons, and polylines, as shown in Figure 15.4.

Think of a GUI component as a piece of paper and the Graphics object as a pencil or paintbrush. You can apply the methods in the Graphics class to draw things on a GUI component.

FIGURE 15.4 The Graphics class contains the methods for drawing strings and shapes.

The Graphics class–an abstract class—provides a device-independent graphics interface for displaying figures and images on the screen on different platforms. Whenever a component (e.g., a button, a label, a panel) is displayed, the JVM automatically creates a Graphics object for the component on the native platform and passes this object to invoke the paintComponent method to display the drawings.

The signature of the paintComponent method is as follows:

protected void paintComponent(Graphics g)

This method, defined in the JComponent class, is invoked whenever a component is first displayed or redisplayed.

500 Chapter 15 Graphics

In order to draw things on a component, you need to define a class that extends JPanel and overrides its paintComponent method to specify what to draw. Listing 15.1 gives an example that draws a line and a string on a panel, as shown in Figure 15.5.

LISTING 15.1 TestPaintComponent.java

1 import javax.swing.*;

2 import java.awt.Graphics;

3

4 public class TestPaintComponent extends JFrame {

10TestPaintComponent frame = new TestPaintComponent();

11frame.setTitle("TestPaintComponent");

12frame.setSize(200, 100);

13frame.setLocationRelativeTo(null); // Center the frame

14frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

15frame.setVisible(true);

16}

17}

inside a frame

FIGURE 15.5 A line and a string are drawn on a panel.

The paintComponent method is automatically invoked to paint graphics when the component is first displayed or whenever the component needs to be redisplayed. Invoking super.paintComponent(g) (line 21) invokes the paintComponent method defined in the superclass. This is necessary to ensure that the viewing area is cleared before a new drawing is displayed. Line 22 invokes the drawLine method to draw a line from (0, 0) to (50, 50). Line 23 invokes the drawString method to draw a string.

All the drawing methods have parameters that specify the locations of the subjects to be drawn. All measurements in Java are made in pixels. The string “Banner” is drawn at location (0, 40).

The JVM invokes paintComponent to draw things on a component. The user should never invoke paintComponent directly. For this reason, the protected visibility is sufficient for paintComponent.

Panels are invisible and are used as small containers that group components to achieve a desired layout. Another important use of JPanel is for drawing. You can draw things on any Swing GUI component, but normally you should use a JPanel as a canvas upon which to draw things. What happens if you replace JPanel with JLabel in line 19 as follows?

class NewPanel extends JLabel {