radius

(xCenter, yCenter)

FIGURE 15.16 The program uses the drawPolygon method to draw a polygon.

15.8 Centering a String Using the FontMetrics Class

You can display a string at any location in a panel. Can you display it centered? To do so, you need to use the FontMetrics class to measure the exact width and height of the string for a particular font. FontMetrics can measure the following attributes for a given font (see Figure 15.17):

■Leading, pronounced ledding, is the amount of space between lines of text.

■Ascent is the distance from the baseline to the ascent line. The top of most characters in the font will be under the ascent line, but some may extend above the ascent line.

■Descent is the distance from the baseline to the descent line. The bottom of most descending characters (e.g., j, y, and g) in the font will be above the descent line, but some may extend below the descent line.

■Height is the sum of leading, ascent, and descent.

Leading

Ascent line

Descent line

FIGURE 15.17 The FontMetrics class can be used to determine the font properties of characters for a given font.

FontMetrics is an abstract class. To get a FontMetrics object for a specific font, use the following getFontMetrics methods defined in the Graphics class:

■ public FontMetrics getFontMetrics(Font font)

Returns the font metrics of the specified font.

■ public FontMetrics getFontMetrics()

Returns the font metrics of the current font.

38g.drawString("Welcome to Java", xCoordinate, yCoordinate);

39}

40}

The methods getWidth() and getHeight() (lines 35–36) defined in the Component class return the component’s width and height, respectively.

Since the message is centered, the first character of the string should be positioned at (xCoordinate, yCoordinate), as shown in Figure 15.18.

FIGURE 15.19 MessagePanel displays a message on the panel.

15.9 Case Study: The MessagePanel Class 513

FIGURE 15.20 TestMessagePanel uses MessagePanel to display four message panels.

LISTING 15.7 TestMessagePanel.java

1 import java.awt.*;

2 import javax.swing.*;

3

4 public class TestMessagePanel extends JFrame {

5public TestMessagePanel() {

9MessagePanel messagePanel4 = new MessagePanel("I love Java");

11messagePanel2.setFont(new Font("Courier", Font.BOLD, 20));

12messagePanel3.setFont(new Font("Times", Font.ITALIC, 20));

13messagePanel4.setFont(new Font("Californian FB", Font.PLAIN, 20));

15messagePanel2.setBackground(Color.CYAN);

16messagePanel3.setBackground(Color.GREEN);

17messagePanel4.setBackground(Color.WHITE);

18messagePanel1.setCentered(true);

22add(messagePanel2);

23add(messagePanel3);

24add(messagePanel4);

25}

26

27public static void main(String[] args) {

28TestMessagePanel frame = new TestMessagePanel();

29frame.setSize(300, 200);

30frame.setTitle("TestMessagePanel");

31frame.setLocationRelativeTo(null); // Center the frame

32frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

33frame.setVisible(true);

34}

35}

The rest of this section explains how to implement the MessagePanel class. Since you can use

the class without knowing how it is implemented, you may skip the implementation if you wish. skip implementation? The MessagePanel class is implemented in Listing 15.8. The program seems long but is

actually simple, because most of the methods are get and set methods, and each method is relatively short and easy to read.

LISTING 15.8 MessagePanel.java

1 import java.awt.FontMetrics;

2 import java.awt.Dimension;

3 import java.awt.Graphics;