Q:

How to calculate the font's width?

I am using java to draw some text, but it is hard for me to calculate the string's width.
for example:
zheng中国...
How long will this string occupy?

A:

For a single string, you can obtain the metrics for the given drawing font, and use that to calculate the string size.  For example:
String      message = new String("Hello, StackOverflow!");
Font        defaultFont = new Font("Helvetica", Font.PLAIN, 12);
FontMetrics fontMetrics = new FontMetrics(defaultFont);
//...
int width = fontMetrics.stringWidth(message);

If you have more complex text layout requirements, such as flowing a paragraph of text within a given width, you can create a java.awt.font.TextLayout object, such as this example (from the docs):
Graphics2D g = ...;
Point2D loc = ...;
Font font = Font.getFont("Helvetica-bold-italic");
FontRenderContext frc = g.getFontRenderContext();
TextLayout layout = new TextLayout("This is a string", font, frc);
layout.draw(g, (float)loc.getX(), (float)loc.getY());

Rectangle2D bounds = layout.getBounds();
bounds.setRect(bounds.getX()+loc.getX(),
              bounds.getY()+loc.getY(),
              bounds.getWidth(),
              bounds.getHeight());
g.draw(bounds);

A:

See Graphics.getFontMetrics() and FontMetrics.stringWidth().

A:

here is a simple app that can show you how to use FontMetrics when testing the width of a String:
import java.awt.*;
import java.awt.event.*;

import javax.swing.*;

public class GUITest {
    JFrame frame;
    public static void main(String[] args){
        new GUITest();
    }
    public GUITest() {
        frame = new JFrame("test");
        frame.setSize(300,300);
        addStuffToFrame();
        SwingUtilities.invokeLater(new Runnable(){
            public void run() {
                frame.setVisible(true);
            }
        });
    }           

    private void addStuffToFrame() {    
        JPanel panel = new JPanel(new GridLayout(3,1));
        final JLabel label = new JLabel();
        final JTextField tf = new JTextField(); 
        JButton b = new JButton("calc sting width");
        b.addActionListener(new ActionListener(){
            public void actionPerformed(ActionEvent e) {
                FontMetrics fm = label.getFontMetrics(label.getFont());
                String text = tf.getText();
                int textWidth = fm.stringWidth(text);
                label.setText("text width for \""+text+"\": " +textWidth);
            }
        });
        panel.add(label);
        panel.add(tf);
        panel.add(b);
        frame.setContentPane(panel);
    }

}

