16.6

习题标准答案：

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class Exercise16_6 extends JFrame {
  private DisplayPanel panel = new DisplayPanel();

  public Exercise16_6() {
    add(panel, BorderLayout.CENTER);
    panel.setFocusable(true);
  }

  /** Main method */
  public static void main(String[] args) {
    JFrame frame = new Exercise16_6();
    frame.setTitle("Exercise16_6");
    frame.setSize(300, 300);
    frame.setLocationRelativeTo(null); // Center the frame
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setVisible(true);
  }

  class DisplayPanel extends MessagePanel {
    private String m1 = "Java is fun";
    private String m2 = "Java is powerful";
    private boolean isM1;

    public DisplayPanel() {
      setCentered(true);
      this.addMouseListener(new MouseAdapter() {
        public void mouseClicked(MouseEvent e) {
          if (isM1) setMessage(m1);
          else setMessage(m2);

          isM1 = !isM1;
        }
      });
    }
  }
}

我的答案：

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class Test_16_6 extends JFrame {
    MessagePanel mPanel = new MessagePanel();
    
  public Test_16_6() {      
      add(mPanel);
      mPanel.addMouseListener(new mouseListener());
  }

  /** Main method */
  public static void main(String[] args) {
    JFrame frame = new Test_16_6();
    frame.setTitle("Exercise16_6");
    frame.setSize(300, 300);
    frame.setLocationRelativeTo(null); // Center the frame
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setVisible(true);
  } 
  
  class mouseListener implements MouseListener{
      boolean clicked = false;
    @Override
    public void mouseClicked(MouseEvent e) {
        // TODO Auto-generated method stub
        if(clicked == false)
            { mPanel.setMessage("Java is fun"); clicked = true; }
            else if(clicked == true)
            { mPanel.setMessage("Java is powerful"); clicked = false;}
        }
    @Override
    public void mouseEntered(MouseEvent e) {
        // TODO Auto-generated method stub
        
    }
    @Override
    public void mouseExited(MouseEvent e) {
        // TODO Auto-generated method stub
        
    }
    @Override
    public void mousePressed(MouseEvent e) {
        // TODO Auto-generated method stub
        
    }
    @Override
    public void mouseReleased(MouseEvent e) {
        // TODO Auto-generated method stub
        
    }      
  }
}

需要用到的类：

// MessagePanel.java: Display a message on a JPanel
import java.awt.Font;
import java.awt.FontMetrics;
import java.awt.Dimension;
import java.awt.Graphics;
import javax.swing.JPanel;

public class MessagePanel extends JPanel {
  /** The message to be displayed */
  private String message = "Welcome to Java";

  /** The x coordinate where the message is displayed */
  private int xCoordinate = 20;

  /** The y coordinate where the message is displayed */
  private int yCoordinate = 20;

  /** Indicate whether the message is displayed in the center */
  private boolean centered;

  /** The interval for moving the message horizontally and vertically */
  private int interval = 10;

  /** Default constructor */
  public MessagePanel() {
  }

  /** Constructor with a message parameter */
  public MessagePanel(String message) {
    this.message = message;
  }

  /** Return message */
  public String getMessage() {
    return message;
  }

  /** Set a new message */
  public void setMessage(String message) {
    this.message = message;
    repaint();
  }

  /** Return xCoordinator */
  public int getXCoordinate() {
    return xCoordinate;
  }

  /** Set a new xCoordinator */
  public void setXCoordinate(int x) {
    this.xCoordinate = x;
    repaint();
  }

  /** Return yCoordinator */
  public int getYCoordinate() {
    return yCoordinate;
  }

  /** Set a new yCoordinator */
  public void setYCoordinate(int y) {
    this.yCoordinate = y;
    repaint();
  }

  /** Return centered */
  public boolean isCentered() {
    return centered;
  }

  /** Set a new centered */
  public void setCentered(boolean centered) {
    this.centered = centered;
    repaint();
  }

  /** Return interval */
  public int getInterval() {
    return interval;
  }

  /** Set a new interval */
  public void setInterval(int interval) {
    this.interval = interval;
    repaint();
  }

  /** Paint the message */
  protected void paintComponent(Graphics g) {
    super.paintComponent(g);

    if (centered) {
      // Get font metrics for the current font
      FontMetrics fm = g.getFontMetrics();

      // Find the center location to display
      int stringWidth = fm.stringWidth(message);
      int stringAscent = fm.getAscent();
      // Get the position of the leftmost character in the baseline
      xCoordinate = getWidth() / 2 - stringWidth / 2;
      yCoordinate = getHeight() / 2 + stringAscent / 2;
    }

    g.drawString(message, xCoordinate, yCoordinate);
  }

  /** Move the message left */
  public void moveLeft() {
    xCoordinate -= interval;
    repaint();
  }

  /** Move the message right */
  public void moveRight() {
    xCoordinate += interval;
    repaint();
  }

  /** Move the message up */
  public void moveUp() {
    yCoordinate -= interval;
    repaint();
  }

  /** Move the message down */
  public void moveDown() {
    yCoordinate -= interval;
    repaint();
  }

  /** Override get method for preferredSize */
  public Dimension getPreferredSize() {
    return new Dimension(200, 30);
  }
}

效果图：