原理：

時序圖：

代碼清單：

command.Command

public interface Command {
    void execute();
}

command.MacroCommand

public class MacroCommand implements Command{
    private Stack commands = new Stack();

    @Override
    public void execute() {
        Iterator it = commands.iterator();
        
 
while(it.hasNext()){
            ((Command)it.next()).execute();
        }
    }

    public void append(Command cmd){
        if(cmd!=null){
            commands.push(cmd);
        }
    }
    public void undo(){
        if(!commands.empty()){
            commands.pop();
        }
    }
    
 
public void clear(){
        commands.clear();
    }
}

drawer.Drawable

public interface Drawable {
    void draw(int x,int y);
}

drawer.DrawCanvas

public class DrawCanvas extends Canvas implements Drawable{
    //顏色
    private Color color = Color.red;
    //要繪制元的半徑
    private int radius = 6
 
;
    //命令歷史記錄
    private MacroCommand history;
    public DrawCanvas(int width,int height,MacroCommand history){
        setSize(width,height);
        setBackground(Color.WHITE);
        this.history = history;
    }

    //繪制
    @Override
    public void draw(int x, int y) {
       Graphics g = getGraphics();
       g.setColor(color);
       g.fillOval(x-radius,y-radius,radius*2,radius*2);
    }

    public void point(Graphics g){
        history.execute();
    }

}

drawer.DrawCommand

public class DrawCommand implements Command{
    //繪制對象
    protected  Drawable drawable;
    //繪制位置
    private Point position;

    public DrawCommand(Drawable drawable, Point position) {
        this.drawable = drawable;
        this.position = position;
    }

    @Override
    public void execute() {
        drawable.draw(position.x,position.y);
    }
}

Main

public class Main extends JFrame implements ActionListener,MouseMotionListener,WindowListener{
    //繪制的歷史記錄
    private MacroCommand history = new MacroCommand();
    //繪制區域
    private DrawCanvas canvas = new DrawCanvas(400,400,history);
    //刪除按鈕
    private JButton clearButton = new JButton("clear");

    public Main(String title){
        super(title);

        this.addWindowListener(this);
        canvas.addMouseMotionListener(this);
        clearButton.addActionListener(this);
        Box buttonBox = new Box(BoxLayout.X_AXIS);
        buttonBox.add(clearButton);
        Box mainBox = new Box(BoxLayout.Y_AXIS);
        mainBox.add(buttonBox);
        mainBox.add(canvas);
        getContentPane().add(mainBox);
        pack();
        show();
    }
    public static void main(String[] args){
        new Main("命令模式");
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        if(e.getSource() == clearButton){
            history.clear();
            canvas.repaint();
        }
    }

    @Override
    public void mouseDragged(MouseEvent e) {
        Command cmd = new DrawCommand(canvas,e.getPoint());
        history.append(cmd);
        cmd.execute();
    }

    @Override
    public void mouseMoved(MouseEvent e) {

    }

    @Override
    public void windowOpened(WindowEvent e) {

    }

    @Override
    public void windowClosing(WindowEvent e) {
        System.exit(0);
    }

    @Override
    public void windowClosed(WindowEvent e) {

    }

    @Override
    public void windowIconified(WindowEvent e) {

    }

    @Override
    public void windowDeiconified(WindowEvent e) {

    }

    @Override
    public void windowActivated(WindowEvent e) {

    }

    @Override
    public void windowDeactivated(WindowEvent e) {

    }
}

二十二、Command 命令模式