java在点击鼠标后绘制一个椭圆形

Question

我想在面板中单击鼠标时画一个椭圆形。我点击按钮后可以画画。但是出现了一个问题：按钮的副本被绘制在面板的左上角

这是我的代码：

 public class PaintPanel extends JPanel implements MouseListener, MouseMotionListener{

        private boolean draw = false;
        private JButton button;
        private Point myPoint;
        public PaintPanel(){
            this.setSize(800,600);
            button = new JButton("Allow draw");
            button.setSize(30,30);
            this.add(button);
            this.addMouseListener(this);
            this.addMouseMotionListener(this);
            button.addActionListener(new ActionListener() {

                @Override
                public void actionPerformed(ActionEvent arg0) {
                    draw = true;

                }
            });
        }
        @Override
        public void paintComponent(Graphics g){
            g.setFont(new Font("Arial",Font.BOLD,24));
            g.setColor(Color.BLUE);
            if(draw){
                g.drawOval(myPoint.x-10, myPoint.y-10, 20, 20);
            }
        }
        @Override
        public void mousePressed(MouseEvent e) {
            if(draw){
                myPoint = e.getPoint();
                repaint();
            }
        }
        public static void main(String[] agrs){
            JFrame frame = new JFrame("Painting on panel");
            frame.add(new PaintPanel());
            frame.setVisible(true);
            frame.setSize(800,600);
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.setLocationRelativeTo(null);
        }
    }

Answer 1

I want to draw append. previos oval doesn't disappear

参见Custom Painting Approaches了解两种常见的增量绘制方法：

1. 保留要绘制的所有对象的列表，然后在组件的paintComponent（）方法中遍历该列表
2. 使用BuffereImage并在BuffereImage上绘制对象，然后在paintComponent（）方法中绘制BuffereImage

Answer 2

你把油漆链弄断了

在进行任何自定义绘制之前添加super.paintComponent(g)

public void paintComponent(Graphics g){
    super.paintComponent(g);
    g.setFont(new Font("Arial",Font.BOLD,24));
    g.setColor(Color.BLUE);
    if(draw){
        g.drawOval(myPoint.x-10, myPoint.y-10, 20, 20);
    }
}

Graphics是一个共享资源。它被提供给在给定的绘制周期内绘制的所有内容，这意味着，除非您首先清除它，否则它将包含在您之前已经绘制的其他内容

paintComponent的工作之一是为绘画准备Graphics上下文，但要清除它（用组件背景色填充）

更详细地看一下Performing Custom Painting和Painting in AWT and Swing