为什么fillRect命令不会显示在我的JFrame上？

Question

因此，我试图通过创建对象并将其添加到JFrame来单击鼠标时绘制一个矩形。 但是一旦运行该命令就不会显示。 有什么想法吗？

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class Gui3 extends JFrame {
    private JPanel mousepanel;
    private JLabel statusbar;

    public Gui3(){
        super("The title");

        mousepanel = new JPanel();
        mousepanel.setBackground(Color.WHITE);
        add(mousepanel, BorderLayout.CENTER);

        statusbar = new JLabel("Default");
        add(statusbar, BorderLayout.SOUTH);

        HandlerClass handler = new HandlerClass();
        mousepanel.addMouseListener(handler);
        mousepanel.addMouseMotionListener(handler);
    }   
    private class HandlerClass implements MouseListener, MouseMotionListener
        {

这就是问题所在。 该程序及其所有方法均有效，只是绘制出了问题的矩形。 绘制形状的对象在下面。

public void mouseClicked(MouseEvent event) {
        statusbar.setText(String.format("Clicked at %d,%d",event.getX(),event.getY()));
                DrawShapes shapes = new DrawShapes();
                add(shapes);
            }
            public void mousePressed(MouseEvent event){
                statusbar.setText("You pressed down the mouse");
            }
            public void mouseReleased(MouseEvent event){
                statusbar.setText("You released the button");
            }
            public void mouseEntered(MouseEvent event){
                statusbar.setText("You entered the area");
                mousepanel.setBackground(Color.RED);
            }
            public void mouseExited(MouseEvent event){
                statusbar.setText("The mouse has left the window");
                mousepanel.setBackground(Color.WHITE);
            }
            //These are mouse motion events
            public void mouseDragged(MouseEvent event){
                statusbar.setText("You are dragging the mouse");
            }
            public void mouseMoved(MouseEvent event){
                statusbar.setText("You are moving the mouse");
            }
        }
    }

这是绘制矩形的对象

import java.awt.*;
import javax.swing.*;
public class DrawShapes extends JPanel {
    public void PaintComponent(Graphics g){
        g.setColor(Color.BLUE);
        g.fillRect(0,0,30,30);
    }
}

Answer 1

关于

public void PaintComponent(Graphics g){
    g.setColor(Color.BLUE);
    g.fillRect(0,0,30,30);
}

明白

PaintComponent != paintComponent

确保使用@Override批注让您知道何时覆盖或不覆盖您认为的方法。

正确的方法如下所示：

@Override
protected void paintComponent(Graphics g) {
    super.paintComponent(g);  // don't forget this!
    g.setColor(Color.BLUE);
    g.fillRect(0, 0, 30, 30);
}

另外，如果您要替换原始的JPanel，请使用CardLayout帮助您轻松完成此操作。 否则，在交换容器中的组件之后，必须确保自己调用revalidate()和repaint() 。

例如，

    @Override
    public void mouseClicked(MouseEvent event) {
        statusbar.setText(String.format("Clicked at %d,%d", event.getX(), event.getY()));
        remove(mousepanel);
        DrawShapes shapes = new DrawShapes();
        getContentPane().add(shapes, BorderLayout.CENTER);
        getContentPane().revalidate();
        getContentPane().repaint();
    }