直到MouseOver，JButton才会出现

Question

I'm trying to get JButtons which have been added to a JPanel to show up during the execution of a program, however they only appear when I hover the mouse of them, they remain invisible until then.

Below is my code, I've tried repaint() and revalidate() with no luck.

There also seems to be an issue with the height of the JPanel, it seems to be larger than the main Window for some reason

import java.awt.Color;
import java.awt.Graphics;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;


public final class SideMenu extends JPanel implements ActionListener{
    private final int width;
    private final int height;

    public SideMenu(int width, int height){        
        this.width = width;
        this.height = height;
        this.setLayout(new GridLayout(0,1));
        this.add(new JButton("button1"));
        this.add(new JButton("button2"));
        this.add(new JButton("button3"));
        this.revalidate();
        this.repaint(); 
    }

    @Override
    public void paint(Graphics g) {
        super.paintComponent(g);
        g.setColor(Color.black);
        g.fillRect(0, 0, width, height);
    }


    @Override
    public void actionPerformed(ActionEvent e) {        
        repaint();  
    }

    public static void main(String[] args){
        int width = 300, height = 400;

        JFrame jf = new JFrame();
        jf.setTitle("Fish Tank");
        jf.setSize(width, height);
        jf.setVisible(true);
        jf.setLayout(null);
        jf.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        ///jf.setResizable(false);

        SideMenu side_menu = new SideMenu(100,height);        
        jf.add(side_menu);
        side_menu.setBounds(200, 0, 100, height);
    }
}

Answer 1

Use paintComponent(..) method instead of paint(..) :

@Override
protected void paintComponent(Graphics g) {
    super.paintComponent(g);
    g.setColor(Color.black);
    g.fillRect(0, 0, width, height);
}

Read more about custom paintings .

Also call jf.setVisible(true); at the end of construction of GUI, when you add all components to JFrame .