JButton and JPanel not being shown on JFrame

Question

I have made a program where there is a "login" button at the bottom, this was working fine for sometime before it started not showing background of the JPanel on which the JButton is placed. After trying for sometime I had discovered that the JButton gets displayed only when I hover over it.

I tried excluding specific areas of the code and I realized this glitch occurred when I initialized the variable JButton, even excluding the addAll(); method (from the code given below) makes the JPanel visible. The code is very basic and simple

public Panel() {
        this.setBackground(Color.darkGray);
        this.setSize(600,400);
        this.setVisible(true);
        addAll();
    }

    private void addAll() {
        String app = "res/files/debugging.jar";
        JButton b = new JButton();
        
        b.addActionListener(new ActionListener() {

            public void actionPerformed(ActionEvent evt) {
                 if (app!=null) {
                     File file = new File(app);
                     try {
                        Desktop.getDesktop().open(file);
                        
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                 }
                }});
        b.setText("Login");
        Dimension size = b.getPreferredSize();
        b.setBounds(250,290,(int)size.getWidth(),(int)size.getHeight());
        this.setLayout(null);
        this.add(b);
    }

Panel extends the class JPanel. The JFrame being used is provided below

public static void main(String[] args) {
        JFrame frame = new JFrame();
        
        
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setResizable(false);
        frame.setTitle("Login to your Account");
        //frame.setIconImage(frameIcon);
        frame.setSize(600,400);
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
        frame.add(new Panel());
    }

Answer 1

Try this.

• I added a private class that implements the actionListener.
• Moved file and button declarations as instance fields.
• Returns the preferred size when getPreferredSize() is called.
• added JFrame.pack() to size the components.

Also, check out The Java Tutorials for many examples of using Swing as well as how to paint.

import java.awt.Color;
import java.awt.Desktop;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.File;
import java.io.IOException;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;

public class FrameDemo extends JPanel {
    JFrame frame = new JFrame();
    JButton b = new JButton();
    String app = "res/files/debugging.jar";

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> new FrameDemo().start());
    }

    public void start() {
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        b.setText("Login");
        b.addActionListener(new MyActionListener());
        Dimension size = b.getPreferredSize();
        this.setLayout(new FlowLayout());
        add(b);
        this.add(b);
    }
    public Dimension getPreferredSize() {
        return new Dimension(600,400);
    }
    public FrameDemo() {
        frame.add(this);
        this.setBackground(Color.darkGray);
        frame.setVisible(true);
        frame.pack();
    }

    private class MyActionListener implements ActionListener {
        public void actionPerformed(ActionEvent evt) {
            if (app != null) {
                File file = new File(app);
                try {
                    Desktop.getDesktop().open(file);

                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}