Swing Application not appearing

Question

Hi im am working on my Java project which I have to create a login page for users to enter, and when login is successful, an alert is supposed to tell them that the login was ok. However, I am having some troubles as my application is not showing up at all, I think it is something to do with the code that I have written. Below is the code:

public class UserLoginPage implements ActionListener {
    //Put all JLabels,Frames and buttons here etc
    JPanel panel = new JPanel();
    JFrame frame = new JFrame();
    JLabel userLabel = new JLabel("Username");
    JLabel passwordLabel = new JLabel("Password");
    JTextField userText = new JTextField();
    JTextField passwordText = new JTextField();
    JButton loginButton = new JButton("Login");

    //Label for successful login
    JLabel success = new JLabel("Login Successful");

    //Default Constructor to add the frames and panels etc
    public UserLoginPage(){
        panel.setLayout(null);
        userLabel.setBounds(10,20,80,25);
        panel.add(userLabel);
        passwordLabel.setBounds(10,50,80,25);
        panel.add(passwordLabel);

        userText.setBounds(100,20,165,25);
        panel.add(userText);
        passwordText.setBounds(100,50,165,25);
        panel.add(passwordText);

        loginButton.setBounds(10,80,80,25);
        loginButton.addActionListener(new UserLoginPage());
        panel.add(loginButton);

        success.setBounds(10,110,300,25);
        panel.add(success);
        //success.setText();

        frame.setSize(500,500);
        frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        frame.setVisible(true);
        frame.add(panel);
    }


    public static void main(String[] args){
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new UserLoginPage();
            }
        });
    }


    @Override
    public void actionPerformed(ActionEvent e) {
        System.out.println("Button Clicked");
    }

I think the problem lies with loginButton.addActionListener(new UserLoginPage()); But I might be wrong, do let me know of how to solve the problem, thank you.

Answer 1

There are multiple issues in your program:

1. panel.setLayout(null); and .setBounds(...). You shouldn't be using null-layouts, as Swing has to deal with multiple PLAFs, OS, screen sizes and resolutions. You might end up with issues like this one. Pixel-perfect layouts might seem like the easiest way to create complex UIs but it's not, it'll just lead you to endless issues. Instead use layout managers or combinations of them.

2. loginButton.addActionListener(new UserLoginPage()); You're creating a new instance of your program every time, and on every instance of it you're creating a new object because all your code is inside the constructor. Just, don't! It's a recursive call that finally creates a java.lang.StackOverflowError, to solve this use this instead of new UserLoginPage()

3. frame.setVisible(true); this line should always be the last line in your program, after you've added everything to your JFrame, not before.

With the above recommendations, here's an updated version of your code:

import java.awt.BorderLayout;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JPasswordField;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;

public class UserLoginPage implements ActionListener {
    // Put all JLabels,Frames and buttons here etc
    private JPanel panel;
    private JFrame frame;
    private JLabel userLabel;
    private JLabel passwordLabel;
    private JTextField userText;
    private JTextField passwordText;
    private JButton loginButton;

    // Label for successful login
    private JLabel success;

    // Default Constructor to add the frames and panels etc
    public UserLoginPage() {
        
    }
    
    private void createAndShowGUI() {
        frame = new JFrame(getClass().getSimpleName());
        panel  = new JPanel();
        
        panel.setLayout(new GridLayout(0, 2));
        
        userLabel = new JLabel("Username");
        passwordLabel = new JLabel("Password");
        
        userText = new JTextField(10);
        passwordText = new JPasswordField(10);
        
        loginButton = new JButton("Login");
        success = new JLabel("Login Successful");
        
        loginButton.addActionListener(this);
        
        panel.add(userLabel);
        panel.add(userText);
        panel.add(passwordLabel);
        panel.add(passwordText);
        panel.add(loginButton);
        
        frame.add(panel);
        frame.add(success, BorderLayout.SOUTH);
        frame.pack();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new UserLoginPage().createAndShowGUI();;
            }
        });
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        System.out.println("Button Clicked");
    }
}

Answer 2

You were correct. loginButton.addActionListener(new UserLoginPage()) causes a java.lang.StackOverflowError. The constructor is always calling itself without a base case to default to. You should be passing that very instance of the UserLoginPage as a parameter, not a new instance.

Use this code instead:

loginButton.addActionListener(this);