How to add components to a JFrame after initialization?

I have a JFrame. When it first launches it won't load a button (on purpose) because it depends on a boolean value, sessionExists, that is false by default. When the user clicks add to launch a JDialog and clicks Ok in that JDialog, it will set the JFrame's sessionExists value to true.

What I want the code to do:

After Ok is clicked, I want the JFrame to update itself and display that button with the text "new button" since sessionExists is now true.

Current behavior:

Currently, the user clicks "ok", the JDialog will exit, but the parent JFrame will remain the same and the button doesn't appear.

Solution I tried:

I tried using .validate and .repaint() in the JFrame to refresh it so the button will be shown, but it doesn't seem to work.

How should I achieve the intended behavior? Should I be using CardLayout?

I reproduced the problem using the code below. My actual program relies on whether a file exists rather than a boolean variable.

The JFrame

import java.awt.BorderLayout;
import java.awt.EventQueue;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.border.EmptyBorder;
import javax.swing.JButton;
import javax.swing.JDialog;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
public class MainMenu extends JFrame {
private JPanel contentPane;
MainMenu thisFrame = this;
boolean sessionExists = false;
public static void main(String[] args) {
EventQueue.invokeLater(new Runnable() {
public void run() {
try {
MainMenu frame = new MainMenu();
frame.setVisible(true);
} catch (Exception e) {
e.printStackTrace();
}
}
});
}
public MainMenu() {
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setBounds(100, 100, 450, 300);
contentPane = new JPanel();
contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
contentPane.setLayout(new BorderLayout(0, 0));
setContentPane(contentPane);
if (sessionExists == true) {
JButton btnNewButton = new JButton("New button");
contentPane.add(btnNewButton, BorderLayout.CENTER);
}
JButton btnNewButton_1 = new JButton("Add Button");
btnNewButton_1.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent arg0) {
Add newTask = new Add(thisFrame);
newTask.setDefaultCloseOperation(JDialog.DISPOSE_ON_CLOSE);
newTask.setTitle("Add Task");
newTask.setVisible(true);
}
});
contentPane.add(btnNewButton_1, BorderLayout.SOUTH);
}
public void refresh() {
this.validate();
this.repaint();
}
}

The "Add" JDialog

import java.awt.BorderLayout;
import java.awt.FlowLayout;
import javax.swing.JButton;
import javax.swing.JDialog;
import javax.swing.JPanel;
import javax.swing.border.EmptyBorder;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
public class Add extends JDialog {
private final JPanel contentPanel = new JPanel();
public Add(MainMenu parentJFrame) {
setBounds(100, 100, 450, 300);
getContentPane().setLayout(new BorderLayout());
contentPanel.setLayout(new FlowLayout());
contentPanel.setBorder(new EmptyBorder(5, 5, 5, 5));
getContentPane().add(contentPanel, BorderLayout.CENTER);
{
JPanel buttonPane = new JPanel();
buttonPane.setLayout(new FlowLayout(FlowLayout.RIGHT));
getContentPane().add(buttonPane, BorderLayout.SOUTH);
{
JButton okButton = new JButton("OK");
okButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
parentJFrame.sessionExists = true;
parentJFrame.refresh();
dispose();
}
});
okButton.setActionCommand("OK");
buttonPane.add(okButton);
getRootPane().setDefaultButton(okButton);
}
{
JButton cancelButton = new JButton("Cancel");
cancelButton.setActionCommand("Cancel");
buttonPane.add(cancelButton);
}
}
}
}

You are not calling if(session == true) when you trying to add it. Instead of this, make new function like adding(), and call it from the Add JDialog class and it will be work.(You will notice you don't have to use session == true because you can call the function by this way directly. But for your understanding I didn't change it.)
Example Code:

MainMenu:

public class MainMenu extends JFrame {
private JPanel contentPane;
MainMenu thisFrame = this;
boolean sessionExists = false;
public static void main(String[] args) {
EventQueue.invokeLater(new Runnable() {
public void run() {
try {
MainMenu frame = new MainMenu();
frame.setVisible(true);
} catch (Exception e) {
e.printStackTrace();
}
}
});
}
public MainMenu() {
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setBounds(100, 100, 450, 300);
contentPane = new JPanel();
contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
contentPane.setLayout(new BorderLayout(0, 0));
setContentPane(contentPane);
JButton btnNewButton_1 = new JButton("Add Button");
btnNewButton_1.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent arg0) {
Add newTask = new Add(thisFrame);
newTask.setDefaultCloseOperation(JDialog.DISPOSE_ON_CLOSE);
newTask.setTitle("Add Task");
newTask.setVisible(true);
}
});
contentPane.add(btnNewButton_1, BorderLayout.SOUTH);
}
public void adding() {
if (sessionExists == true) {
JButton btnNewButton = new JButton("New button");
contentPane.add(btnNewButton, BorderLayout.CENTER);
}
}
public void refresh() {
this.validate();
this.repaint();
}
}

Add:

public class Add extends JDialog {
private final JPanel contentPanel = new JPanel();
public Add(MainMenu parentJFrame) {
setBounds(100, 100, 450, 300);
getContentPane().setLayout(new BorderLayout());
contentPanel.setLayout(new FlowLayout());
contentPanel.setBorder(new EmptyBorder(5, 5, 5, 5));
getContentPane().add(contentPanel, BorderLayout.CENTER);
{
JPanel buttonPane = new JPanel();
buttonPane.setLayout(new FlowLayout(FlowLayout.RIGHT));
getContentPane().add(buttonPane, BorderLayout.SOUTH);
{
JButton okButton = new JButton("OK");
okButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
parentJFrame.sessionExists = true;
parentJFrame.adding();
parentJFrame.refresh();
dispose();
}
});
okButton.setActionCommand("OK");
buttonPane.add(okButton);
getRootPane().setDefaultButton(okButton);
}
{
JButton cancelButton = new JButton("Cancel");
cancelButton.setActionCommand("Cancel");
buttonPane.add(cancelButton);
}
}
}
}