This is the main file(Example.java) and the second class called "ChildExample.java". Both are in this file you have to divide to two java files.

Example.java

import java.awt.EventQueue;

import javax.swing.JFrame;
import javax.swing.JButton;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;

public class Example {

    private JFrame frame;
    
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                	Example window = new Example();
                    window.frame.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }

    public Example() {
        initialize();
    }

    private void initialize() {
        frame = new JFrame();
        frame.setBounds(100, 100, 450, 300);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().setLayout(null);

		JButton btnClicked = new JButton("Clicked");
		//ChildExample obj=new ChildExample();
		btnClicked.addActionListener(new ActionListener() {
		    public void actionPerformed(ActionEvent arg0) {
		    	ChildExample obj=new ChildExample();
		        //obj.getJavaProject2_28();
		    }
		});
        btnClicked.setBounds(150, 99, 89, 23);
        frame.getContentPane().add(btnClicked);
    }
}

//-------------------------------------------------------Second class--------------------------------------------------------

import java.awt.EventQueue;
import javax.swing.JFrame;

public class ChildExample {

    private JFrame frame;

//    public static void main(String[] args) {
//        EventQueue.invokeLater(new Runnable() {
//            public void run() {
//                try {
//                	ChildExample window = new ChildExample();
//                    window.frame.setVisible(true);
//                } catch (Exception e) {
//                    e.printStackTrace();
//                }
//            }
//        });
//    }

    public ChildExample() {
        initialize();
    }

    public void getJavaProject2_28()
    {
        frame.setVisible(true);
    }

    private void initialize() {
        frame = new JFrame();
        frame.setBounds(100, 100, 450, 300);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }

}

Have any problem leave a comment here.