[Rotkiv]

use JFrame and JPanel instead of just normal Frame and Panel by using

import javax.swing.*;

and get rid of container, in the end your main function should look like this.

public static void main(String[] args) {
     JFrame frame = new JFrame();
    frame.setTitle("Calculator");
    frame.setSize(200, 200);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    frame.add(new Calculator());
    frame.setVisible(true);
}

JFrame has the .setDefaultCloseOperation() method so that takes the place of your listener and 5 lines of code.

with all those changes it works fine for me and looks great. GJ.