Make A Calculator Using Java GUI

I am trying to make a calculator using Java GUI. I've managed to make an ActionListener and add it to a button, but I've made an error in my code that I'm unsure of how to solve. Because of how I've written the code, only one number can be placed in the text field. For example, the an ActionListener for the three button on the calculator was added to the button, but no matter how many times the user presses the button, only one 3 will appear in the text field. The code is below:

import javax.swing.*;//import the packages needed for gui
import java.awt.*;
import java.awt.event.*;
public class Calculator {
public static void main(String[] args) {
JFrame window = new JFrame("Window");//makes a JFrame
window.setSize(300,350);

[code].....

As you can see, because the compiler forces the String variable to be final, so when the user presses the button, the code simply shows how a space character and three character would look like, because the String variable can't change. How do I write my code so that every time the user presses the button, a character is added to the text field?