gistfile1.java

public class CalculatorWindow extends JFrame {
  // ...
  JButton button0;
  JButton button1;
  // ...
  
  public void doWhateverSetup() {
    // ...
    button0.addActionListener(new KeypadButtonAction(0, this));
    button1.addActionListener(new KeypadButtonAction(1, this));
    // ...
  }
  
  // ...
  
}

class KeypadButtonAction implements ActionListener {
  // I can't remember the specific declaration details of ActionListener
  // look them up
  
  private int value;
  private CalculatorWindow window;
  
  public KeypadButtonAction(int value, CalculatorWindow window) {
    this.value = value;
    this.window = window;
  }
  
  public void onAction(...) {
    window.editBox.append(value + "");
    // or whatever you want here
  }
}