Quick Help With Calculator

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.io.*;

public class caculatorButtons extends JFrame implements ActionListener
{
    private JTextField displayText = new JTextField(30);
    private JButton[] button = new JButton[20];
    private String[] keys = {"MC", "MR", "M-", "M+",
                             "7", "8", "9", "/",
                             "4", "5", "6", "*",
                             "1", "2", "3", "-",
                             "0", "C", "=", "+"};

    private String numStr1 = "";
    private String numStr2 = "";

    private char op;
    private boolean firstInput = true;

    public  caculatorButtons()
    {
        setTitle ("My Calculator");
        setSize (270, 300);
        Container pane = getContentPane();
        pane.setLayout(null);

        displayText.setSize (240, 30);
        displayText.setHorizontalAlignment(SwingConstants.  RIGHT);
        displayText.setLocation (10,10);
        pane.add(displayText);

        int x, y;

        x = 10;
        y = 40;
        for (int ind = 0; ind < 20; ind++)
        {
            button[ind] = new JButton (keys[ind]);
            button[ind].addActionListener(this);
            button[ind].setSize(60, 30);
            button[ind].setLocation (x, y);
            pane.add(button[ind]);
            x = x + 60;
            if ((ind + 1) % 4 == 0 )
            {
                x = 10;
                y = y + 30;
            }
        }

        this.addWindowListener(new WindowAdapter()
            {
                public void windowClosing(WindowEvent e)
                {
                    System.exit(0);
                }
            }
        );

        setVisible (true);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
    }

    public void actionPerformed (ActionEvent e)
    {
        String resultStr;

        String str = String.valueOf (e.getActionCommand());

        char ch = str.charAt(0);

        switch (ch)
        {
        case '0': case '1': case '2':
        case '3': case '4': case '5':
        case '6': case '7': case '8':
        case '9':

            if (firstInput)
            {
                numStr1 = numStr1 + ch;
                displayText.setText (numStr1);
            }
            else
            {
                numStr2 = numStr2 + ch;
                displayText.setText(numStr2);
            }
            break;

        case '+': case '-': case '*': case '/':
        op = ch;
        firstInput = false;
            break;
        case '=': resultStr = evaluate();
                  displayText.setText(resultStr);
                  numStr1 = resultStr;
                  numStr2 = "";
                  firstInput = false;
            break;
        case 'C': displayText.setText("");
                  numStr1 = "";
                  numStr2 = "";
                  firstInput = true;
        }
    }
    private String evaluate()
    {
        final char beep = '\u0007';

        try
        {
            int num1 = Integer.parseInt(numStr1);
            int num2 = Integer.parseInt(numStr2);
            double result = 0;

            switch (op)
            {
                case '+': result = num1 + num2;
            }
            return String.valueOf(result);
        }
        catch (ArithmeticException e)
        {
            System.out.print (beep);
            return " E R R O R:" + e.getMessage();
        }
        catch (NumberFormatException e)
        {
            System.out.print (beep);
            if (numStr1.equals(""))
               return "E R R O R: Invalid First Number";
            else
               return "E R R O R: Invalid Second Number";
        }
        catch (Exception e)
        {
            System.out.print (beep);
            return "E R R O R";
        }
    }
    public static void main(String[] args)
    {
        caculatorButtons C = new caculatorButtons();
    }
}

            switch (op)
            {
                case '+': result = num1 + num2; break;
                case '-': do something; break;
                case '*': do something; break;
                case '/': do something; break;
            }