How do I add a quit button to this program? How about "clear"?

Question

I need to add a button that "clears the calculator" and also one that quits the program on the butPanel. It also needs to be very basic Java code because I am a beginner and have an awful comp.sci. teacher. I have a sample of code that has the quit button, but I am unsure of how to put it into MY program. I've tried so much. Also if there is a better way to "error-check" that would be much appreciated.

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

public class Calculator2 extends JFrame implements ActionListener
{
    JLabel heading = new JLabel ("2. Calculator");
    JLabel intro1 = new JLabel ("This program stimulates a four-function calculator.");
    JLabel intro2 = new JLabel ("It takes an operator and a number. It can add, subtract,");
    JLabel intro3 = new JLabel (" multiply and divide. Enter in the format eg. '+35'. ");
    JLabel inLabel = new JLabel ("        Operation: ");
    JLabel outLabel = new JLabel ("       Total:           ");
    JTextField inOper = new JTextField (7);
    JTextField outTota = new JTextField (7); // intro

//panels
JPanel titlePanel = new JPanel ();
JPanel intro1Panel = new JPanel ();
JPanel intro2Panel = new JPanel ();
JPanel intro3Panel = new JPanel ();
JPanel operPanel = new JPanel ();
JPanel totaPanel = new JPanel ();
JPanel butPanel = new JPanel ();

String operTemp;
String totaTemp;

public Calculator2 ()
{
    setTitle ("C - 2.");
    inOper.addActionListener (this);
    outTota.setEditable (false);
    getContentPane ().setLayout (new FlowLayout ());

    titlePanel.add (heading);
    intro1Panel.add (intro1);
    intro2Panel.add (intro2);
    intro3Panel.add (intro3);
    operPanel.add (inLabel);
    operPanel.add (inOper);
    totaPanel.add (outLabel);
    totaPanel.add (outTota); //adds components to panels

    getContentPane ().add (titlePanel);
    getContentPane ().add (intro1Panel);
    getContentPane ().add (intro2Panel);
    getContentPane ().add (intro3Panel);
    getContentPane ().add (operPanel);
    getContentPane ().add (totaPanel); //Adds panels to Frame

    setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE);
}


public static int isInteger (String input)
{
    try
    {
        Integer.parseInt (input);
        return Integer.parseInt (input);
    }
    catch (NumberFormatException nfe)
    {
        return 0;
    }
} //isInteger method


// The application
public String calculate (String operation, String newtotal)
{
    int total = isInteger (newtotal);
    String totalS;
    char operator;
    int number = 0;
    operator = operation.charAt (0);

    if (operator == '+')
    {
        number = isInteger (operation.substring (1));
        total = total + number;
        totalS = Integer.toString (total);
    }


    else if (operator == '-')
    {
        number = isInteger (operation.substring (1));
        total = total - number;
        totalS = Integer.toString (total);
    }


    else if (operator == '*')
    {
        number = isInteger (operation.substring (1));
        total = total * number;
        totalS = Integer.toString (total);
    }


    else if (operator == '/')
    {
        number = isInteger (operation.substring (1));
        total = total / number;
        totalS = Integer.toString (total);
    }
    else
    {
        totalS = ("ERROR");
    }

    if (number == 0)
    {
        totalS = ("ERROR");
    }
    return totalS;
} // calculate method


public void actionPerformed (ActionEvent evt)
{
    String userIn = inOper.getText ();
    String totalIn = outTota.getText ();
    try
    {
        totaTemp = calculate (userIn, totalIn);
        outTota.setText (totaTemp + "");
    }
    catch (Exception ex)
    {
        outTota.setText ("ERROR");
    }
    repaint ();
}


public static void main (String[] args)
{
    Calculator2 calc = new Calculator2 ();
    calc.setSize (350, 350);
    calc.setResizable (false);
    calc.setVisible (true);
}
}

Answer 1

First declare the buttons and initialize them:

JButton exitButton = new JButton("Exit");
JButton clearButton = new JButton("Clear");

Then, make sure the butPanel has a layout and the panel is added to the content pane:

butPanel.setLayout(new FlowLayout());
getContentPane().add(butPanel);

Then add the action listeners on the new buttons:

exitButton.addActionListener(new ActionListener() {
  public void actionPerformed(ActionEvent e) {
    System.exit(0);
  }
});
clearButton.addActionListener(new ActionListener() {
  public void actionPerformed(ActionEvent e) {
    // TODO: clear the stuff here
  }
});