Sample: SimpleCalculator

import java.awt.*;

import java.awt.event.*;

import javax.swing.*;

public class SimpleCalculator extends JFrame {

  // Text fields for Number 1, Number 2, and Result

  private JTextField jtfNum1, jtfNum2, jtfResult;

  // Buttons "Add", "Subtract", "Multiply" and "Divide"

  private JButton jbtAdd, jbtSub, jbtMul, jbtDiv;

  // Main Method

  public static void main(String[] args) {

    SimpleCalculator frame = new SimpleCalculator();

    frame.pack();

    frame.setTitle("SimpleCalculator");

    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    frame.setLocationRelativeTo(null); // Center the frame

    frame.setVisible(true);

  }

  // Default Constructor

  public SimpleCalculator() {

    // Panel p1 to hold text fields and labels

    JPanel p1 = new JPanel();

    p1.setLayout(new FlowLayout());

    p1.add(new JLabel("Number 1"));

    p1.add(jtfNum1 = new JTextField(3));

    p1.add(new JLabel("Number 2"));

    p1.add(jtfNum2 = new JTextField(3));

    p1.add(new JLabel("Result"));

    p1.add(jtfResult = new JTextField(8));

    jtfResult.setEditable(false);

    jtfResult.setHorizontalAlignment(SwingConstants.RIGHT);

    // Panel p2 to hold buttons

    JPanel p2 = new JPanel();

    p2.setLayout(new FlowLayout());

    p2.add(jbtAdd = new JButton("Add"));

    p2.add(jbtSub = new JButton("Subtract"));

    p2.add(jbtMul = new JButton("Multiply"));

    p2.add(jbtDiv = new JButton("Divide"));

    // Set mnemonic keys

    jbtAdd.setMnemonic('A');

    jbtSub.setMnemonic('S');

    jbtMul.setMnemonic('M');

    jbtDiv.setMnemonic('D');

    // Add panels to the frame

    setLayout(new BorderLayout());

    add(p1, BorderLayout.CENTER);

    add(p2, BorderLayout.SOUTH);

    // Register listeners

    jbtAdd.addActionListener(new Listener());

    jbtSub.addActionListener(new Listener());

    jbtMul.addActionListener(new Listener());

    jbtDiv.addActionListener(new Listener());

  }

  class Listener implements ActionListener {

    // Handle ActionEvent from buttons and menu items

    public void actionPerformed(ActionEvent e) {

      String actionCommand = e.getActionCommand();

      if ("Add".equals(actionCommand))

          calculate('+');

      else if ("Subtract".equals(actionCommand))

        calculate('-');

      else if ("Multiply".equals(actionCommand))

        calculate('*');

      else if ("Divide".equals(actionCommand))

        calculate('/');

    }

  }

  // Calculate and show the result in jtfResult

  private void calculate(char operator) {

    // Obtain Number 1 and Number 2

    double num1 = new Double(jtfNum1.getText().trim()).doubleValue();

    double num2 = new Double(jtfNum2.getText().trim()).doubleValue();

    double result = 0;

    // Perform selected operation

    switch (operator) {

      case '+': result = num1 + num2;

                break;

      case '-': result = num1 - num2;

                break;

      case '*': result = num1 * num2;

                break;

      case '/': result = num1 / num2;

    }

    // Set result in jtfResult

    jtfResult.setText(String.valueOf(result));

  }

}