import java.awt.*;
import java.awt.event.*;

/**
 * A very simple calculator.  This will read two integers from the user 
 * and display their sum.
 *
 * Uses a simple graphical user interface, rather than textual 
 * input and output.
 * 
 * The interface will present two text areas in which the user can
 * enter numbers, an ``equals'' button that the user presses to
 * request a result, and an area in which the program can write
 * output.
 * 
 * The class serves as the listener for its own frame and button,
 * providing actionPerformed and windowClosing methods.
 *
 * @author Samuel A. Rebelsky
 * @version 1.1 of February 1999
 */
public class SimpleCalc 
  extends WindowAdapter
  implements ActionListener 
{
  // +--------+--------------------------------------------------
  // | Fields |
  // +--------+

  /** The frame used for interaction. */
  protected Frame gui;
  /** The field used for input of the first value. */
  protected TextField val1;
  /** The field used for input of the second value. */
  protected TextField val2;
  /** The area used to show the plus sign. */
  protected Label plus;
  /** The area used for output. */
  protected Label out;
  /** The area used for messages (errors and others). */
  protected Label messages;
  /** The button used to tell it to compute. */
  protected Button equals;

  // +--------------+--------------------------------------------
  // | Constructors |
  // +--------------+

  /** Set up the frame for interaction. */
  protected SimpleCalc() {
    // Create the frame.
    gui = new Frame("Add");
    gui.setLayout(new FlowLayout());
    gui.addWindowListener(this);
    // Create the output field.  Leave sufficient space for output.
    out = new Label("     ");
    // Create the messages field.  Begin with instructions.
    messages = new Label("Enter two integers and click the = button.");
    // Create the input fields.  Note that we initialize them to have
    // some spaces to ensure that the layout manager gives us enough
    // space.  A little later, we'll clear them out.
    val1 = new TextField("   ");
    val2 = new TextField("   ");
    // Create the addition symbol.
    plus = new Label("+");
    // Create the button.
    equals = new Button("=");
    equals.addActionListener(this);
    // Add the various components
    gui.add(messages);
    gui.add(val1);
    gui.add(plus);
    gui.add(val2);
    gui.add(equals);
    gui.add(out);
    gui.pack();
    // Clear the text fields (see above).
    val1.setText("");
    val2.setText("");
    // And we're ready to go.
    gui.show();
  } // SimpleCalc()

  // +----------------+------------------------------------------
  // | Helper Methods |
  // +----------------+

  /** Do the simple computation. */
  public void doComputation() {
    // The two values we'll use to compute.
    int num1 = 0;
    int num2 = 0;
    // Read the strings the user entered.
    String str1 = val1.getText();
    String str2 = val2.getText();
    // Convert the first value to a number.
    try {
      num1 = Integer.valueOf(str1).intValue();
    }
    catch (Exception e) {
      // Whoops!  Bad input.  Tell the user.
      messages.setText("The left operand is not an integer!");
      // Reset the input.
      val1.setText("");
      // Give up.
      return;
    }
    // Convert the second value to a number.
    try {
      num2 = Integer.valueOf(str2).intValue();
    }
    catch (Exception e) {
      // Whoops!  Bad input.  Tell the user.
      messages.setText("The right operand is not an integer!");
      // Reset the input.
      val2.setText("");
      // Give up.
      return;
    }
    // Display the result.
    out.setText(Integer.toString(num1+num2));
  } // doComputation()

  // +----------------+------------------------------------------
  // | Event Handlers |
  // +----------------+

  /** React to a click on the button. */
  public void actionPerformed(ActionEvent evt) {
    doComputation();
  } // actionPerformed(ActionEvent)

  /** React to the closing of the window. */
  public void windowClosing(WindowEvent event) {
    gui.dispose();
    System.exit(0);
  } // windowClosing()

  // +------+----------------------------------------------------
  // | Main |
  // +------+

  /* Start the ball rolling by creating an object. */
  public static void main(String[] args) {
    new SimpleCalc();
  } // main(String[])
} // class SimpleCalc