In this applet, the user can type in two real numbers.  The
   user can click on buttons labled +, - , *, and / to perform
   basic arithmetic operations on the numbers.  When the user
   clicks on a button the answer is displayed.  The applet
   should be about 200 by 120 pixels.

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

public class SimpleCalculator extends JApplet implements ActionListener 

   JTextField xInput, yInput;  // Input boxes for the numbers.
   JLabel answer;  // JLabel for displaying the answer, or an 
                   //    error message if appropriate.

   public void init() 
      /* Since I will be using the content pane several times,
         declare a variable to represent it.  Note that the
         return type of getContentPane() is Container. */
      Container content = getContentPane();
      /* Assign a background color to the applet and its
         content panel.  This color will show through between
         components and around the edges of the applet. */
      /* Create the input boxes, and make sure that the background
         color is white.  (They are likely to be white by default.) */
      xInput = new JTextField("0");
      yInput = new JTextField("0");
      /* Create panels to hold the input boxes and labels "x =" and
         "y = ".  By using a BorderLayout with the TextField in the
         Center position, the TextField will take up all the space
         left after the label is given its preferred size. */
      JPanel xPanel = new JPanel();
      xPanel.setLayout(new BorderLayout());
      xPanel.add( new Label(" x = "), BorderLayout.WEST );
      xPanel.add(xInput, BorderLayout.CENTER);
      JPanel yPanel = new JPanel();
      yPanel.setLayout(new BorderLayout());
      yPanel.add( new Label(" y = "), BorderLayout.WEST );
      yPanel.add(yInput, BorderLayout.CENTER);
      /* Create a panel to hold the four buttons for the four
         operations.  A GridLayout is used so that the buttons
         will all have the same size and will fill the panel. 
         The applet servers as ActionListener for the buttons. */
      JPanel buttonPanel = new JPanel();
      buttonPanel.setLayout(new GridLayout(1,4));

      JButton plus = new JButton("+");

      JButton minus = new JButton("-");

      JButton times = new JButton("*");

      JButton divide = new JButton("/");
      /* Create the label for displaying the answer in red
         on a white background.  The label is set to be
         "opaque" to make sure that the white background
         is painted. */
      answer = new JLabel("x + y = 0", JLabel.CENTER);
      /* Set up the layout for the applet, using a GridLayout,
          and add all the components that have been created. */

      content.setLayout(new GridLayout(4,1,2,2));
      /* Try to give the input focus to xInput, which is the natural
         place for the user to start. */
   }  // end init()
   public Insets getInsets() 
           // Leave some space around the borders of the applet.
      return new Insets(2,2,2,2);

   public void actionPerformed(ActionEvent evt) 
           // When the user clicks a button, get the numbers
           // from the input boxes and perform the operation
           // indicated by the button.  Put the result in
           // the answer label.  If an error occurs, an
           // error message is put in the label.
      double x, y;  // The numbers from the input boxes.
      /* Get a number from the xInput TextField.  Use 
         xInput.getText() to get its contents as a String.
         Convert this String to a double.  The try...catch
         statement will check for errors in the String.  If 
         the string is not a legal number, the error message
         "Illegal data for x." is put into the answer and
         the actionPerformed() method ends. */

         String xStr = xInput.getText();
         x = Double.parseDouble(xStr);
      catch (NumberFormatException e) 
            // The string xStr is not a legal number.
         answer.setText("Illegal data for x.");
      /* Get a number from yInput in the same way. */

         String yStr = yInput.getText();
         y = Double.parseDouble(yStr);
      catch (NumberFormatException e) 
         answer.setText("Illegal data for y.");
      /* Perfrom the operation based on the action command
         from the button.  Note that division by zero produces
         an error message. */

      String op = evt.getActionCommand();
      if (op.equals("+"))
         answer.setText( "x + y = " + (x+y) );
      else if (op.equals("-"))
         answer.setText( "x - y = " + (x-y) );
      else if (op.equals("*"))
         answer.setText( "x * y = " + (x*y) );
      else if (op.equals("/")) 
         if (y == 0)
            answer.setText("Can't divide by zero!");
            answer.setText( "x / y = " + (x/y) );
   } // end actionPerformed()

}  // end class SimpleCalculator

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