Temperature Conversion Example

Computer Science Example
Applet with TextFields, Labels, Buttons, a Panel and GridLayout

// To run this click on TempConvert.html

import java.awt.*;
import java.awt.event.*;
import java.applet.Applet;
import java.text.*;

// A Java applet that coverts Fahrenheit temperatures to Celsius temperatures.
public class TempConversion extends Applet
{
     private Label lblFahrenheit, lblCelsius;
     private TextField txtFahrenheit, txtCelsius;
     private Button Calculate;
     private Panel panel;

     // Method that gets the components and puts them on the applet.
     public void init ()
     {
          setBackground (Color.cyan);
          panel = new Panel ();
          panel.setLayout (new GridLayout (5, 1, 12, 12));

          lblFahrenheit = new Label ("Fahrenheit");
          txtFahrenheit = new TextField (10);
          lblCelsius = new Label ("Celsius");
          txtCelsius = new TextField (10);
          Calculate = new Button ("Calculate");
          Calculate.addActionListener (new CalculateListener ());

          panel.add (lblFahrenheit); panel.add (txtFahrenheit);
          panel.add (lblCelsius); panel.add (txtCelsius);
          panel.add (Calculate);
          add (panel);
     } // method init

     // Inner class to listen for the Calculate button.
     class CalculateListener implements ActionListener
     {
          public void actionPerformed (ActionEvent event)
          {
               double fahrenheit, celsius;
               fahrenheit = AppletIO.getDouble (txtFahrenheit);
               celsius = (fahrenheit - 32) * 5 / 9;
               txtCelsius.setText ("" + AppletIO.decimals (celsius));
          } // method actionPerformed
     } // class CalculateListener
} // class TempConversion

// A class that handles IO for an applet.
class AppletIO
{
     // Gets a double from the TextField and returns it to the calling method.
     public static double getDouble (TextField box)
     {
          double number;
          try
          {
               number = Double.valueOf (box.getText ()).doubleValue ();
          } catch (NumberFormatException e) { number = 0;}
          return number;
     } // method getDouble

     // Formats a double for string output with two decimal places.
     public static String decimals (double number)
     {
          DecimalFormat decFor = new DecimalFormat ();
          decFor.setMaximumFractionDigits (2);
          decFor.setMinimumFractionDigits (2);
          return decFor.format (number);
     } // method decimals

     // Gets an integer from the TextField and returns it to the calling method.
     public static int getInteger (TextField box)
     {
          int number;
          try
          {
               number = Integer.parseInt (box.getText());
          } catch (NumberFormatException e) { number = 0;}
          return number;
     } // method getInteger
} // class AppletIO

<HTML>
    <HEAD>
        <TITLE>Temperature Conversion</TITLE>
    </HEAD>
    <BODY>
        <H1>Temperature Conversion</H1>
        <APPLET CODE="TempConversion.class" WIDTH=300 HEIGHT=300></APPLET>
    </BODY>
</HTML>