A Simple Introductory Java Program

• Start BlueJ
• Create a new, empty project

• Create a new "class" in your project
• Make sure to name that class '"Junk", including the capitalization - you have no choice here, "Junk" is it!
  
• Open that class and erase everything
• Copy everything below the line --- COPY HERE --- into the "Junk" class
• Compile everything

/*
 * This is the program we discussed in class, but I added some commments to it.
 * Comments are enclosed in "slash-star to star-slash" symbols and are simply
 * ignored by the compiler.
 */

/*
 * First, we have some 'import' statement to import those classes we will need in
 * our program. You can use a "star" to import all classes in a group, or
 * package, which is convenient sometimes, or import specifically named classes
 * only, which is more efficient for the compiler.
 */
import javax.swing.*;
import java.awt.FlowLayout;
import java.awt.event.*;

/*
 * Next comes the basic name of the 'public class'. It usually starts with a
 * capital letter and can not include any spaces. In our case we define a new
 * class called 'Junk' that extends, or "is a", JFrame, and implements, or 
 * "acts as" an ActionListener. Virtually every Java program starts out like 
 * this.
 */
public class Junk extends JFrame implements ActionListener
{
    /*
     * Next come the fieldds, used to store things. Everything that a class
     * "has" should be defined in fields at the top of the class. In our case
     * the class - aka program - has two buttons and one text field, properly
     * initialized according to the Java API.
     */
    JButton button1 = new JButton("Click me");
    JButton button2 = new JButton("No, me!");
    JTextField display = new JTextField("Some text goes here");
    
    /*
     * Next comes the 'constructor', i.e. a method that has the same name as
     * the class name. This is where all initialization and layout code goes.
     */
    public Junk()
    {
        super("My First Program");

        /*
         * As our layout we define a 'FlowLayout', which means that everything
         * is going to be arranged in a row.
         */        
        getContentPane().setLayout(new FlowLayout());

        /*
         * Now we add the various GUI elements.
         */
        getContentPane().add(button1);
        getContentPane().add(button2);
        getContentPane().add(display);
        
        /*
         * We activate the buttons via the 'addActionListener' command.
         */
        button1.addActionListener(this);
        button2.addActionListener(this);
        
        /*
         * Finally we define the window size and make the window visible.
         */
        setSize(400, 400);
        setVisible(true);
    }

    /*
     * The next method is called when an activated button is clicked. In it
     * we decide what is actually supposed to happen when a button is clicked.
     * The 'if' statement inside this method allows us to react differently
     * depending on which button was clicked.
     */
    public void actionPerformed(ActionEvent e)
    {
        if (e.getSource() == button1)
            display.setText("it tickles ..");
        else if (e.getSource() == button2)
            display.setText("ohh, stop ...");
    }

    /*
     * Last is the 'main' method. Every Java program must have one such 
     * method, which is usually short and creates an object of the current
     * type, passing execution to the constructor.
     */
    public static void main(String args[])
    {
        Junk j = new Junk();
    }
}