What Are Threads?

• Here's a program that spits out 500 random numbers, pausing 20 milliseconds between each number.
• The numbers are added in a method of the JFrame, addRandoms().
• All of our logic is in one thread, so we do not consider this a threaded program.
• Note, this doesn't include other JVM threads, such as the garbage collector.  If our program only uses one thread, then we consider it a single-threaded program.
• Watch what happens when we click the Start button.  It stays 'clicked' until all 500 random numbers have been generated and added to the list.  If we want to stop it, we have to wait until it is finished.
• Don't be confused by the call to Thread.sleep(20);.
• That is telling the current thread of execution to sleep for 20 milliseconds.  Since we're only running in one thread, it is telling that entire thread to sleep.  Thus, nothing can happen, the Start button can't be released, and the Stop button can't be clicked in that time.
• The point is, just because I have Thread in there does not mean that we're running in a separate thread, because we're not.
• Visually:

• The source code (Generated from both the GUI Editor and the Source Editor):

package Threads;

import javax.swing.DefaultListModel;

/**
 *
 * @author  default
 */
public class RandomNumber extends javax.swing.JFrame {

    DefaultListModel model;
 
    /** Creates new form RandomNumber */
    public RandomNumber() {
        initComponents();
 
        // Instantiate the model, add it to the list.
        model = new DefaultListModel();
        lstNumbers.setModel(model);
 
    }

    public void addRandoms() {
        try {
            for (int j = 0; j < 500; j++) {
                // Generate a random number between 0 and 1, multiply it by 1000 to make it intersting.
                int intRandom = (int) (Math.random() * 1000);

                // add the random number to the model.
                model.addElement(j + ": " + intRandom);

                // Sleep for 20 ms.
                Thread.sleep(20);
            }

        } catch (InterruptedException e) {
            System.out.println("Interrupted");
        }
    }
 
    /** This method is called from within the constructor to
     * initialize the form.
     * WARNING: Do NOT modify this code. The content of this method is
     * always regenerated by the Form Editor.
     */
    private void initComponents() {
        jScrollPane1 = new javax.swing.JScrollPane();
        lstNumbers = new javax.swing.JList();
        jPanel1 = new javax.swing.JPanel();
        btnStart = new javax.swing.JButton();
        btnStop = new javax.swing.JButton();
        jLabel1 = new javax.swing.JLabel();

        addWindowListener(new java.awt.event.WindowAdapter() {
            public void windowClosing(java.awt.event.WindowEvent evt) {
                exitForm(evt);
            }
        });

        jScrollPane1.setViewportView(lstNumbers);

        getContentPane().add(jScrollPane1, java.awt.BorderLayout.CENTER);

        btnStart.setText("Start");
        btnStart.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                btnStartActionPerformed(evt);
            }
        });

        jPanel1.add(btnStart);

        btnStop.setText("Stop");
        btnStop.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                btnStopActionPerformed(evt);
            }
        });

        jPanel1.add(btnStop);

        getContentPane().add(jPanel1, java.awt.BorderLayout.SOUTH);

        jLabel1.setText("Random Numbers");
        getContentPane().add(jLabel1, java.awt.BorderLayout.NORTH);

        pack();
    }

    private void btnStartActionPerformed(java.awt.event.ActionEvent evt) {
        // Start the random number generation.
        addRandoms();
 
    }

    private void btnStopActionPerformed(java.awt.event.ActionEvent evt) {
        // Just close the program.
        System.exit(0);
 
    }

    /** Exit the Application */
    private void exitForm(java.awt.event.WindowEvent evt) {
        System.exit(0);
 
    }

    /**
    * @param args the command line arguments
    */
    public static void main(String args[]) {
        new RandomNumber().show();
    }
 

    // Variables declaration - do not modify
    private javax.swing.JButton btnStop;
    private javax.swing.JList lstNumbers;
    private javax.swing.JScrollPane jScrollPane1;
    private javax.swing.JLabel jLabel1;
    private javax.swing.JPanel jPanel1;
    private javax.swing.JButton btnStart;
    // End of variables declaration

}
 

• Now, let's thread this up.

Created by:  Brandan Jones December 17, 2001