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A Simple Thread Example

The simple example shown in full on the first page of this lesson defines two classes: SimpleThread and TwoThreadsTest. Let's begin our exploration of the application with the SimpleThread class--a subclass of the Thread class, which is provided by the java.lang package:
class SimpleThread extends Thread {
    public SimpleThread(String str) {
    public void run() {
        for (int i = 0; i < 10; i++) {
            System.out.println(i + " " + getName());
            try {
                sleep((int)(Math.random() * 1000));
            } catch (InterruptedException e) {}
        System.out.println("DONE! " + getName());
The first method in the SimpleThread class is a constructor that takes a String as its only argument. This constructor is implemented by calling a superclass constructor and is interesting to us only because it sets the Thread's name, which is used later in the program.

The next method in the SimpleThread class is the run() method. The run() method is the heart of any Thread and where the action of the Thread takes place. The run() method of the SimpleThread class contains a for loop that iterates ten times. In each iteration the method displays the iteration number and the name of the Thread, then sleeps for a random interval of up to 1 second. After the loop has finished, the run() method prints "DONE!" along with the name of the thread. That's it for the SimpleThread class.

The TwoThreadsTest class provides a main() method that creates two SimpleThread threads: one is named "Jamaica" and the other is named "Fiji". (If you can't decide on where to go for vacation you can use this program to help you decide--go to the island whose thread prints "DONE!" first.)

class TwoThreadsTest {
    public static void main (String[] args) {
        new SimpleThread("Jamaica").start();
        new SimpleThread("Fiji").start();
The main() method also starts each thread immediately following its construction by calling the start() method. To save you from typing in this program, click here for the source code to the SimpleThread class and here for the source code to the TwoThreadsTest program. Compile and run the program and watch your vacation fate unfold. You should see output similar to the following:
0 Jamaica
0 Fiji
1 Fiji
1 Jamaica
2 Jamaica
2 Fiji
3 Fiji
3 Jamaica
4 Jamaica
4 Fiji
5 Jamaica
5 Fiji
6 Fiji
6 Jamaica
7 Jamaica
7 Fiji
8 Fiji
9 Fiji
8 Jamaica
DONE! Fiji
9 Jamaica
DONE! Jamaica
(Looks like I'm going to Fiji!!) Notice how the output from each thread is intermingled with the output from the other. This is because both SimpleThread threads are running concurrently. Thus, both run() methods are running at the same time and each thread is displaying its output at the same time as the other.

Try This: Change the main program so that it creates a third thread with the name "Bora Bora". Compile and run the program again. Does this change the island of choice for your vacation? Here's the code for the new main program, which is now named ThreeThreadsTest.

Keep Going

This page glosses over many of the details of threads such as the start() and sleep() methods. Don't worry, the next several pages of this lesson explain these concepts and others in detail. The important thing to understand from this page is that a Java program can have many threads, and that those threads can run concurrently.

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