How to create a task running as a Java Thread? Thread and Runnable are the most basic concurrent types to use. In this post we show how to use Thread to execute concurrent tasks.

In the previous post you could learn how to correctly use Runnable interface. In this post we show how to create a task that directly extends from java.lang.Thread class and run two instances to do their jobs concurrently:

package com.farenda.java.lang;

public class ThreadExample {

    private static class Counter extends Thread {

        // it's only to generate unique ids for each task:
        private static int threads = 0;

        private int id = ++threads;
        private final int number;

        public Counter(int number) {
            this.number = number;
        }

        @Override
        public void run() {
            for (int i = 0; i < number; ++i) {
                System.out.printf("counter %d, value: %d%n", id, i);
            }
        }
    }

    public static void main(String[] args) {
        Counter counter1 = new Counter(5);
        Counter counter2 = new Counter(5);

        System.out.println("Starting counters:");
        counter1.start();
        counter2.start();
        System.out.println("main thread ends here");
    }
}

Whole task implementation is performed run() method. Note that threads are not started when instantiated, but only after call to start() method.

And here’s the output of the above program:

Starting counters:
counter 1, value: 0
counter 1, value: 1
counter 1, value: 2
counter 2, value: 0
counter 2, value: 1
counter 2, value: 2
counter 2, value: 3
counter 2, value: 4
main thread ends here
counter 1, value: 3
counter 1, value: 4

As you can see the execution of threads is non-deterministic and can occur in any order – here part of the first thread job has been executed after thread two.

Also note that the program finishes only when all the threads have finished the work.