JAVA - Multithreading by Extending Thread Class - About Creating Objects

Question

public class game extends Thread{
    public void run() {
        System.out.println("Sub-class must override the void run() method of Thread class!");
    }
    public static void main(String args[]) {
        Thread t = new Thread();
        t.start();
    }
}

For these lines of code above I got nothing in the console. But for these codes below:

public class game extends Thread{
    public void run() {
        System.out.println("Sub-class must override the void run() method of Thread class!");
    }
    public static void main(String args[]) {
        game g = new game();
        g.start();
    }
}

I got "Sub-class must override the void run() method of Thread class!" shown in the console.

Could you help me, why I need to create an object of the sub-class rather than an object of the Thread class? What's the difference? Sorry I am a total novice.

John Kane · Accepted Answer · 2018-06-04 13:43:30Z

2

If you create an instance of the parent class, the compiler will not know about its child. That is why you need to instantiate the sub class oppesed to the parent.

answered Jun 4, 2018 at 13:43

John Kane

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1 Comment

Mr.Snail Over a year ago

Thank you sir! I seems to get it.

Antho Christen · Accepted Answer · 2018-06-05 03:49:41Z

2

That's because in the first code the object is a default thread that has no task to run. You could have given the thread object a task like this,

public class game implements Runnable{
    public void run() {
        System.out.println("Sub-class must override the void run() method of Thread class!");
    }
    public static void main(String args[]) {
        Thread t = new Thread(new game());
        t.start();
    }
}

Whereas in the second case you give the Thread (its Sub-class Game) a default task to print in its run method. See more about threads here

edited Jun 5, 2018 at 3:49

answered Jun 4, 2018 at 13:43

Antho Christen

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2 Comments

Mr.Snail Over a year ago

Sir thank you! Thread t = new Thread(game); Here what is the game inside the ()? Is it a String name or runnable r? I am confused at here for days... Thank you!

Antho Christen Over a year ago

Its a Runnable, when you don't pass a Runnable to a normal Thread it does nothing. A thread is spawned to do a task, by default a thread doesn't know what task to do, unless, you give it one through runnable task (passing a Runnable on construction). Or you create your custom thread class to do something when run (by overriding the run method)

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