JAVA-通过扩展线程类进行多线程-关于创建对象

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.

Answer 1

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

Answer 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.