confusion while creating a thread by extending thread class

Question

we knew that we can create a new thread by creating a new class thats extends thread and then to create an instance of that thread..while going through this topic i saw an example in my book which is as follows.

 class NewThread extends Thread{
    NewThread(){
        super("demo thread");
        System.out.println("child thread:"+this);
        start();
    }
    public void run(){
        try{
            for(int i=5;i>0;i--){
                System.out.println("child thread"+i);
                Thread.sleep(500);
            }
         } catch(InterruptedException e){
             System.out.println("child interrupted");
         }
         System.out.println("exiting child thread");
      }
  }

in this example i am able to understand all those things except the constructor part in which we are not using any instance(thread) to call start().so my question is how the start() method is called without any thread.

If you do not understand why the `start()` call in your constructor does not need an explicit object reference, then you probably could make better use of your time by learning more about the Java language before you try to understand Java threads. — Solomon Slow, May 12 '16 at 13:41

score 1 · Answer 1 · answered May 12 '16 at 06:58

You are inheriting the start() method from the class Thread that your NewThread class extends. So you can call it like any other method.

Same goes for the run() method which could use the @Override annotation in order to make the concept of inheritance clearer.

@Override
public void run() {
        try{
            for(int i = 5; i > 0; i--) {
                System.out.println("child thread" + i);
                Thread.sleep(500);
            }
        } catch(InterruptedException e) {
           System.out.println("child interrupted");
        }
        System.out.println("exiting child thread");
    }

score 0 · Answer 2 · answered May 12 '16 at 06:51

0

It is valid because method start() is inherited from Thread class, so you can call it like any other class method.

answered May 12 '16 at 06:51

redMist

219
2
12

score 0 · Answer 3 · answered May 12 '16 at 06:52

0

This start() method is called on the thread instance, which has just been created in this constructor.

answered May 12 '16 at 06:52

Artem Larin

142
6

score 0 · Answer 4 · edited May 23 '17 at 12:16

Below code is responsible for calling start() method on your NewThread instance in NewThread constructor, which calls start() method in Thread class, which calls run() method.

 NewThread(){
    super("demo thread");
    System.out.println("child thread:"+this);
    start();
}

Flow:

NewThread() -> start() -> Thread start() -> native start0() ->  run()

Refer to this SE question for internals of Thread class start() method :

What's the difference between Thread start() and Runnable run()

confusion while creating a thread by extending thread class

4 Answers4