Overriding start() method of Threads

[Kavita Tipnis]

Is there any reason behind not overriding the start() method in a subclass of java.lang.Thread?
I found that instead of the thread class having to invoke the run() , you will have to do it explicitly.

Thanks!

[Ulf Dittmer]

You should almost never subclass Thread, and almost always implement Runnable. It's extremely rare that you'd need to override methods in the Thread class: ExtendingThreadVsImplementingRunnable

[Kavita Tipnis]

Thanks! I was just going through Marcus Green's exam # 1 and it had one question where it was overriding.
Also invoking start() without a thread object does not throw a compiler error, it just runs the current executing thread's run().

[victor kamat]

Does it do it in the same thread or another thread ?

[Ankitt Gupta]

Also invoking start() without a thread object does not throw a compiler error, it just runs the current executing thread's run()

it must be .start()

[Kavita Tipnis]

Reference : Marcus Greens exam no 1, question 19 ( it is not 1.6 but just wanted to clarify)

 
public class Tux extends Thread{

        static String sName = "vandeleur";
        public static void main(String argv[]){
        Tux t = new Tux();
        t.piggy(sName);
        System.out.println(sName);
        
        }
        public void piggy(String sName){
                sName = sName + " wiggy";
        start();
        }
        public void run(){
        
        for(int i=0;i  <  4; i++){
                sName = sName + " " + i;
                
        }
        }

}
   

This is not overriding, but I wrote a simple class to check it out. But invoking start() in piggy() is confusing

[Ruben Soto]

Yes, it is confusing.

Here are some things to think about:

1. When you do start(), that actually means this.start(). What this means is that the current class must have a start() method (which all Thread instances have.) If you call start() in the code for a class which doesn't have a start() method you will get a compiler error.

2. You can only call java.lang.Thread.start() once on a given Thread instance, or you will get an IllegalThreadStateException What that means is that when you call start() within the code of a subclass of Thread, the call itself must take place in a thread other than the one that you are attempting to start. In other words, you are calling code inside a Thread class from a thread of execution that must be different than the thread of execution originated by the start() invocation. It also means that once the thread is alive, it can't call the start() code again or it will throw the exception (of course, you could always catch the exception and do something with it.)

I hope this didn't make things even more confusing!

[Kavita Tipnis]

Thank you for the pointers, this.start() would make it very clearer.