Little confused on the thread behaviour

Question

I have written a basic code in Thread and the output which i got is pretty surprising.

public class ThreadImp implements Runnable{

    public static void main(String[] args) {
        ThreadImp threadImp = new ThreadImp();
        Thread t =new Thread(threadImp);
        t.setName("Fred");
        t.start();
        threadImp.run();
        t.run();
    
    }
    public void run(){
        System.out.println("Current Thread: "+ Thread.currentThread());
    }
}

The output here i am expecting is that it will be print

Current Thread: Thread[main,5,main]

Current Thread: Thread[Fred,5,main]

Current Thread: Thread[main,5,main]

This result i can understand, that there is only one thread which i am creating i.e Thread Fred. Other two calls to run method is just like calling a normal method.

But what i can't understand is sometimes when i execute the same program i am getting the following output. So can anyone explain it to me why is it behaving like this.

Current Thread: Thread[main,5,main]

Current Thread: Thread[Fred,5,main]

Answer 1

You call Thread.run() (which you normally shouldn't call directly), and that does nothing if the Thread has completed and cleaned itself up:

@Override
public void run() {
    if (target != null) {
        target.run();
    }
}

Source: Java 7 update 79

Where target is the provided runnable. The field target is set to null when the thread has ended:

/**
 * This method is called by the system to give a Thread
 * a chance to clean up before it actually exits.
 */
private void exit() {
    if (group != null) {
        group.threadTerminated(this);
        group = null;
    }
    /* Aggressively null out all reference fields: see bug 4006245 */
    target = null;
    /* Speed the release of some of these resources */
    threadLocals = null;
    inheritableThreadLocals = null;
    inheritedAccessControlContext = null;
    blocker = null;
    uncaughtExceptionHandler = null;
}

Source: Java 7 update 79

So the exact output (including order, and if you get two or three messages) depends on timing, scheduling, etc.