CODEWITHSUNDEEP
javamultithreadingrunnable
Chuck Worker
Chuck Worker

Reputation: 267

Unclear execution of small program with threads

I created a test program uses threads:

class First implements Runnable {
    public void run() {
        System.out.print("One ");
    }
}

class Second implements Runnable {
    Thread other;
    Second(Thread t) {other = t;}
    public void run() {
        System.out.print("Two");
    }
}

public class ThreadTest {
    void go() {
        First first = new First();
        Thread t = new Thread(first);
        Second second = new Second(t);
        Thread u = new Thread(second);
        t.start();
        u.start();
    }

    public static void main(String[] args) {
        new ThreadTest().go();
    }
}

I expected that it will print:

One One

But it suddenly for me had printed:

One Two

I don't understand why this is happening. I thought that in both threads, the method run() from First class must be called. Because I passed First instance into Thread constructor and then pass this created thread into Second constructor. So method run() from First class has priority over run() method from Second class. Where am I wrong?

Upvotes: 0

Views: 66

Answers (2)

rgettman
rgettman

Reputation: 178333

There is no reason why the second thread should print "One". It may have a reference to the first Thread, but it does nothing with it. Nothing overrides Second's run method, so it just prints Two.

To make the second Thread print One, have run call the other Thread's run method.

class Second implements Runnable {
    Thread other;
    Second(Thread t) {other = t;}
    public void run() {
        other.run();
    }
}

But it's unusual to call one Thread's run method from the run method of another Thread. In fact, it's unusual to call run directly at all.

Upvotes: 2

Gray
Gray

Reputation: 116918

I expected that it will print: One One But it suddenly for me had printed: One Two

You are starting 2 threads. The First.run() method prints out "One". The Second.run() method prints out "Two". Just because you assign other = t; in the Second constructor does not change it's run() method.

Because I passed First instance into Thread constructor and then pass this created thread into Second constructor. So method run() from First class has priority over run() method from Second class. Where am I wrong

Maybe in the Second.run() method you meant to call other.run() or something? That would be a strange pattern however that is not recommended. You should not link Thread classes like that. You should probably pass in the First method and then do:

class Second implements Runnable {
    First first;
    Second(First first) { this.first = first; }
    public void run() {
        first.run();
    }
}

But even then, it is a bit of a strange pattern to call one run() method from another.

Upvotes: 1

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