Run Java Threads sequentially

Question

How will you execute Three threads sequentially? For eg. Thread1, Thread2, Thread3. It is not possible to pass the reference of one Thread to the other and invoke from the run() method.

So code should be like this:

 Thread1.start();
 Thread2.start();
 Thread3.start();

and out put should be

 Printing Thread1
 Printing Thread2
 Printing Thread3

This can be possible by using ThreadPoolExecutor and using a blocking queue but even that is not an acceptable answer.

Answer 1

Since this is an interview question, they're looking for specific knowledge, not a "well it's obviously better to do it this way" answer. It also seems that they'll likely strike out solution after solution until they get the answer they want.

Odds are they want to see if you can implement inter-thread communications yourself. But they don't want you to do it the easy way (thread references available). Otherwise, you could just do thread.join().

So have all three threads grab some bit of shared memory (synchronized static class). Have each thread check a public static int nextThread(). Upon successful comparison that they are the next thread, they should do their work and update public static setNextThread(int value) with the value of the next thread to be processed.

The key is to do this in a thread-safe manner; however, if you can guarantee unique thread identifiers and ensure that no two threads have the same identifier, you can (with careful coding) even manage to do this without synchronization.

Answer 2

The simplest answer is

Thread1.run();
Thread2.run();
Thread3.run();

The problem with unrealistic questions is they often have an uninformative answer. ;)

The whole point of having threads is to run them concurrently. If you are not doing that at all, don't use threads.

You might say that; you cannot call the run() method, in which case you cannot use ThreadPoolExecutor because it calls the run() method for you. i.e. thats what submit() eventually does.

EDIT: The results are completely deterministic, becaus ethe fact that there is a Thread involved is irrelivent.

static class PrintThread extends Thread {
    public PrintThread(String name) {
        super(name);
    }

    @Override
    public void run() {
        for (int i = 0; i < 100; i++)
            System.out.println(getName() + ": " + i);
    }
}

public static void main(String args[]) {
    Thread thread1 = new PrintThread("A");
    Thread thread2 = new PrintThread("B");
    Thread thread3 = new PrintThread("C");

    thread1.run();
    thread2.run();
    thread3.run();
}

Prints

A: 0
A: 1
.. deleted ..
C: 98
C: 99

as expected.