Java Threads: How to print alphabets and numbers using two threads one at a time

Question

I am trying to work around with threads in java. Though I understand that threads output are unpredictable, However was wondering if there is a way to do that.

I have to implement two threads, one prints alphabets(a,b,c...z) and other prints numbers(1,2,3....26). Have to implement it in such a way that the output should be a,1,b,2,c,3,d,4......z,26. Below is my code but it doesn't give the desired output.

public class ThreadsExample {

  public static void main(String[] args) {
    Runnable r = new Runnable1();
    Thread t = new Thread(r);
    Runnable r2 = new Runnable2();
    Thread t2 = new Thread(r2);
    t.start();
    t2.start();
  }
}

class Runnable2 implements Runnable{
  public void run(){
    for(char i='a';i<='z';i++) {
        System.out.print(i+",");
    }
  }
}

 class Runnable1 implements Runnable{
  public void run(){
    for(int i=1;i<=26;i++) {
       System.out.print(i+",");
    }
 }
}

What tweak should I make in the code to get the desired output? How does synchronization helps here? Or is it really possible when working with Threads at all?

PS: This is not an assignment or some exercise. Its self learning.

Answer 1

The whole idea of threads: it represents a "stream of activity" that executes code independent of other threads.

In your case, you want that these two threads go in "lockstep". Thread A does one step, then Thread B, then A, then B.

In order to get there, the two threads need something "synchronize" on - in other words: A sends a signal to B when it has done its steps - and B has to wait for that signal. Then B does its thing, signals to A, ...

For starters, a simple boolean value would do. One thread sets it to true, the other to false (to indicate when it has made its step). Then the thread waits for the boolean to toggle again.

As you intend to learn things, I would just start experimenting from there. In case you want to take detours, look here for example. This might help as well.