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.

GhostCat · Answer 1 · 2017-08-17T12:14:24.970

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.

Thank you for the explanation. The links clears the air around synchronization. — roger_that, Aug 17 '17 at 18:05

stefan bachert · Accepted Answer · 2017-08-17T12:16:14.850

It is possible. You need to synchronize it well.

Approach Pseudocode

query some (synchronized) state state will tell whether nums or chars are allowed

if state allows char and caller will put chars, do it now and change state and wake up waiting threads

if not, wait

if state allows numbers and caller will put numbers, do it now and change state and wake up waiting threads

if not, wait

Java code

public class ThreadsExample {
  public static ThreadsExample output = new 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();
  }

  private Object syncher = new Object ();  // we use an explicit synch Object, you could use annotation on methods, too. like ABHISHEK did.
             // explicit allows to deal with more complex situations, especially you could have more the one locking Object
  private int state = 0; // 0 allows chars, 1 allows ints

  public void print (char pChar) {
    synchronized (syncher) {      // prevent the other print to access state
        while (true) {
            if (state == 0) {     // char are allowed
                System.out.print(pChar + ","); // print it
                state = 1;        // now allow ints
                syncher.notify(); // wake up all waiting threads
                return;
            } else {              // not allowed for now
                try {
                    syncher.wait();  // wait on wake up
                } catch (InterruptedException e) {
                }
            }
        }
    }
  }

  public void print (int pInt) {
    synchronized (syncher) {
        while (true) {
            if (state == 1) {
                System.out.print(pInt + ",");
                state = 0;
                syncher.notify();
                return;
            } else {
                try {
                    syncher.wait();
                } catch (InterruptedException e) {
                }
            }
        }
     }
  }
}

class Runnable2 implements Runnable{
  public void run(){
    for(char i='a';i<='z';i++) {
        ThreadsExample.output.print(i);
    }
  }
}

 class Runnable1 implements Runnable{
  public void run(){
    for(int i=1;i<=26;i++) {
        ThreadsExample.output.print(i);
    }
 }
}

Output

a,1,b,2,c,3,d,4,e,5,f,6,g,7,h,8,i,9,j,10,k,11,l,12,m,13,n,14,o,15,p,16,q,17,r,18,s,19,t,20,u,21,v,22,w,23,x,24,y,25,z,26,

Got a better understanding now looking at both the codes and other links. Thank you. — roger_that, Aug 17 '17 at 18:06

score 2 · Answer 3 · answered Aug 17 '17 at 12:00

HERE IS THE CODE:: You need to create 2 threads and implement wait and notify methods correctly you can also refer "Create two threads, one display odd & other even numbers" for your answer.

    public class ThreadClass {
    volatile int i = 1;
    volatile Character c = 'a';
    volatile boolean state = true;

    synchronized public void printAlphabet() {
        try {
            while (!state) {
                wait();
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(Thread.currentThread().getName() + " " +c);
        state = false;
        c++;
        notifyAll();
    }
    synchronized public void printNumbers() {
        try {
            while (state) {
                wait();
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(Thread.currentThread().getName() + " " + i);
        state = true;
        i++;
        notifyAll();
    }
    public static void main(String[] args) {
        ThreadClass threadClass = new ThreadClass();
        Thread t1 = new Thread() {
            int k = 0;
            @Override
            public void run() {
                while (k < 26) {
                    threadClass.printAlphabet();
                    k++;
                }
            }
        };
        t1.setName("Thread1");
        Thread t2 = new Thread() {
            int j = 0;

            @Override
            public void run() {
                while (j < 26) {
                    threadClass.printNumbers();
                    j++;
                }
            }
        };
        t2.setName("Thread2");

        t1.start();

        t2.start();
    }
}

Aleksandar Zoric · Answer 4 · 2017-08-17T11:37:18.457

Your threads are running at the same time. But not the way you want it, as mentioned above. You will see blocks of data from thread 1 and then a block of data from thread 2; and this is because of thread scheduling. Thread 1 is just queuing its output before thread 2.

To test this theory, increase your output to a 1000 records for example as the alphabet and 26 numbers are not as large to see this.

By doing so, you will see these 'blocks' of data. There is a way to do what you mentioned, but it is not advisable as this is not demonstrating how threads actually work but rather you forcing it to work that way.

score 0 · Answer 5 · answered Apr 30 '20 at 11:40

With less Code:

class MyRunnable implements Runnable {
    private static int n = 1;
    private static char c = 'a';

    public void run() {
        for (int i = 1; i <= 26; i++) {
            synchronized (this) {
                try {
                    notifyAll();
                    if (Thread.currentThread().getName().equals("A")) {
                        System.out.print(c + ",");
                        c++;
                    } else {
                        System.out.print(n + ",");
                        n++;
                    }
                    if (i != 26) {
                        wait();
                    }
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }

        }
    }
}

public class PrintAlphabetNumberJob {

    public static void main(String[] args) throws InterruptedException {
        MyRunnable r = new MyRunnable();
        Thread tAlphabet = new Thread(r, "A");
        Thread tNumber = new Thread(r, "N");
        tAlphabet.start();
        Thread.sleep(100);
        tNumber.start();
    }

}

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

5 Answers5

Approach Pseudocode

Java code

Output