Problem

Create three threads

• one that print '1' for 5 times.

• one that print '2' for 5 times.

• one that print '3' for 5 times.

Need to implement coordination between them so that  1, 2 and 3 should be printed in order from the three threads:

1

2

3

1

2

3

...

Hint: The order should be same theoretically and practically. Putting a sleep while printing might work in some cases, but it cannot be guaranteed always.

Solution

We will use an enum to implement the state pattern and use it for coordination.

package com.javajee.threads;

public class ThreeThreadSynchronization {

  public static State currentState = State.ONE;
  public static Object lockObject = new Object();

  public static void main(String[] args) {
    Thread t1 = new Thread(new MyRunnable(State.ONE));
    Thread t2 = new Thread(new MyRunnable(State.TWO));
    Thread t3 = new Thread(new MyRunnable(State.THREE));
    t1.start();
    t2.start();
    t3.start();

  }

}

class MyRunnable implements Runnable {

  State allowedState;

  public MyRunnable(State allowedState) {
    this.allowedState = allowedState;
  }

  @Override
  public void run() {
    for (int i = 0; i < 10; i++) {
      synchronized (ThreeThreadSynchronization.lockObject) {

        while (allowedState != ThreeThreadSynchronization.currentState) {
          try {
            ThreeThreadSynchronization.lockObject.wait();
          } catch (InterruptedException e) {
            e.printStackTrace();
          }
        }

        System.out.println(allowedState.getVal());
        ThreeThreadSynchronization.currentState = ThreeThreadSynchronization.currentState.nextState();
        ThreeThreadSynchronization.lockObject.notifyAll();
      }
    }

  }

}

enum State {
  ONE(1) {
    @Override
    public State nextState() {
      return TWO;
    }

  },
  TWO(2) {
    @Override
    public State nextState() {
      return THREE;
    }
  },
  THREE(3) {
    @Override
    public State nextState() {
      return ONE;
    }
  };

  private int val;

  State(int val) {
    this.val = val;
  }

  public abstract State nextState();

  public int getVal() {
    return val;
  }
}