Implementing two THREADS to print EVEN-ODD numbers in order
Sahil Agarwal
1462
Javainterview questionMultithreadingInterview

Print Even and Odd Numbers Using 2 Threads
There will be two threads such that one thread only prints even numbers while the other thread only prints odd numbers .
This problem is generally asked in interviews and its quite important with view of multithreading concept as it covers two main concept in the topic.

  1. Synchronization
  2. Inter Thread Communication

Why we need synchronization?

Here we will be achieving synchronization by using the synchronized methods . What this does is when one of the thread is executing the method then the other thread cannot execute that method .

Why we need Inter thread Communication?

Inter-thread communication allows synchronized threads to communicate with each other using a set of methods.The methods are wait, notify, and notifyAll.
Wait() causes the current thread to wait indefinitely until some other thread calls notify() or notifyAll() on the same object.

The code is pasted below . I have added some comments , please try to go through it , and if you have any doubts please feel free to comment.

	import java.util.*;
	import java.lang.*;
	import java.io.*;

//Sub class of thread class
	class EvenOddThread extends Thread{
		Integer start;
		Integer N;
		Printer print;
		Boolean isEven;
		EvenOddThread(Integer start , Integer N, Printer print , Boolean isEven){
			this.N = N;
			this.start = start;
			this.print = print;
			this.isEven = isEven;
		}
		@Override
		public void run(){
			while(start <= N){
				if(isEven)
					print.evenPrinter(start);
				else
					print.oddPrinter(start);
				start += 2;
			}
		}
	}
	//Helper Class
	class Printer{
		public volatile boolean isOdd = false; //This variable is Volatile since multiple threads are accessing it

		//Method Synchronization using synchronized keyword
		public synchronized void evenPrinter(int num){
			if(!isOdd){
				try {
	                wait();
	            } catch (InterruptedException e) {
	                Thread.currentThread().interrupt();
            }
			}
			System.out.println(num);
			isOdd = false ;
			notify();
		}

		public synchronized void oddPrinter(int num){
			if(isOdd){
				try {
	                wait();
	            } catch (InterruptedException e) {
	                Thread.currentThread().interrupt();
            }
			}
			System.out.println(num);
			isOdd = true ;
			notify();
		}

	}
	class Test
	{
		public static void main (String[] args) throws java.lang.Exception
		{
			 Printer printerObj = new Printer();
			 EvenOddThread t1 = new EvenOddThread(1,10,printerObj,false); // Odd thread
			 EvenOddThread t2 = new EvenOddThread(2,10,printerObj,true);  // Even thread
			 System.out.println("Thread execution starts.....");
			 t1.start(); //Starting thread1 which calls run method
			 t2.start(); //Starting thread2 which calls run method


		}
	}

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