Print even and odd numbers in increasing order using two threads in Java

Given an integer N, the task is to write Java Program to print the first N natural numbers in increasing order using two threads.

Prerequisite: Multithreading

Examples:

Input: N = 10
Output: 1 2 3 4 5 6 7 8 9 10
Input: N = 18
Output: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Approach: The idea is to create two threads and print even numbers with one thread and odd numbers with another thread. Below are the steps:

Create two threads T1 and T2 using the below syntax, where T1 and T2 are used to print odd and even numbers respectively.

Thread T1 = new Thread(new Runnable() {
public void run() { mt.printEvenNumber(); }
});
Thread T2 = new Thread(new Runnable() {
public void run() { mt.printOddNumber(); }
});
where, printOddNumber() is used to print all the odd numbers till N, printEvenNumber() is used to print all the even numbers till N.

Maintain a global counter variable and start both threads using the below function:

T1.start(); T2.start();

If the counter is even in the Thread T1, then wait for the thread T2 to print that even number. Otherwise, print that odd number, increment the counter and notify to the Thread T2 using the function notify().
If the counter is odd in the Thread T2, then wait for the thread T1 to print that odd number. Otherwise, print that even number, increment the counter and notify the Thread T1 using the function notify().

Below is the implementation of the above approach:

Java

// Java program for the above approach

public class GFG {

    // Starting counter
  	//will force all threads to update and use the latest copy of this counter, and not use locally cached copies
    volatile static int counter = 1; 
    
  	int limit;

    GFG (int limit) {this.limit = limit;}

  //function to print odd numbers
    public synchronized void printOddNum () {
        while(counter<=limit) {
            if(counter%2 == 1) { //counter is odd, print it
              	// remove thread name and use System.out.print() to print in one line, as per the sample output format
                System.out.println(Thread.currentThread().getName()+": "+counter); 
                counter++;
                notifyAll();
            } else {
                try {
                    wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }

    // Function to print even numbers
    public synchronized void printEvenNum () {
        while (counter<=limit) {
            if(counter%2 == 0) { //counter is even, print it
              // remove thread name and use System.out.print() to print in one line, as per the sample output format
                System.out.println(Thread.currentThread().getName()+": "+counter);
                counter++;
                notifyAll();
            } else {
                try {
                    wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }

    // Driver Code
    public static void main(String[] args) {
        GFG printer = new GFG (10);

        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                printer.printOddNum();
            }
        });
      
        t1.setName("Odd"); // for clearer verification

        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                printer.printEvenNum();
            }
        });
      
        t2.setName("Even"); // for clearer verification

        t1.start();
        t2.start();
    }
}

Output

Odd: 1
Even: 2
Odd: 3
Even: 4
Odd: 5
Even: 6
Odd: 7
Even: 8
Odd: 9
Even: 10

Time Complexity: O(N)
Auxiliary Space: O(1)

Print even and odd numbers in increasing order using two threads in Java

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