Learn Java by Example: Java Program to Check Whether a Number is Prime or Not

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Java Program to Check Whether a Number is Prime or Not

In this article, you’ll learn to check whether a number is prime or not. This is done using a for loop and while loop in Java.

 


A prime number is a number which is divisible by only two numbers: 1 and itself. So, if any number is divisible by any other number, it is not a prime number.

Example 1: Program to Check Prime Number using a for loop


public class Main{

    public static void main(String[] args){

        int num = 29;
        boolean flag = false;
        for(int i = 2; i <= num/2; ++i)
        {
            // condition for nonprime number
            if(num % i == 0)
            {
                flag = true;
                break;
            }
        }

        if (!flag)
            System.out.println(num + " is a prime number.");
        else
            System.out.println(num + " is not a prime number.");
    }
}

Output

29 is a prime number.

In the above program, for loop is used to determine if the given number num is prime or not.

Here, note that we are looping from 2 to num/2. It is because a number is not divisible by more than its half.

Inside the for loop, we check if the number is divisible by any number in the given range (2...num/2).

  • If num is divisible, flag is set to true and we break out of the loop. This determines num is not a prime number.
  • If num isn’t divisible by any number, flag is false and num is a prime number.

Example 2: Program to Check Prime Number using a while loop


public class Main{

    public static void main(String[] args){

        int num = 33, i = 2;
        boolean flag = false;
        while(i <= num/2)
        {
            // condition for nonprime number
            if(num % i == 0)
            {
                flag = true;
                break;
            }

            ++i;
        }

        if (!flag)
            System.out.println(num + " is a prime number.");
        else
            System.out.println(num + " is not a prime number.");
    }
}

Output

33 is not a prime number.

In the above program, while loop is used instead of a for loop. The loop runs until i <= num/2. On each iteration, whether num is divisble by i is checked and the value of i is incremented by 1.

 

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