Java program to check whether number is prime or not

Let’s learn java program to check whether number is prime or not.

Java program to check whether number is prime or not

Prime numbers are those numbers which cannot be divided by other numbers than 1 or itself.

Example : 2, 3, 5, 7, 11, 13, 17….

NOTE: 0 and 1 are not prime numbers. 2 is only even prime number.

Here’s the java code to check if number is prime or not using if else.

public class PrimeNumberInJava
{
   public static void main(String[] args)
   {
      int a, b = 0, flag = 0;
      int number = 66;
      b = number / 2;
      if(number == 0 || number == 1)
      {
         System.out.println(number + " is not prime number");
      }
      else
      {
         for(a = 2; a <= b; a++)
         {
            if(number % a == 0)
            {
               System.out.println(number + " is not prime number.");
               flag = 1;
               break;
            }
         }
         if(flag == 0)
         {
            System.out.println(number + " is prime number.");
         }
      }
   }
}

Output:

Prime number program in java using while loop

Here’s the program to check whether a number is prime or not using while loop.

import java.util.Scanner;
public class PrimeNumberUsingWhileLoop
{
   public static void main(String[] args)
   {
      boolean checkPrime = true;
      Scanner sc = new Scanner(System.in);
      System.out.println("Please enter any number: ");
      int number = sc.nextInt();
      int a = 2;
      while(a <= number / 2)
      {
         if(number % a == 0)
         {
            checkPrime = false;
            break;
         }
         a++;
      }
      if(checkPrime)
      {
         System.out.println(number + " is a prime number.");
      }
      else
      {
         System.out.println(number + " is not a prime number.");
      }
      sc.close();
   }
}

Output:

Please enter any number: 17
17 is a prime number.

Program to check whether number is prime or not using for loop

In the below java program inside for loop we checking if the given number is divisible by any number in given range from 2 to number / 2. Because given number is not divisible by its half.

Now if number is divisible, bool is set to true and break out of loop. Else it is not divisible by any number, bool is false and the given number is prime number.

Here’s the java program.

public class CheckPrimeNumberForLoop
{
   public static void main(String[] args)
   {
      int number = 17;
      boolean bool = false;
      for(int a = 2; a <= number / 2; ++a)
      {
         // checking for a non-prime number
         if(number % a == 0)
         {
            bool = true;
            break;
         }
      }
      if(!bool)
      {
         System.out.println(number + " is a prime number.");
      }
      else
      {
         System.out.println(number + " is not a prime number.");
      }
   }
}

Output:

17 is a prime number.