Java Program to Check Prime Number

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In the previous article, we have discussed Java Program to Check Niven Number

In this article we are going to understand what Prime number is and how we can check whether a number is Prime or not in Java with examples.

Java Program to Check Prime Number

Prime numbers are numbers which have no factors except one and itself.

Example:

19-> Prime number
678 -> Not a Prime number
99 -> Not a Prime number

In the above examples the number 19 is only a prime number.

Let’s see different ways to check prime number.

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Approach:

  1. We ask the user to enter a number which we store in an integer variable num.
  2. We then use a while loop to iterate from 2 to the number itself and check if the number is divisible. If divisible then break out of the loop after setting flag = false.
  3. If flag is true, it is said to be a Prime number or else it is not.

Method-1: Java Program to Check Prime Number By Using Static Value

import java.util.Scanner;
public class PrimeNumber
{
    public static void main(String args[])
    {
        //A number is declared
        int num = 7;

        //Iter is the iterator which starts from 2 as 1 divided every number
        int iter = 2;
        //Flag is used to record if the number is divisible after each iteration
        boolean flag = true;
        while (num > iter)
        {
            if (num % iter == 0)
            {
                flag = false;
                break;
            }
            iter++;
        }
        if (flag)
        {
            System.out.println(num+" is prime");
        }
        else
        {
            System.out.println(num+" is not prime");
        }
    }
}

Output:

7 is prime

Method-2: Java Program to Check Prime Number By Using User Input Value

import java.util.Scanner;
public class PrimeNumber
{
    public static void main(String args[])
    {
        //Taking the number as input from the user using scanner class
        Scanner scan = new Scanner(System.in);
        System.out.print("Enter a number : ");
        int num = scan.nextInt();

        //Iter is the iterator which starts from 2 as 1 divided every number
        int iter = 2;
        //Flag is used to record if the number is divisible after each iteration
        boolean flag = true;
        while (num > iter)
        {
            if (num % iter == 0)
            {
                flag = false;
                break;
            }
            iter++;
        }
        if (flag)
        {
            System.out.println(num+" is prime");
        }
        else
        {
            System.out.println(num+" is not prime");
        }
    }
}

Output:

Enter a number : 97
97 is prime

Method-3: By C Language

#include <stdio.h>
int main() {
  int num, i, flag = 0;
  printf("Enter a positive integer: ");
  scanf("%d", &num);

  for (i = 2; i <= num / 2; ++i) {
    //if it is divisible by any other number
    //thenit breaks
    if (num % i == 0) {
      flag = 1;
      break;
    }
  }

  if (num == 1) {
    printf("1 is unique number");
  } 
  else {
    if (flag == 0)
      printf("Prime number.");
    else
      printf("Not a prime number.");
  }

  return 0;
}

 

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