Java program to check for prime and find next Prime in Java

Any whole number which is greater than 1 and has only two factors that is 1 and the number itself, is called a prime number. Other than these two number it has no positive divisor. For example: 7 = 1 × 7

Following is the algorithm to find whether a number is prime or not −

• Take integer variable A.

• Divide the variable A with (A-1 to 2).

• If A is divisible by any value (A-1 to 2) it is not prime.

• Else it is prime.

Example

Following Java program accepts an integer from the user, finds whether the given number is prime and, prints the next prime number.

import java.util.Scanner;
public class NextNumberisPrime {
   public static int isPrime(int num){
      int prime = 1;
      for(int i = 2; i < num; i++) {
         if((num % i) == 0) {
            prime = 0;
         }
      }
      return num;
   }
   public static int nextPrime(int num) {
      num++;
      for (int i = 2; i < num; i++) {
         if(num%i == 0) {
            num++;
            i=2;
         } else {
            continue;
         }
      }
      return num;
   }
   public static void main(String args[]){
      Scanner sc = new Scanner(System.in);
      System.out.println("Enter a number ::");
      int num = sc.nextInt();
      int result = 0;
      int prime = isPrime(num);
      if (prime == 1) {
         System.out.println(num+" is a prime number");
      } else {
         System.out.println(num+" is not a prime number");
      }
      System.out.println("Next prime number is: "+nextPrime(num));
   }
}

Output

Enter a number ::
25
25 is not a prime number
Next prime number is: 29