Java Program to Check Whether Number Prime or Not

This post shows a Java program to check whether the passed number is a prime number or not.

A number is a prime number if can be divided either by 1 or by the number itself. So the logic for your program should be to run a for loop and divide the passed number every time in that loop, if it completely divides any time then the passed number is not a prime number. You only need to run your loop from 2 till N/2 (where N is the passed number), reason being no number is completely divisible by a number more than its half.

Java program to check whether the number is prime or not

import java.util.Scanner;

public class PrimeNumChecker {
  public static void main(String[] args) {
    // Using Scanner class to take input
    Scanner sc = new Scanner(System.in);
    System.out.println("Enter number - ");
    int num = sc.nextInt();
    boolean flag = isNumberPrime(num);
    if(flag){
      System.out.println(num + " is a prime number");
    }else{
      System.out.println(num + " is not a prime number");
    }
  }

  private static boolean isNumberPrime(int num){
    boolean flag = true;
    for(int i = 2; i <= num/2; i++){
      // No remainder means completely divides 
      if(num % i == 0){
        flag = false;
        break;
      }
    }
    return flag;
  }
}

Output

Enter number - 
7
7 is a prime number

Enter number - 
10
10 is not a prime number

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