Prime.java

public class Prime {
  public static boolean isPrime(int num) {
      if (num > Math.pow(2, 31)) return false;
      if (num < 0 || num == 0 || num == 1) return false;
     
      // A not prime num = a * b, a will always smaller or equal to b (vice versa) 
      // & a and b can't both larger then square root of num(e.g. 36 = 6 * 6 = 4 * 9)
      int n = (int)Math.sqrt(num);
      for(int i = 2; i <= n ; i++){  
            if(num % i == 0) return false; 
        }
        return true;
  }
}