program logic of printing the prime numbers

Question

Can any body help to understand this java program?

It just prints prime numbers, as you enter how many you want and it works well.

class PrimeNumbers
{           


        

Answer 1

The factor of a number can lie only from 1 to sqrt of num. So instead of checking from 1 till num for its factors, we check for all numbers from 1 to sqrt(num) so see if any of them divides num. If any divides num, it is not prime, else it is. This improves efficiency of code.