用循环查找素数

Question

I'am new to Java and my homework was to make a program where the user input a number and the program check if the number is a prime number or not.

I have to use any loop and JOptionPane for input.

I've managed to write this code and it works:

import static javax.swing.JOptionPane.*;
public class Programmeringsoppgave5 {

    public static void main(String[] args) {
        int number = 0; 
        String readNumber = showInputDialog("Write a number and exit with ESC");

        while (readNumber != null){
        number = Integer.parseInt(readNumber);
            if (number != 0 && (number == 1 || number == 2)){
                System.out.println("Number " + readNumber + " is a prime number.");
            } else if (number % 2 == 0){
                System.out.println("Number " + readNumber + " is not a prime number.");
            } else if (number % 2 == 1){
                System.out.println("Number " + readNumber + " is a prime number.");
            }//end if-test
            readNumber = showInputDialog("Write a number and exit with ESC");
        }//end while    
    } //end method
}//end class

I was wondering if there was any easier or better ways to write this program?

Answer 1

To make sure a number is prime, you need to see whether it's divisible by any of the prime numbers below it.

So you can do something more like having an ArrayList of primes you've found so far, which starts with a hard-coded 2.

Then, you start looping from 3 up, and you test whether each new number is divisible by any of the primes already in your ArrayList. If it's not, you add it and move on to the next. If it is, then you just move forward.

You do this until you get to the number in question.

There are a few optimizations you can perform. For example, when checking for numbers that are the factors of the current number, you only need to go up to sqrt(currNum).

If you want to get fancy, you could implement the Sieve of Eratosthenes: http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes