Sophomore Java Homework-P4.18 v2

/**
 * Name: Prime numbers-loop method
 * @author LinChuWen
 * Date: 2014.10.14
 *
 * NCHU EE,course number:2335
 * course name: Object Oriented Language
 * Textbook: Big Java:Late Objects-Cay S. Horstmann
 * Problem: P4.18
 * Description: Enter an integer "n", then prints out all prime numbers up to that integer.
 */
import java.util.*;
public class HW3_P4_18_v2 {

    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        int n,n_sqrt;
        int dividend,divisor;
        boolean prime;

        System.out.print("Please enter an integer: ");

        while(input.hasNext()){
            n=input.nextInt();
            n_sqrt=(int)Math.sqrt(n);

            if(n<0){
                System.out.println("Please enter a positive integer!");
                System.out.print("Please enter an integer: ");
                continue;
            } //if end

            else if(n>=0 && n<4){
                if(n==0 || n==1)
                    System.out.println("No prime numbers up to " + n);
                else if(n==2)
                    System.out.println(2);
                else if(n==3)
                    System.out.printf("2\n3\n");

                System.out.print("Please enter an integer: ");
                continue;
            } //else if end

            else{
                System.out.printf("2\n3\n");
                for(dividend=5;dividend<=n;dividend+=2){
                    prime=true;
                    for(divisor=3;divisor<=n_sqrt;divisor+=2){
                        if(dividend%divisor==0 && dividend!=divisor){
                            prime=false;
                            break;
                        } //if end
                    } //small for end

                    if(prime)
                        System.out.println(dividend);
                } //big for end
            } //else end

            System.out.print("Please enter an integer: ");

        } //while end

        input.close();
    } //main end
} //class end