Compiled and tested here: http://www.browxy.com/
import java.util.Scanner;
class Main {
public static void main(String[] args) {
Scanner reader = new Scanner(System.in);
System.out.println("Enter a number: ");
int N = reader.nextInt(); // user input.
// Array
// 0 - prime, 1 - composite
int[] arr = new int[N + 1];
arr[0] = arr[1] = 1;
// loop from 2 to square root of N
for (int i = 2; i <= Math.sqrt(N); i++) {
// check if prime
if (arr[i] == 0) {
// add all factors of i to composite numbers to N
for (int j = i * i; j <= N; j += i) {
arr[j] = 1;
}
}
}
// Print all positions for which fields are not reset
for (int i = 0; i < arr.length; i++) {
if (arr[i] == 0) {
System.out.print(i + ", ");
}
}
}
}