Prime Number

import java.util.Iterator;
import java.util.SortedSet;
import java.util.TreeSet;

public class Prime implements Iterable<Long> {
  private final SortedSet<Long> primes;

  public Prime() {
    primes = new TreeSet<Long>();
    primes.add((long )2);
    primes.add((long )3);
  }

  public SortedSet<Long> primes() {
    return primes;
  }

  public boolean isPrime(long n) {
    for (long i = primes().last() + 2; i <= n; i += 2) {
      boolean is_prim = true;
      for (long j: primes()) {
        if (i % j == 0) {
          is_prim = false;
          break;
        }
      }
      if (is_prim) primes().add(i);
    }
    for (long i: primes())
      if (n % i == 0) return false;
    return true;
  }

  @Override
  public Iterator<Long> iterator() {
    return primes.iterator();
  }

  private static void print(Prime p) {
    int i = 0;
    StringBuilder sb = new StringBuilder();
    Iterator<Long> it = p.primes().iterator();
    while (it.hasNext()) {
      sb.append(it.next());
      if (it.hasNext()) sb.append(" ");
      if (++i % 20 == 0) sb.append("\n");
    }
    System.out.println(sb.toString());
  }

  public static void main(String[] args) {
    Prime p = new Prime();
    print(p);
    if (p.isPrime(100)) System.out.println("100 is prime number.");
    print(p);
    if (p.isPrime(1000)) System.out.println("1000 is prime number.");
    print(p);
  }
}