P37 { public static int gcd(int first, int second) { if (first == 0) { return second; } else if (second == 0) { return first; } else if (first > second) { return gcd(first - second, second); } else { return gcd(first, second - first); } }  static int gcd(int first, int second);  }
@Test public void gcdOf36And63Is9() throws Exception { int gcd = P37.gcd(36, 63); assertThat(gcd, equalTo(9)); }