code: public static String join(String one, String sep, String two) {
          one = one == null ? "" : one;
          two = two == null ? "" : two;
          boolean oneEnds = one.endsWith(sep);
          boolean twoStarts = two.startsWith(sep);
          if (oneEnds && twoStarts) {
              return one + two.substring(sep.length());
          } else if (oneEnds || twoStarts) {
              return one + two;
          } else {
              return one + sep + two;
          }
      }
docs: Joins two strings with the given separator, unless the first string ends with the separator, or the second
  string begins with it. For example, the output <code>one/two</code> would be returned from <code>join("one", "two", "/")</code>
  or <code>join("one/", "/two", "/")</code> or <code>join("one/", "two", "/")</code> or
  <code>join("one", "/two", "/")</code>
  
  @param one The prefix
  @param sep The separator to put between the two strings, if it is not there already
  @param two The suffix
  @return The joined strings