code: public static String multiply(CharSequence self, Number factor) {
          String s = self.toString();
          int size = factor.intValue();
          if (size == 0)
              return "";
          else if (size < 0) {
              throw new IllegalArgumentException("multiply() should be called with a number of 0 or greater not: " + size);
          }
          StringBuilder answer = new StringBuilder(s);
          for (int i = 1; i < size; i++) {
              answer.append(s);
          }
          return answer.toString();
      }
docs: Repeat a CharSequence a certain number of times.
  
  @param self   a CharSequence to be repeated
  @param factor the number of times the CharSequence should be repeated
  @return a String composed of a repetition
  @throws IllegalArgumentException if the number of repetitions is &lt; 0
  @since 1.8.2