code: public static String getAsterisks(int length) {
  		if (length < 1) return "";
  		StringBuilder sb = new StringBuilder();
  		for (int i = 0; i < length; i++) {
  			sb.append("*");
  		}
  		return sb.toString();
  	}
docs: 生成长度为<tt>length</tt>的星号字符串。
  @param length e.g. 4
  @return e.g. <code>****</code>