code: public static String asterisked(String str, int keepSize, boolean keepStart) {
  		if (StringUtils.isBlank(str)) return "";
  		
  		int length = str.length();
  		if (length < keepSize) return str;
  		if (keepSize < 1) return getAsterisks(length);
  		
  		int notKeepSize = length - keepSize;
  		String keep;
  		if (keepStart) {
  			keep = str.substring(0, keepSize);
  			return keep + getAsterisks(notKeepSize);
  		} else {
  			keep = str.substring(notKeepSize);
  			return getAsterisks(notKeepSize) + keep;
  		}
  	}
docs: 将字符串内容替换成星号.<br>
  Replace to asterisk.
  @param str e.g. 123456
  @param keepSize e.g. 4
  @param keepStart e.g. false
  @return e.g. **3456