code: public static int count(final String source, final String target) {
  
      if (isEmpty(source) || isEmpty(target)) {
        return 0;
      }
      int targetLength = target.length();
      int replacedLength = source.length() - source.replace(target, "").length();
      return replacedLength / targetLength;
    }
docs: Count how much target in souce string.</br>统计target在source中出现的次数。
  
  @param source source string
  @param target target string
  @return the count of target in source string.