code: public boolean matchesDigit() {
          return !isEmpty() && (Character.isDigit(queue.charAt(pos))
                  || queue.charAt(pos) == '-' && remainingLength() >= 2 && Character.isDigit(queue.charAt(pos + 1)));
      }
docs: 测试是否以数字开头,负数也是数字,但是不支持科学表达式模型
  
  @return if matches a digit character