code: public static <T> Holder<T> of(T value) throws NullPointerException{
  		if(null == value){
  			throw new NullPointerException("Holder can not hold a null value!");
  		}
  		return new Holder<>(value);
  	}
docs: 新建Holder类，持有指定值，当值为空时抛出空指针异常
  
  @param <T> 被持有的对象类型
  @param value 值，不能为空
  @return Holder