编程语言为什么有null
色即是空，空即是色
色不异空，空不异色 ------ 《心经》
千百年来，常有人拿这句话调侃和尚看见女色不断默念此经，以达到两眼空空的境界，连《西游记》里的女儿国一集都有这个桥段，可这唐玄奘法师，哪能两眼空空呢？如果你血气方刚，那么坦率承认和表达袒露眼前人的美，是空；如果你参悟大道，如庄子一般感悟到，天地有大美而不言，人的美丑掺杂着主观判断，也是空。
回到计算机世界，开始悟"空"。
一、null是谁发明的？
用Java写业务代码时，如果时间匆忙，我经常忘记做null值的特例判断，这给我带来了不少的问题和麻烦，这令人又爱又恨的null值，究竟从何而来？
不像中文，中文的字词大多已经创造好，但英文这种表音文字，是有可能随时随地用字母造一个新词的。因此，null这个单词是否诞生于计算机发明之前？答案是肯定的。[1]
托尼·霍尔（Tony Hoare），编程世界中，null的创造者，也是快排算法（Quick Sort）的发明人，但身处历史洪流中的人物总有局限性，他在2009年的Qcon技术会议上，发表了一篇名为 NULL: The Billion Dollar Mistake 的演讲[2]
I call it my billion-dollar mistake...At that time, I was designing the first comprehensive type system for references in an object-oriented language. My goal was to ensure that all use of references should be absolutely safe, with checking performed automatically by the compiler.But I couldn't resist the temptation to put in a null reference, simply because it was so easy to implement. This has led to innumerable errors, vulnerabilities, and system crashes, which have probably caused a billion dollars of pain and damage in the last forty years.
-- Tony Hoare, inventor of ALGOL W.
不过，也有人说，null的概念并不是由霍尔博士发明的，也许在机器语言程序员将指针指向0时，编程世界里，这个概念就诞生了。
With all due respect to Dr Hoare, I call hubris. 😉 Machine-language programmers invented the null reference the first time they set a pointer to zero. -- Steven A. Lowe Nov 11, 2010 at 6:39
------ 来自StackOverflow的一条评论[3]
但也有编程语言是没有null这个概念的，如Haskell，这是门函数式编程语言，但我对函数式编程不懂。
Some programming languages (mostly functional programming languages like Haskell) don't support the concept of
null. Instead, they use the Maybe/Optional Pattern to represent the 'absence of a value'. The compiler ensures that the 'no value' case is handled explicitly. Hence, null pointer errors cannot occur.[4]
二、我所理解的null存在的意义
集合里，空集(empty set)并不等于数字0，但空集是一个重要的概念，计算机中的null，也可以此类比，死亡，是宇宙的尽头，null，是编程语言的尽头，这里空无一物，没有给你分配的内存，不要再往下走了。就目前为止，我只能从这个层面出发，去理解为什么有null。
0能够表达非数值概念的空无，无法表达数值的空无。
null能够表达一切范围的空无，却不能表达自己的空无。这个矛盾，源于自指。
References
1\] ['null' in dictionary](https://www.etymonline.com/word/null) \[2\] [Null References: The Billion Dollar Mistake](https://www.infoq.com/presentations/Null-References-The-Billion-Dollar-Mistake-Tony-Hoare/?ref=hackernoon.com) \[3\] [what is the purpose of null?](https://stackoverflow.com/questions/584507/what-is-the-purpose-of-null) \[4\] [A quick and thorough guide to 'null': what it is, and how you should use it](https://www.freecodecamp.org/news/a-quick-and-thorough-guide-to-null-what-it-is-and-how-you-should-use-it-d170cea62840/) \[5\] [If null is a billion dollar mistake, what is the solution to represent a non-initialized object?](https://softwareengineering.stackexchange.com/questions/413149/if-null-is-a-billion-dollar-mistake-what-is-the-solution-to-represent-a-non-ini) \[6\] [NULL: The Billion Dollar Mistake](https://hackernoon.com/null-the-billion-dollar-mistake-8t5z32d6)