一点思考|工作十几年了，竟从未用过do-while！

{"type":"doc","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"最近在看 Java 的基础知识，其中有部分是关于循环的，在 Java 中，循环的语法总共分为 3 种： "},{"type":"codeinline","content":[{"type":"text","text":"for"}]},{"type":"text","text":"、 "},{"type":"codeinline","content":[{"type":"text","text":"while"}]},{"type":"text","text":"、 "},{"type":"codeinline","content":[{"type":"text","text":"do-while"}]},{"type":"text","text":"，如下图所示："}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/40/4014dd8484d3a1de98d6b44262f2adcb.png","alt":null,"title":"","style":[{"key":"width","value":"50%"},{"key":"bordertype","value":"none"}],"href":"","fromPaste":false,"pastePass":false}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"但我惊奇的发现，在之前的职业生涯中（11 年），竟从未用过 "},{"type":"codeinline","content":[{"type":"text","text":"do-while"}]},{"type":"text","text":"（尴尬），于是问了群里的小伙伴，发现也是鲜有人用。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/09/09a77f277926ebff723aea942a9d0c49.png","alt":null,"title":"","style":[{"key":"width","value":"50%"},{"key":"bordertype","value":"none"}],"href":"","fromPaste":false,"pastePass":false}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"do-while 语法分析"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我们先来了解一下 do-while 的语法："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"do {\n // statements\n} while (expression);"}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"注意："},{"type":"text","marks":[{"type":"strong"}],"text":"最后一个冒号不能省略，否则会提示编译出错"},{"type":"text","text":"。"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"它的执行流程如下图所示："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/dc/dc56034fd1533d359090da86787755ad.png","alt":null,"title":"","style":[{"key":"width","value":"25%"},{"key":"bordertype","value":"none"}],"href":"","fromPaste":false,"pastePass":false}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"那它究竟在什么地方用呢？"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"do-while 使用场景"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在我多次的搜索和请教下，终于找到了两个相对满意的使用场景，接下来一起来看。"}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"使用场景一：抢票"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"对于抢票业务来说，无论三七二十一，先抢了再说，然后在判断是否抢票成功，如果抢票成功则退出循环，否则继续执行抢票，实现的伪代码如下所示："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"do {\n // 抢票代码...\n} while (没抢到票);"}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"思路提供者：贾总"}]}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"使用场景二：进制转换"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"经过大量搜索发现，在 JDK 的源码中，也存在少量使用 "},{"type":"codeinline","content":[{"type":"text","text":"do-while"}]},{"type":"text","text":" 的场景，比如 "},{"type":"codeinline","content":[{"type":"text","text":"Integer"}]},{"type":"text","text":" 中进制转换，相关源码如下："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"static int formatUnsignedInt(int val, int shift, char[] buf, int offset, int len) {\n int charPos = len;\n int radix = 1 << shift;\n int mask = radix - 1;\n do {\n buf[offset + --charPos] = Integer.digits[val & mask];\n val >>>= shift;\n } while (val != 0 && charPos > 0);\n\n return charPos;\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"比如十进制转二进制就会执行此方法，在进制转换的业务中，无论任何情况，一定会至少执行一次进制转换的，因此这种业务场景就非常适合 "},{"type":"codeinline","content":[{"type":"text","text":"do-while"}]},{"type":"text","text":"。"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"总结"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"孔子说：温故而知新。当我们学完很多知识之后，回过头来再琢磨这些知识，发现很有趣，这就是知识的一大乐趣吧。本文我们介绍了两种 "},{"type":"codeinline","content":[{"type":"text","text":"do-while"}]},{"type":"text","text":" 的使用场景，抢票和进制转换，你还知道哪些 "},{"type":"codeinline","content":[{"type":"text","text":"do-while"}]},{"type":"text","text":" 的使用场景吗？欢迎评论区告诉我~"}]}]}