177 lines
60 KiB
JavaScript
177 lines
60 KiB
JavaScript
import{_ as d}from"./chunks/ArticleMetadata.Sb1DYAHo.js";import{_ as A,D as p,o as t,c as g,I as s,w as y,k,a as D,R as i,b as C,e as c}from"./chunks/framework.FVQzxbLi.js";import"./chunks/md5.RtphNWHi.js";const o="/assets/202010071229226.u6t1oD4Z.png",B="/assets/202010071230720.tk45jvyd.png",F="/assets/202010071230999.F-m2rdiQ.png",E="/assets/202010071232376.hFgK6mfi.png",u="/assets/202010071232956.9Azuj1-R.png",R=JSON.parse('{"title":"if选择结构","description":"","frontmatter":{"title":"if选择结构","author":"查尔斯","date":"2020/10/07 13:20","categories":["Java基础快速入门"],"tags":["Java","Java基础"]},"headers":[],"relativePath":"courses/java/01-Java语法入门/08-if选择结构.md","filePath":"courses/java/01-Java语法入门/08-if选择结构.md","lastUpdated":1663999704000}'),m={name:"courses/java/01-Java语法入门/08-if选择结构.md"},v=k("h1",{id:"if选择结构",tabindex:"-1"},[D("if选择结构 "),k("a",{class:"header-anchor",href:"#if选择结构","aria-label":'Permalink to "if选择结构"'},"")],-1),f=i('<h2 id="前言" tabindex="-1">前言 <a class="header-anchor" href="#前言" aria-label="Permalink to "前言""></a></h2><p><strong>C：</strong> 上一篇我们介绍了流程控制语句的概念，本篇我们要正式开始学习其中的选择控制语句。那么在 Java 中，如果要实现选择控制语句有哪些语法呢？</p><p>本篇我们就来看看其中，在以后用的最多的一种选择结构语法：if 系列选择结构。</p><p><img src="'+o+`" alt="202010071229226"></p><h2 id="基础if选择结构" tabindex="-1">基础if选择结构 <a class="header-anchor" href="#基础if选择结构" aria-label="Permalink to "基础if选择结构""></a></h2><p><strong>基础 if 选择结构</strong> 是 if 系列选择结构中，最基础的结构语法！它是最简单的选择结构，也只能用于一种流程分支的处理。</p><div class="language-java vp-adaptive-theme"><button title="Copy Code" class="copy"></button><span class="lang">java</span><pre class="shiki shiki-themes github-light github-dark-dimmed vp-code"><code><span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 语法</span></span>
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<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 条件往往是由关系运算符来组成的判断</span></span>
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<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 条件的结果是一个boolean值，要么为true(真)，要么为false(假)</span></span>
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<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">if</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> (条件) {</span></span>
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<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;"> // 条件为真时执行的语句</span></span>
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<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">}</span></span></code></pre></div>`,7),_=i(`<p><strong>案例需求：如果佩奇 Java 成绩大于 90 分，笔者将会奖励她一部 OnePlus 8 Pro。</strong></p><div class="language-java vp-adaptive-theme"><button title="Copy Code" class="copy"></button><span class="lang">java</span><pre class="shiki shiki-themes github-light github-dark-dimmed vp-code"><code><span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 声明一个变量用来存储佩奇的Java成绩</span></span>
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<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">int</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> score</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> =</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 91</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">;</span></span>
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<span class="line"></span>
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<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 套用基础if语法</span></span>
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<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">if</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> (score </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">></span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 90</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">) {</span></span>
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<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> System.out.</span><span style="--shiki-light:#6F42C1;--shiki-dark:#DCBDFB;">println</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">(</span><span style="--shiki-light:#032F62;--shiki-dark:#96D0FF;">"笔者奖励了一部OnePlus 8 Pro。"</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">);</span></span>
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<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">}</span></span>
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<span class="line"></span>
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<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">System.out.</span><span style="--shiki-light:#6F42C1;--shiki-dark:#DCBDFB;">println</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">(</span><span style="--shiki-light:#032F62;--shiki-dark:#96D0FF;">"程序结束！"</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">);</span></span></code></pre></div><div class="tip custom-block"><p class="custom-block-title">笔者说</p><p>套用下语法，用用关系运算符，就实现了一个很简单的选择案例，是不是很 easy。</p></div><h3 id="复杂条件的基础if" tabindex="-1">复杂条件的基础if <a class="header-anchor" href="#复杂条件的基础if" aria-label="Permalink to "复杂条件的基础if""></a></h3><p>接下来再看看下方案例中的条件，它就没有那么容易了。</p><p><strong>案例需求：如果乔治 Java 成绩大于 90 分，并且音乐成绩大于 80 分时，或者 Java 成绩等于 100 分，音乐成绩大于 70 分时，笔者会奖励他一部 OnePlus 8 Pro。</strong></p><p>面对这种场景，单靠普通的关系运算符显然无法实现，结合逻辑运算符就可以很好解决。</p><div class="language-java vp-adaptive-theme"><button title="Copy Code" class="copy"></button><span class="lang">java</span><pre class="shiki shiki-themes github-light github-dark-dimmed vp-code"><code><span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 声明变量分别存储乔治的Java成绩和音乐成绩</span></span>
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<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">int</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> jScore</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> =</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 60</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">;</span></span>
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<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">int</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> mScore</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> =</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 81</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">;</span></span>
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<span class="line"></span>
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<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 关系运算符结合逻辑运算符</span></span>
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<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 考虑到运算符的优先级，可以使用()来提升部分表达式的优先级，也是复杂条件下增加可读性的方法</span></span>
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<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// ()的添加应该成为一种本能，它是义务教育阶段学数学后的条件反射</span></span>
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<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">if</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> ((jScore </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">></span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 90</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> &&</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> mScore </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">></span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 80</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">) </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">||</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> (jScore </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">==</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 100</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> &&</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> mScore </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">></span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 70</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">)) {</span></span>
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<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> System.out.</span><span style="--shiki-light:#6F42C1;--shiki-dark:#DCBDFB;">println</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">(</span><span style="--shiki-light:#032F62;--shiki-dark:#96D0FF;">"笔者奖励了一部OnePlus 8 Pro。"</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">);</span></span>
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<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">}</span></span>
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<span class="line"></span>
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<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">System.out.</span><span style="--shiki-light:#6F42C1;--shiki-dark:#DCBDFB;">println</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">(</span><span style="--shiki-light:#032F62;--shiki-dark:#96D0FF;">"程序结束！"</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">);</span></span></code></pre></div><h2 id="if-else选择结构" tabindex="-1">if-else选择结构 <a class="header-anchor" href="#if-else选择结构" aria-label="Permalink to "if-else选择结构""></a></h2><p>我们敲完上方的代码发现，如果条件不满足的话，好像什么处理情况也没有，这怎么可能，只有老师奖励学生，没有学生受罚？我们再来看个案例。</p><p><strong>案例需求：如果苏西 Java 考试成绩大于 90 分，笔者就奖励她一部 OnePlus 8 Pro，否则就罚她给笔者买一部 OnePlus 8 Pro。</strong></p><p>面对这个案例，我们还是可以采用基础If语法。</p><div class="language-java vp-adaptive-theme"><button title="Copy Code" class="copy"></button><span class="lang">java</span><pre class="shiki shiki-themes github-light github-dark-dimmed vp-code"><code><span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 条件成立情况</span></span>
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<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">if</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> (score </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">></span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 90</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">) {</span></span>
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<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> System.out.</span><span style="--shiki-light:#6F42C1;--shiki-dark:#DCBDFB;">println</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">(</span><span style="--shiki-light:#032F62;--shiki-dark:#96D0FF;">"笔者奖励了一部OnePlus 8 Pro。"</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">);</span></span>
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<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">}</span></span>
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<span class="line"></span>
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<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 条件不成立情况</span></span>
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<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// if (score <= 90) {</span></span>
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<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;"> // System.out.println("苏西哭着给笔者买了一部OnePlus 8 Pro，笔者很感动。");</span></span>
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<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// }</span></span>
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<span class="line"></span>
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<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 还可以这么写</span></span>
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<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// !表示取反，也就是某种情况的相反情况</span></span>
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<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">if</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> (</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">!</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">(score </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">></span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 90</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">)) {</span></span>
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<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> System.out.</span><span style="--shiki-light:#6F42C1;--shiki-dark:#DCBDFB;">println</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">(</span><span style="--shiki-light:#032F62;--shiki-dark:#96D0FF;">"苏西哭着给笔者买了一部OnePlus 8 Pro，笔者很感动。"</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">);</span></span>
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<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">}</span></span></code></pre></div><p>但是明知道两个结果是互斥的，还要再写一次判断，并且条件越复杂，写起来就越麻烦。</p><p>例如：如果要基于刚才复杂条件的if案例来写的话。</p><div class="language-java vp-adaptive-theme"><button title="Copy Code" class="copy"></button><span class="lang">java</span><pre class="shiki shiki-themes github-light github-dark-dimmed vp-code"><code><span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 声明变量分别存储乔治的Java成绩和音乐成绩</span></span>
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<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">int</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> jScore</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> =</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 60</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">;</span></span>
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<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">int</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> mScore</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> =</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 81</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">;</span></span>
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<span class="line"></span>
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<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 成立</span></span>
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<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">if</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> ((jScore </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">></span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 90</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> &&</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> mScore </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">></span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 80</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">) </span></span>
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<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> ||</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> (jScore </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">==</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 100</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> &&</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> mScore </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">></span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 70</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">)) {</span></span>
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<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> System.out.</span><span style="--shiki-light:#6F42C1;--shiki-dark:#DCBDFB;">println</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">(</span><span style="--shiki-light:#032F62;--shiki-dark:#96D0FF;">"笔者奖励了一部OnePlus 8 Pro。"</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">);</span></span>
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<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">}</span></span>
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<span class="line"></span>
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<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 不成立的情况</span></span>
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<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">if</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> (</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">!</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">((jScore </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">></span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 90</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> &&</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> mScore </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">></span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 80</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">) </span></span>
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<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> ||</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> (jScore </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">==</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 100</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> &&</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> mScore </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">></span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 70</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">))) {</span></span>
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<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> System.out.</span><span style="--shiki-light:#6F42C1;--shiki-dark:#DCBDFB;">println</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">(</span><span style="--shiki-light:#032F62;--shiki-dark:#96D0FF;">"奖励乔治给笔者买了一部OnePlus 8 Pro。"</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">);</span></span>
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<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">}</span></span>
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<span class="line"></span>
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<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">System.out.</span><span style="--shiki-light:#6F42C1;--shiki-dark:#DCBDFB;">println</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">(</span><span style="--shiki-light:#032F62;--shiki-dark:#96D0FF;">"程序结束！"</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">);</span></span></code></pre></div><p><strong>头疼</strong> ，这时候我们可以使用 <code>if-else</code> 语法来轻松解决这种条件互斥的情况。<code>if-else</code> 选择结构可以处理简单的互斥分支情况。</p><div class="language-java vp-adaptive-theme"><button title="Copy Code" class="copy"></button><span class="lang">java</span><pre class="shiki shiki-themes github-light github-dark-dimmed vp-code"><code><span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 语法</span></span>
|
||
<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">if</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> (条件) {</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;"> // 语句1</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">} </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">else</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> {</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;"> // 语句2</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">}</span></span></code></pre></div><p><img src="`+B+'" alt="202010071230720"></p>',19),b=i(`<p>使用 <code>if-else</code> 来解决上述案例需求。</p><div class="language-java vp-adaptive-theme"><button title="Copy Code" class="copy"></button><span class="lang">java</span><pre class="shiki shiki-themes github-light github-dark-dimmed vp-code"><code><span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 声明一个变量存储苏西的Java考试成绩</span></span>
|
||
<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">int</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> score</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> =</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 90</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">;</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 套用if-else语法</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// if：如果 else：否则</span></span>
|
||
<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">if</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> (score </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">></span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 90</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">) {</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> System.out.</span><span style="--shiki-light:#6F42C1;--shiki-dark:#DCBDFB;">println</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">(</span><span style="--shiki-light:#032F62;--shiki-dark:#96D0FF;">"笔者奖励了一部iPhone 11 Pro。"</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">);</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">} </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">else</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> {</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> System.out.</span><span style="--shiki-light:#6F42C1;--shiki-dark:#DCBDFB;">println</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">(</span><span style="--shiki-light:#032F62;--shiki-dark:#96D0FF;">"苏西哭着给笔者买了一部OnePlus 8 Pro，笔者很感动。"</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">);</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">}</span></span></code></pre></div><div class="tip custom-block"><p class="custom-block-title">笔者说</p><p>实际上我们学的这些语法，在你念着案例时就能把代码写出来，如果（if）苏西的 Java 考试成绩（score）大于90分（> 90），笔者就奖励她一部 OnePlus 8 Pro（System.out.println("xxx");），否则（else）就罚她给笔者买一部 OnePlus 8 Pro（System.out.println("xxx");）。</p></div><h2 id="多重if选择结构" tabindex="-1">多重if选择结构 <a class="header-anchor" href="#多重if选择结构" aria-label="Permalink to "多重if选择结构""></a></h2><p>我们掌握了 <code>基础if</code> 来处理单分支选择，也掌握了 <code>if-else</code> 来处理互斥分支，但是如果出现了更多种的分支情况怎么办？</p><p><strong>案例需求：如果佩奇 Java 考试成绩大于等于 80 分，笔者就奖励她一部 OnePlus 8 Pro，如果佩奇 Java 考试成绩大于等于 60 分，笔者就奖励她一部一加云耳，否则就罚她给笔者买一部 iPhone 11 Pro。</strong></p><p>面对此需求，<code>if-else</code> 只能处理互斥分支所以不能采用，那使用 <code>基础if</code> 的方案如下。</p><div class="language-java vp-adaptive-theme"><button title="Copy Code" class="copy"></button><span class="lang">java</span><pre class="shiki shiki-themes github-light github-dark-dimmed vp-code"><code><span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 声明一个变量存储佩奇的Java考试成绩</span></span>
|
||
<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">int</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> score</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> =</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 90</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">;</span></span>
|
||
<span class="line"></span>
|
||
<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">if</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> (score </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">>=</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 80</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">) {</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> System.out.</span><span style="--shiki-light:#6F42C1;--shiki-dark:#DCBDFB;">println</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">(</span><span style="--shiki-light:#032F62;--shiki-dark:#96D0FF;">"笔者奖励她一部OnePlus 8 Pro。"</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">);</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">}</span></span>
|
||
<span class="line"></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 如果不添加&&来进行限制，那么佩奇考试成绩>80分时，笔者将会破产</span></span>
|
||
<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">if</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> (score </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">>=</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 60</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> &&</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> score </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"><</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 80</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">) {</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> System.out.</span><span style="--shiki-light:#6F42C1;--shiki-dark:#DCBDFB;">println</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">(</span><span style="--shiki-light:#032F62;--shiki-dark:#96D0FF;">"笔者奖励她一部一加云耳。"</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">);</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">}</span></span>
|
||
<span class="line"></span>
|
||
<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">if</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> (score </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"><</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 60</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">) {</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> System.out.</span><span style="--shiki-light:#6F42C1;--shiki-dark:#DCBDFB;">println</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">(</span><span style="--shiki-light:#032F62;--shiki-dark:#96D0FF;">"佩奇哭着给笔者买了一部iPhone 11 Pro，笔者很感动。"</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">);</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">}</span></span></code></pre></div><p>毫无疑问，<code>基础if</code> 依然需要添加多次条件，但对于范围型判断来讲，如果不把范围控制好，那么后续的 if 条件就会出现多余判断的情况，因为每个 <code>基础if</code> 都是独立的，Java 为此提供了一种简化的方案：<code>多重if选择结构</code>。</p><p><strong>多重if选择结构：</strong> 可以处理分段的条件分支情况，<strong>它是自上而下进行选择判断，只要有一个条件满足，剩下的条件都不再执行判断</strong> 。所以不要随意排列条件，把容易满足的条件放在下方，不然容易满足的条件判断为 <code>true</code> 后，后续其他条件都不会再判断了。</p><div class="language-java vp-adaptive-theme"><button title="Copy Code" class="copy"></button><span class="lang">java</span><pre class="shiki shiki-themes github-light github-dark-dimmed vp-code"><code><span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 语法</span></span>
|
||
<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">if</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> (条件1) {</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;"> // 语句1</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">} </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">else</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> if</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">(条件2) { </span><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 可以有多个else if</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;"> // 语句2</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">} </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">else</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> { </span><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 可以省略</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;"> // 语句3</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">}</span></span></code></pre></div>`,11),q=i(`<div class="language-java vp-adaptive-theme"><button title="Copy Code" class="copy"></button><span class="lang">java</span><pre class="shiki shiki-themes github-light github-dark-dimmed vp-code"><code><span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 存储考试成绩</span></span>
|
||
<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">int</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> score</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> =</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 90</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">;</span></span>
|
||
<span class="line"></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 套用if - else if - else语句</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">/*</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;"> * 注意事项：</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;"> * 1.else if 必须配合if使用，if只能写一个，else if可以写很多个</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;"> * 2.if-else if结构是自上而下进行判断选择的，只要上方的一个条件成立，下方的其他条件不再执行</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;"> * 3.可以结合else来使用，当上方所有条件都不成立时，就会执行else语句内容</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;"> */</span></span>
|
||
<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">if</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> (score </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">>=</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 80</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">) {</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> System.out.</span><span style="--shiki-light:#6F42C1;--shiki-dark:#DCBDFB;">println</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">(</span><span style="--shiki-light:#032F62;--shiki-dark:#96D0FF;">"笔者奖励她一部OnePlus 8 Pro。"</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">);</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">} </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">else</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> if</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> (score </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">>=</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 60</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">) { </span><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 不需要限制 < 80的条件了，因为只要符合>=80，就不再向下判断</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> System.out.</span><span style="--shiki-light:#6F42C1;--shiki-dark:#DCBDFB;">println</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">(</span><span style="--shiki-light:#032F62;--shiki-dark:#96D0FF;">"笔者奖励她一部一加云耳。"</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">);</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">} </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">else</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> {</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> System.out.</span><span style="--shiki-light:#6F42C1;--shiki-dark:#DCBDFB;">println</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">(</span><span style="--shiki-light:#032F62;--shiki-dark:#96D0FF;">"佩奇哭着给笔者买了一部iPhone 11 Pro，笔者很感动。"</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">);</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">}</span></span></code></pre></div><h2 id="嵌套if选择结构" tabindex="-1">嵌套if选择结构 <a class="header-anchor" href="#嵌套if选择结构" aria-label="Permalink to "嵌套if选择结构""></a></h2><p>生活在北上广深的各位同学们，对早晚高峰的地铁站印象很深了吧？在乘坐地铁前，我们需要有两次检查，第一次是安全检查，第二次是车票检查。火车站也是类似的。</p><p>面对这种多次检查，它的逻辑不是 <code>多重If</code> 这种自上而下依次执行的判断，而是当通过第一层判断后才可以进入第二层判断，面对这种场景，我们可以把 if 语句再灵活使用一点。</p><p><img src="`+F+`" alt="202010071230999"></p><p><strong>嵌套if</strong> 的应用场景：<strong>在之前条件满足的前提下，再增加额外的判断</strong> 。它可以通过外层语句和内层语句的协作，增强程序的灵活性。在笔者看来，当业务场景描述清楚的时候，福至心灵下就会自然而然的写出来对应的代码了。</p><div class="language-java vp-adaptive-theme"><button title="Copy Code" class="copy"></button><span class="lang">java</span><pre class="shiki shiki-themes github-light github-dark-dimmed vp-code"><code><span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">if</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> (条件1) {</span></span>
|
||
<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> if</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> (条件2) </span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;"> // 语句1</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> } </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">else</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> {</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;"> // 语句2</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> }</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">} </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">else</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> {</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;"> // 语句3</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">}</span></span></code></pre></div>`,7),P=i(`<p><strong>案例需求：学校举行运动会，百米赛跑跑入10秒内的学生有资格进决赛，然后根据性别进入男子组或女子组</strong></p><div class="language-java vp-adaptive-theme"><button title="Copy Code" class="copy"></button><span class="lang">java</span><pre class="shiki shiki-themes github-light github-dark-dimmed vp-code"><code><span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 声明一个变量存储百米成绩</span></span>
|
||
<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">int</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> score</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> =</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 9</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">;</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 声明一个变量存储性别</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">String</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> gender</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> =</span><span style="--shiki-light:#032F62;--shiki-dark:#96D0FF;"> "女"</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">;</span></span>
|
||
<span class="line"></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 套用嵌套if语句</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 注意事项：如果没有进入决赛，也就不会分配组别了</span></span>
|
||
<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">if</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> (score </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"><</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 10</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">) { </span><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 可以进入决赛</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> System.out.</span><span style="--shiki-light:#6F42C1;--shiki-dark:#DCBDFB;">println</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">(</span><span style="--shiki-light:#032F62;--shiki-dark:#96D0FF;">"进入了决赛！"</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">);</span></span>
|
||
<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> if</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> (</span><span style="--shiki-light:#032F62;--shiki-dark:#96D0FF;">"男"</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">.</span><span style="--shiki-light:#6F42C1;--shiki-dark:#DCBDFB;">equals</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">(gender)) { </span><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 分配组别</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> System.out.</span><span style="--shiki-light:#6F42C1;--shiki-dark:#DCBDFB;">println</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">(</span><span style="--shiki-light:#032F62;--shiki-dark:#96D0FF;">"进入了男子组！"</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">);</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> } </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">else</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> {</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> System.out.</span><span style="--shiki-light:#6F42C1;--shiki-dark:#DCBDFB;">println</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">(</span><span style="--shiki-light:#032F62;--shiki-dark:#96D0FF;">"进入了女子组！"</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">);</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> }</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">}</span></span></code></pre></div><h3 id="字符串的比较" tabindex="-1">字符串的比较 <a class="header-anchor" href="#字符串的比较" aria-label="Permalink to "字符串的比较""></a></h3><p>大多数情况下，我们比较内容时都会采用 <code>==</code> 这个关系运算符，但是如果遇到特殊类型，就不可以了。</p><p><strong>字符串的比较不要使用==，而是采用equals()方法。</strong></p><div class="tip custom-block"><p class="custom-block-title">笔者说</p><p>介绍数据类型时，笔者就说过 String 是个引用数据类型，有点特别。先了解，知道使用equals 方法来比较字符串内容即可。</p></div><div class="language-java vp-adaptive-theme"><button title="Copy Code" class="copy"></button><span class="lang">java</span><pre class="shiki shiki-themes github-light github-dark-dimmed vp-code"><code><span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// == 号可以比较基本数据类型的值是否相等(八大基本数据类型)</span></span>
|
||
<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">int</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> num1</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> =</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 10</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">;</span></span>
|
||
<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">int</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> num2</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> =</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 10</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">;</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// System.out.println(num1 == num2); // true</span></span>
|
||
<span class="line"></span>
|
||
<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">char</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> gender1</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> =</span><span style="--shiki-light:#032F62;--shiki-dark:#96D0FF;"> '男'</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">;</span></span>
|
||
<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">char</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> gender2</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> =</span><span style="--shiki-light:#032F62;--shiki-dark:#96D0FF;"> '男'</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">;</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// System.out.println(gender1 == gender2); // true</span></span>
|
||
<span class="line"></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// ------------------</span></span>
|
||
<span class="line"></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 如果是字符串，不要使用==号比较，而是使用equals()方法</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 因为字符串是引用数据类型，==号如果比较引用数据类型，实际上比较的是数据的地址值而不是比较内容</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 所以==很有可能导致你的字符串内容比较出问题，因为它比较的是地址值</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 即使你发现两个字符串用==也比较为true了，那其实是涉及到常量池的概念了。</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 先记住不要用==比较字符串就可以了！！！后期面向对象阶段学完就知道为啥了。</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">String</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> str1</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> =</span><span style="--shiki-light:#032F62;--shiki-dark:#96D0FF;"> "我是一个字符串"</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">;</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">String</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> str2</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> =</span><span style="--shiki-light:#032F62;--shiki-dark:#96D0FF;"> "我是一个字符串"</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">;</span></span>
|
||
<span class="line"><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">System.out.</span><span style="--shiki-light:#6F42C1;--shiki-dark:#DCBDFB;">println</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">(str1.</span><span style="--shiki-light:#6F42C1;--shiki-dark:#DCBDFB;">equals</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">(str2));</span></span></code></pre></div><h2 id="答题环节" tabindex="-1">答题环节 <a class="header-anchor" href="#答题环节" aria-label="Permalink to "答题环节""></a></h2><p>光看不练假把式，来几个练习，练起来。</p><h3 id="模拟登录" tabindex="-1">模拟登录 <a class="header-anchor" href="#模拟登录" aria-label="Permalink to "模拟登录""></a></h3><details class="details custom-block"><summary>需求：使用 if 编写程序：从键盘获取用户名和密码，如果用户名和密码都正确，那么就显示"欢迎进入xxx的世界！"，否则就提示"用户名或密码错误！"。</summary><p>提示：可以预先设定一个假的用户名和密码。</p></details><p><img src="`+E+'" alt="202010071232376"></p><h3 id="人机猜拳" tabindex="-1">人机猜拳 <a class="header-anchor" href="#人机猜拳" aria-label="Permalink to "人机猜拳""></a></h3><p>从控制台输入要出的拳，1代表石头、2代表剪刀、3代表布，电脑 <strong>随机</strong> 出拳，比较胜负，完成此代码功能。</p><p><img src="'+u+`" alt="202010071232956"></p><h4 id="随机数的生成" tabindex="-1">随机数的生成 <a class="header-anchor" href="#随机数的生成" aria-label="Permalink to "随机数的生成""></a></h4><p>随机数是我们在程序中经常要使用到的，前期我们可以用它模拟很多业务，下面介绍一个 Java 中生成随机数的玩法。</p><div class="language-java vp-adaptive-theme"><button title="Copy Code" class="copy"></button><span class="lang">java</span><pre class="shiki shiki-themes github-light github-dark-dimmed vp-code"><code><span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// Math.random()可以随机生成[0.0,1.0)之间的小数</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// [0.0,1.0)表示一个区间，等价于0.0 <= num1 < 1.0</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 左闭右开就是包含左边数据，不包含右边的数据</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// [0.0,1.0]也表示一个区间，等价于0.0 <= num1 <= 1.0</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 左闭右闭就是包含左边数据，也包含右边的数据</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 想了解更多区间的内容，自己去百度下相关数学知识</span></span>
|
||
<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">double</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> num1</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> =</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> Math.</span><span style="--shiki-light:#6F42C1;--shiki-dark:#DCBDFB;">random</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">();</span></span>
|
||
<span class="line"></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">/*</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;"> * 基于上方的方法，如果想指定生成某个范围内的随机数，可以推导出一个公式。</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;"> * 随机数公式为：(int)(Math.random() * (max - min)) + min;</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;"> */</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 案例1：生成[0,10)之间的随机整数</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// [0.0,1.0) * 10 --> [0,10)</span></span>
|
||
<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">int</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> num2</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> =</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> (</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">int</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">)(Math.</span><span style="--shiki-light:#6F42C1;--shiki-dark:#DCBDFB;">random</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">() </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">*</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 10</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">);</span></span>
|
||
<span class="line"></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 案例2：生成[5,14)之间的随机整数</span></span>
|
||
<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">int</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> num3</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> =</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> (</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">int</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">)(Math.</span><span style="--shiki-light:#6F42C1;--shiki-dark:#DCBDFB;">random</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">() </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">*</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 9</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">) </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">+</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 5</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">;</span></span>
|
||
<span class="line"></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 案例3：如果想生成[6,20]之间的随机整数，那可以将其理解为[6,21)，这样就可以再套上方的公式了</span></span>
|
||
<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">int</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> num4</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> =</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> (</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">int</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">)(Math.</span><span style="--shiki-light:#6F42C1;--shiki-dark:#DCBDFB;">random</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">() </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">*</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 15</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">) </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">+</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 6</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">;</span></span></code></pre></div><h3 id="幸运抽奖" tabindex="-1">幸运抽奖 <a class="header-anchor" href="#幸运抽奖" aria-label="Permalink to "幸运抽奖""></a></h3><details class="details custom-block"><summary>需求：实现幸运抽奖功能：输入4位的会员号，如果会员号的百位数字等于产生的随机数字，那么这个会员就是幸运会员，将奖励一部 iPhone 11 Pro！进行合理结果提示。</summary><p>提示：获取4位数，各个位的数字举例：</p></details><div class="language-java vp-adaptive-theme"><button title="Copy Code" class="copy"></button><span class="lang">java</span><pre class="shiki shiki-themes github-light github-dark-dimmed vp-code"><code><span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">int</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> num</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> =</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 1234</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">;</span></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 千位</span></span>
|
||
<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">int</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> qian</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> =</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> num </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">/</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 1000</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">;</span></span>
|
||
<span class="line"></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 百位</span></span>
|
||
<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">int</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> bai</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> =</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> num </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">/</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 100</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> %</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 10</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">;</span></span>
|
||
<span class="line"></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 十位</span></span>
|
||
<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">int</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> shi</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> =</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> num </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">/</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 10</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> %</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 10</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">;</span></span>
|
||
<span class="line"></span>
|
||
<span class="line"><span style="--shiki-light:#6A737D;--shiki-dark:#768390;">// 个位</span></span>
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<span class="line"><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">int</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> ge</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> =</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;"> num </span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;">%</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 10</span><span style="--shiki-light:#D73A49;--shiki-dark:#F47067;"> %</span><span style="--shiki-light:#005CC5;--shiki-dark:#6CB6FF;"> 10</span><span style="--shiki-light:#24292E;--shiki-dark:#ADBAC7;">;</span></span></code></pre></div><h2 id="后记" tabindex="-1">后记 <a class="header-anchor" href="#后记" aria-label="Permalink to "后记""></a></h2><p>选择结构的使用，一般不会困扰到大家。因为它与我们日常生活是非常贴近的，可读性非常强。当然如果还是不能理解，别忘了流程图哈！</p><div class="info custom-block"><p class="custom-block-title">笔者说</p><p>对于技术的学习，笔者一贯遵循的步骤是：先用最最简单的 demo 让它跑起来，然后学学它的最最常用 API 和 配置让自己能用起来，最后熟练使用的基础上，在空闲时尝试阅读它的源码让自己能够洞彻它的运行机制，部分问题出现的原因，同时借鉴这些技术实现来提升自己的代码高度。</p><p>所以在笔者的文章中，前期基本都是小白文，仅仅穿插很少量的源码研究。当然等小白文更新多了，你们还依然喜欢，后期会不定时专门对部分技术的源码进行解析。</p></div>`,24);function S(n,j,T,x,J,O){const e=d,r=p("ClientOnly"),a=p("Mermaid");return t(),g("div",null,[v,s(r,null,{default:y(()=>{var h,l;return[(((h=n.$frontmatter)==null?void 0:h.aside)??!0)&&(((l=n.$frontmatter)==null?void 0:l.showArticleMetadata)??!0)?(t(),C(e,{key:0,article:n.$frontmatter},null,8,["article"])):c("",!0)]}),_:1}),f,s(a,{id:"mermaid_382ee168",graph:"flowchart%20LR%0A%20%20%20%20A(%5B%E5%BC%80%E5%A7%8B%5D)%20--%3E%20B%7B%E6%9D%A1%E4%BB%B6%E8%A1%A8%E8%BE%BE%E5%BC%8F%7D%0A%20%20%20%20B%20--%3E%7C%E6%98%AF%7C%20C%5B%E8%AF%AD%E5%8F%A5%5D%0A%20%20%20%20C%20--%3E%20D%0A%20%20%20%20B%20--%3E%7C%E5%90%A6%7C%20D(%5B%E7%BB%93%E6%9D%9F%5D)"}),_,s(a,{id:"mermaid_382ee204",graph:"flowchart%20LR%0A%20%20%20%20A(%5B%E5%BC%80%E5%A7%8B%5D)%20--%3E%20B%7B%E6%9D%A1%E4%BB%B6%E8%A1%A8%E8%BE%BE%E5%BC%8F%7D%0A%20%20%20%20B%20--%3E%7C%E6%98%AF%7C%20C%5B%E8%AF%AD%E5%8F%A51%5D%0A%20%20%20%20C%20--%3E%20E%0A%20%20%20%20B%20--%3E%7C%E5%90%A6%7C%20D%5B%E8%AF%AD%E5%8F%A52%5D%0A%20%20%20%20D%20--%3E%20E(%5B%E7%BB%93%E6%9D%9F%5D)"}),b,s(a,{id:"mermaid_64a570de",graph:"flowchart%20LR%0A%20%20%20%20A(%5B%E5%BC%80%E5%A7%8B%5D)%20--%3E%20B%7B%E6%88%90%E7%BB%A9%20%3E%3D%2080%7D%0A%20%20%20%20B%20--%3E%7C%E6%98%AF%7C%20C%5B%E8%AF%AD%E5%8F%A51%5D%0A%20%20%20%20C%20--%3E%20G%0A%20%20%20%20B%20--%3E%7C%E5%90%A6%7C%20D%7B%E6%88%90%E7%BB%A9%20%3E%3D%2060%7D%0A%20%20%20%20D%20--%3E%7C%E6%98%AF%7C%20E%5B%E8%AF%AD%E5%8F%A52%5D%0A%20%20%20%20E%20--%3E%20G%0A%20%20%20%20D%20--%3E%7C%E5%90%A6%7C%20F%5B%E8%AF%AD%E5%8F%A53%5D%0A%20%20%20%20F%20--%3E%20G(%5B%E7%BB%93%E6%9D%9F%5D)"}),q,s(a,{id:"mermaid_64a57064",graph:"flowchart%20LR%0A%20%20%20%20A(%5B%E5%BC%80%E5%A7%8B%5D)%20--%3E%20B%7B%E6%9D%A1%E4%BB%B61%7D%0A%20%20%20%20B%20--%3E%7C%E6%98%AF%7C%20C%7B%E6%9D%A1%E4%BB%B62%7D%0A%20%20%20%20C%20--%3E%7C%E6%98%AF%7C%20D%5B%E8%AF%AD%E5%8F%A51%5D%0A%20%20%20%20D%20--%3E%20F%0A%20%20%20%20C%20--%3E%7C%E5%90%A6%7C%20E%5B%E8%AF%AD%E5%8F%A52%5D%0A%20%20%20%20E%20--%3E%7C%E5%90%A6%7C%20F%0A%20%20%20%20B%20--%3E%7C%E5%90%A6%7C%20F(%5B%E7%BB%93%E6%9D%9F%5D)"}),P])}const M=A(m,[["render",S]]);export{R as __pageData,M as default};
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