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<!DOCTYPE HTML>
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<title>3.6 字符串 · GitBook</title>
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<li class="chapter " data-level="1.1" data-path="./">
<a href="./">
Introduction
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<li class="chapter " data-level="1.2" data-path="Char1.html">
<a href="Char1.html">
第一章：Java简介
</a>
</li>
<li class="chapter " data-level="1.3" data-path="Char3.html">
<a href="Char3.html">
第三章：Java的基础编程结构
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<ul class="articles">
<li class="chapter " data-level="1.3.1" data-path="Char3.1.html">
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3.1 一个简单的Java程序
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<li class="chapter " data-level="1.3.2" data-path="Char3.2.html">
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3.2 注释
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<li class="chapter " data-level="1.3.3" data-path="Char3.3.html">
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3.3 资料的类型
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<li class="chapter " data-level="1.3.4" data-path="Char3.4.html">
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3.4 变量和常量
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<li class="chapter " data-level="1.3.5" data-path="Char3.5.html">
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3.5 运算子
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3.6 字符串
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3.7 输入和输出
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<li class="chapter " data-level="1.3.8" data-path="Char3.8.html">
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3.8 控制流程
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<li class="chapter " data-level="1.3.9" data-path="Char3.9.html">
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3.9 Big Number
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<li class="chapter " data-level="1.3.10" data-path="Char3.10.html">
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3.10 Array
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第四章：对象和类
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4.1 介绍面向对象编程
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4.2 使用之前已经定义好的类
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<li class="chapter " data-level="1.4.3" data-path="Char4.3.html">
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4.3 定义你自己的类
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4.4 静态域和方法
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4.5 方法的参数
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4.6 对象的建构
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4.7 包
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4.8 JAR文件
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4.9 文件的注释
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4.10 类设计的巧思
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Published with GitBook
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<!-- Title -->
<h1>
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<a href="." >3.6 字符串</a>
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<h1 id="36-字串strings">3.6 字串Strings</h1>
<p>Java没有内建的string类型，而是在Java的标准库中加入了一个事先定义的类String，每一个在程序中出现的字串，都是这个String类的实例。例如：</p>
<pre><code class="lang-Java">String e =<span class="hljs-string">""</span>; <span class="hljs-comment">// an empty string</span>
String greeting = <span class="hljs-string">"Hello"</span>;
</code></pre>
<h2 id="361-substrings">3.61 Substrings:</h2>
<p>通过<code>substring</code>这个方法，你可以从一个String中抽取出一个子字串。例如：</p>
<pre><code class="lang-Java">String e = <span class="hljs-string">"Hello"</span>;
String s = s.substring(<span class="hljs-number">0</span>,<span class="hljs-number">3</span>) <span class="hljs-comment">// s = "Hel"</span>
</code></pre>
<ol>
<li>这里第一个参数表示开始的地方，第二个参数减第一个参数为字符串的长度。 </li>
</ol>
<h2 id="362-字符串的连接concatenation：">3.62 字符串的连接Concatenation：</h2>
<ol>
<li>Java可以用<code>+</code>来连接两个字符串，产生一个新的字符串。</li>
<li>当你将一个字符串和一个不是字符串的变量结合在一起的时候，整个输出的结果会变成字符串。例如：<pre><code class="lang-Java"><span class="hljs-keyword">int</span> age = <span class="hljs-number">13</span>;
String tomAge = <span class="hljs-string">"Tom"</span>+age <span class="hljs-comment">//tomAge = "Tom13"</span>
</code></pre>
在java中，你想将多个字符连接在一起，并且中间用符号区别开，你会用到join方法：<pre><code class="lang-Java">String all = String.join(<span class="hljs-string">"/"</span>,<span class="hljs-string">"S"</span>,<span class="hljs-string">"M"</span>,<span class="hljs-string">"L"</span>,<span class="hljs-string">"XL"</span>);
<span class="hljs-comment">// all is the String "S/M/L/XL"</span>
</code></pre>
</li>
</ol>
<h2 id="363-字串的不可变性（！！重要）string-are-immutable">3.63 字串的不可变性（！！重要）String Are Immutable</h2>
<p>对于已经生成的字串，String类没有提供方法来让你改变字符串内容，如果你要改变某个字符串变量a的值，你可以有几种不同的方法：</p>
<ol>
<li>通过concatenation和substring两个来进行删减和增加达到change的目的。</li>
<li>直接将a指向别的字符串，等于直接更改a指向的地址。</li>
</ol>
<h2 id="365-空字串和null">3.65 空字串和Null</h2>
<p>空字串<code>""</code>的长度为0，空字串是一个有着字符串长度（0），并且没有内容的Java对象。String对象还可以有一个特殊的值，叫做<code>null</code>，这表示当前没有任何一个对象指向现在的变数。在Java中，在你测试是不是空字串的时候，你需要以下的程式码：</p>
<pre><code class="lang-Java"><span class="hljs-keyword">if</span> (str != <span class="hljs-keyword">null</span> && str.length() != <span class="hljs-number">0</span>)
</code></pre>
<ol>
<li>要注意，这个顺序不可以换，必须先测试这个string是不是null的，因为null.lenghth()会报错。</li>
</ol>
<h2 id="369-stringbuilder">3.69 StringBuilder</h2>
<p>除了concatenation这个方法，Java还有append方法来合并两个字串。
在将多个string结合在一起的时候，如果使用concatenation这个方法，每一次相加就会产生一个新的String，非常的麻烦。为此，Java用stringbuilder来解决这个问题，利用<code>builder.appened()</code>这个方法，这样不会每次拼接都产生新的string，而是会保存在一个缓存区里面。</p>
<pre><code class="lang-Java">String str = builder.toString();
</code></pre>
<ol>
<li>当你确定已经结束拼接之后，可以将其变回string：</li>
</ol>
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