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id="post-meta"><div class="meta-firstline"><span class="post-meta-date"><i class="far fa-calendar-alt fa-fw post-meta-icon"></i><span class="post-meta-label">发表于</span><time class="post-meta-date-created" datetime="2021-02-10T06:55:37.000Z" title="发表于 2021-02-10 14:55:37">2021-02-10</time><span class="post-meta-separator">|</span><i class="fas fa-history fa-fw post-meta-icon"></i><span class="post-meta-label">更新于</span><time class="post-meta-date-updated" datetime="2021-08-19T03:14:06.918Z" title="更新于 2021-08-19 11:14:06">2021-08-19</time></span><span class="post-meta-categories"><span class="post-meta-separator">|</span><i class="fas fa-inbox fa-fw post-meta-icon"></i><a class="post-meta-categories" href="/categories/java/">java</a><i class="fas fa-angle-right post-meta-separator"></i><i class="fas fa-inbox fa-fw post-meta-icon"></i><a class="post-meta-categories" href="/categories/java/javaSE/">javaSE</a></span></div><div class="meta-secondline"><span class="post-meta-separator">|</span><span class="post-meta-wordcount"><i class="far fa-file-word fa-fw post-meta-icon"></i><span class="post-meta-label">字数总计:</span><span class="word-count">786</span><span class="post-meta-separator">|</span><i class="far fa-clock fa-fw post-meta-icon"></i><span class="post-meta-label">阅读时长:</span><span>3分钟</span></span><span class="post-meta-separator">|</span><span class="post-meta-pv-cv" id="" data-flag-title="4-变量"><i class="far fa-eye fa-fw post-meta-icon"></i><span class="post-meta-label">阅读量:</span><span id="busuanzi_value_page_pv"></span></span></div></div></div></header><main class="layout" id="content-inner"><div id="post"><article class="post-content" id="article-container"><h1 id="变量"><a href="#变量" class="headerlink" title="变量"></a>变量</h1><h2 id="命名规则"><a href="#命名规则" class="headerlink" title="命名规则"></a>命名规则</h2><ul>
<li>变量名只能使用 <code>字母</code>、<code>数字</code>、<code>$</code>、<code>_</code></li>
<li>变量第一个字符 只能使用 <code>字母 、$、 _</code></li>
<li>使用完整单词命名</li>
</ul>
<h2 id="类型转换"><a href="#类型转换" class="headerlink" title="类型转换"></a>类型转换</h2><ul>
<li><p>低精度向高精度转换可以正常转换</p>
</li>
<li><p>高精度向低精度转换有风险</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">HelloWorld</span> </span>&#123;</span><br><span class="line">  </span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>&#123;</span><br><span class="line">  </span><br><span class="line">        <span class="keyword">byte</span> b = <span class="number">5</span>;</span><br><span class="line">        <span class="keyword">int</span> i1 = <span class="number">10</span>;</span><br><span class="line">        <span class="keyword">int</span> i2 = <span class="number">300</span>;</span><br><span class="line">          </span><br><span class="line">        b = (<span class="keyword">byte</span>) i1;</span><br><span class="line">        <span class="comment">//因为i1的值是在byte范围之内，所以即便进行强制转换</span></span><br><span class="line">        <span class="comment">//最后得到的值，也是10</span></span><br><span class="line">        System.out.println(b);</span><br><span class="line">          </span><br><span class="line">        <span class="comment">//因为i2的值是在byte范围之外，所以就会按照byte的长度进行截取</span></span><br><span class="line">        <span class="comment">//i2的值是300，其对应的二进制数是 100101100</span></span><br><span class="line">        <span class="comment">//按照byte的长度8位进行截取后，其值为 00101100 即44</span></span><br><span class="line">        b =(<span class="keyword">byte</span>) i2;</span><br><span class="line">        System.out.println(b);</span><br><span class="line">         </span><br><span class="line">        <span class="comment">//查看一个整数对应的二进制的方法：</span></span><br><span class="line">        System.out.println(Integer.toBinaryString(i2));</span><br><span class="line">         </span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>注意：<code>short + short = int</code></p>
</li>
</ul>
<h2 id="变量分类"><a href="#变量分类" class="headerlink" title="变量分类"></a>变量分类</h2><ul>
<li><p>成员变量（字段、属性、Field）：</p>
<p>当一个变量被声明在类下面，变量就叫做<strong>成员变量</strong>，或<strong>字段</strong> 、<strong>属性</strong>、<strong>Field</strong></p>
<p>比如变量下面例子中，<code>i</code>就是一个属性。那么从第2行这个变量声明的位置开始，整个类都可以访问得到；所以其作用域就是从其声明的位置开始的整个类</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">HelloWorld</span> </span>&#123;</span><br><span class="line">    <span class="keyword">int</span> i = <span class="number">1</span>;</span><br><span class="line">    <span class="keyword">int</span> j = i;  <span class="comment">//其他的属性可以访问i</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">method1</span><span class="params">()</span></span>&#123;</span><br><span class="line">        System.out.println(i); <span class="comment">//方法1里可以访问i</span></span><br><span class="line">    &#125;</span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">method2</span><span class="params">()</span></span>&#123;</span><br><span class="line">        System.out.println(i); <span class="comment">//方法2里可以访问i</span></span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
</li>
<li><p>参数</p>
<p>如果一个变量，是声明在一个方法上的，就叫做<strong>参数</strong>，参数的作用域即为<strong>该方法</strong>内的所有代码，在该方法外面不能使用该参数</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">HelloWorld</span> </span>&#123;</span><br><span class="line"> </span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">method1</span><span class="params">(<span class="keyword">int</span> i)</span></span>&#123; <span class="comment">//参数i的作用域即方法method1</span></span><br><span class="line">        System.out.println(i);</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">method2</span><span class="params">()</span></span>&#123;</span><br><span class="line"> </span><br><span class="line">        System.out.println(i); <span class="comment">//method2 不能访问参数i</span></span><br><span class="line">    &#125;</span><br><span class="line">     </span><br><span class="line">    <span class="keyword">int</span> j = i;  <span class="comment">//类里面也不能访问参数i</span></span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
</li>
<li><p>局部变量</p>
<p>声明在方法内的变量，叫做局部变量，其作用域在声明开始的位置，到其所处于的块结束位置。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">HelloWorld</span> </span>&#123;</span><br><span class="line"> </span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">method1</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="keyword">int</span> i  = <span class="number">5</span>;  <span class="comment">//其作用范围是从声明的第4行，到其所处于的块结束12行位置</span></span><br><span class="line">        System.out.println(i);</span><br><span class="line">        &#123;            <span class="comment">//子块</span></span><br><span class="line">            System.out.println(i); <span class="comment">//可以访问i</span></span><br><span class="line">            <span class="keyword">int</span> j = <span class="number">6</span>;</span><br><span class="line">            System.out.println(j); <span class="comment">//可以访问j</span></span><br><span class="line">        &#125;</span><br><span class="line">        System.out.println(j); <span class="comment">//不能访问j,因为其作用域到第10行就结束了</span></span><br><span class="line">    &#125;</span><br><span class="line"> </span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
</li>
</ul>
<ul>
<li><p>静态变量</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">static</span> <span class="keyword">int</span> age;	<span class="comment">//(类变量)</span></span><br></pre></td></tr></table></figure>
</li>
</ul>
<h2 id="final变量"><a href="#final变量" class="headerlink" title="final变量"></a>final变量</h2><p>用final修饰的变量，只能赋值1次，因此常<strong>用于定义常量</strong></p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">HelloWorld</span> </span>&#123;</span><br><span class="line"> </span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">method1</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="keyword">final</span> <span class="keyword">int</span> i = <span class="number">5</span>;</span><br><span class="line">         </span><br><span class="line">        i = <span class="number">10</span>; <span class="comment">//i在第4行已经被赋值过了，所以这里会出现编译错误</span></span><br><span class="line">         </span><br><span class="line">    &#125;</span><br><span class="line"> </span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>但是对于引用类型的数据，变量存储的是地址，地址不可变，但地址里存放的值可以改变</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">HelloWorld</span> </span>&#123;</span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>&#123;</span><br><span class="line">        <span class="keyword">final</span> <span class="keyword">int</span> []a = &#123;<span class="number">1</span>,<span class="number">2</span>,<span class="number">3</span>&#125;;</span><br><span class="line">        a[<span class="number">1</span>] = <span class="number">10</span>;</span><br><span class="line">        a[<span class="number">2</span>] = <span class="number">100</span>;</span><br><span class="line">        </span><br><span class="line">        System.out.println(a[<span class="number">1</span>]);</span><br><span class="line">        System.out.println(a[<span class="number">2</span>]);</span><br><span class="line">        </span><br><span class="line">        <span class="comment">// 注意这样写：a = &#123;4,5,6&#125;; 是错误的</span></span><br><span class="line"></span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>输出如下<img src="https://gitee.com/Dream-and-dreaming/images/raw/master/img/image-20210122203958650.png" alt="image-20210122203958650"></p>
</article><div class="post-copyright"><div class="post-copyright__title"><span class="post-copyright-info"><h>4-变量</h></span></div><div class="post-copyright__type"><span class="post-copyright-info"><a href="http://ajream.github.io/posts/68f2c599">http://ajream.github.io/posts/68f2c599</a></span></div><div class="post-copyright-m"><div class="post-copyright-m-info"><div class="post-copyright-a" style="display: inline-block;width: 120px"><h>作者</h><div class="post-copyright-cc-info"><h>aJream</h></div></div><div class="post-copyright-c" style="display: inline-block;width: 120px"><h>发布于</h><div class="post-copyright-cc-info"><h>2021-02-10</h></div></div><div class="post-copyright-u" style="display: inline-block;width: 120px"><h>更新于</h><div class="post-copyright-cc-info"><h>2021-08-19</h></div></div><div class="post-copyright-c" style="display: inline-block;width: 120px"><h>许可协议</h><div class="post-copyright-cc-info"><a class="icon" rel="noopener" target="_blank" title="Creative Commons" 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