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<!DOCTYPE html>
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<a class="post-title-link" href="/2019/05/03/java编程思想-十-内部类/" itemprop="url">java编程思想(十): 内部类</a></h1>
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<span class="post-meta-item-text">发表于</span>
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2019-05-03
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<span id="/2019/05/03/java编程思想-十-内部类/" class="leancloud_visitors" data-flag-title="java编程思想(十): 内部类">
<span class="post-meta-divider">|</span>
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</span>
<span class="post-meta-item-text">阅读次数:</span>
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<i class="fa fa-file-word-o"></i>
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<span class="post-meta-item-text">字数统计:</span>
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115
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<i class="fa fa-clock-o"></i>
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<span class="post-meta-item-text">阅读时长 ≈</span>
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1
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<p>将一个类的定义放在另一个类的内部，这就是内部类。</p>
<p>内部类拥有外部类对象的引用。</p>
<p>在一个类中可以有多个内部类，以不同的方式实现同一个接口，或者继承同一个类。</p>
<p>有了内部类，一个类可以有任意多种方式实现任意多个接口，可以有任意多种方式继承任意多个类。</p>
<h2 id="成员内部类"><a href="#成员内部类" class="headerlink" title="成员内部类"></a>成员内部类</h2>
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<a class="post-title-link" href="/2019/05/03/java编程思想-七-复用类/" itemprop="url">java编程思想(七): 复用类</a></h1>
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2019-05-03
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<span id="/2019/05/03/java编程思想-七-复用类/" class="leancloud_visitors" data-flag-title="java编程思想(七): 复用类">
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<i class="fa fa-file-word-o"></i>
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<span class="post-meta-item-text">字数统计:</span>
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734
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<span class="post-meta-divider">|</span>
<span class="post-meta-item-icon">
<i class="fa fa-clock-o"></i>
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<span class="post-meta-item-text">阅读时长 ≈</span>
<span title="阅读时长">
3
</span>
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<h2 id="1-final-关键字"><a href="#1-final-关键字" class="headerlink" title="1. final 关键字"></a>1. final 关键字</h2><p>final关键字主要的含义就是“无法改变”。final可以用于数据、方法和类。</p>
<h3 id="1-数据"><a href="#1-数据" class="headerlink" title="1. 数据"></a>1. 数据</h3><p>final作用于数据上表示数据本身不可以改变。<br><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></pre></td><td class="code"><pre><span class="line"><span class="keyword">final</span> <span class="keyword">int</span> a = <span class="number">10</span>;</span><br><span class="line">a = <span class="number">11</span>; <span class="comment">// 错误 </span></span><br><span class="line"></span><br><span class="line"><span class="keyword">static</span> <span class="keyword">final</span> <span class="keyword">int</span> C = <span class="number">2</span>; <span class="comment">// 编译期常量</span></span><br><span class="line"><span class="keyword">final</span> List<String> list = <span class="keyword">new</span> ArrayList<String>();</span><br><span class="line">list.add(<span class="string">"a"</span>); <span class="comment">// 正确</span></span><br><span class="line">list.add(<span class="string">"b"</span>); <span class="comment">// 正确</span></span><br><span class="line">list = <span class="keyword">new</span> ArrayList<String>(); <span class="comment">// 错误，list不能指向其他对象</span></span><br></pre></td></tr></table></figure></p>
<p>如上，final作用于引用表示引用值本身不能变，即不能指向其他对象。</p>
<h4 id="final修饰参数"><a href="#final修饰参数" class="headerlink" title="final修饰参数"></a>final修饰参数</h4><p>在方法体的匿名内部类中，参数需要用final修饰。<br><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></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">interface</span> <span class="title">adder</span></span>{<span class="function"><span class="keyword">void</span> <span class="title">addXYZ</span><span class="params">()</span></span>;}</span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">outer</span></span>{</span><br><span class="line"> <span class="function"><span class="keyword">public</span> adder <span class="title">getAdder</span><span class="params">(<span class="keyword">final</span> <span class="keyword">int</span> x)</span></span>{</span><br><span class="line"> <span class="keyword">final</span> <span class="keyword">int</span> y = <span class="number">10</span>;</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">new</span> adder(){</span><br><span class="line"> <span class="keyword">int</span> z = <span class="number">10</span>;</span><br><span class="line"> <span class="meta">@Override</span></span><br><span class="line"> <span class="function"><span class="keyword">void</span> <span class="title">addXYZ</span><span class="params">()</span></span>{<span class="keyword">return</span> x + y + z;}</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure></p>
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<h1 id="访问权限控制"><a href="#访问权限控制" class="headerlink" title="访问权限控制"></a>访问权限控制</h1><p>访问权限控制主要是为了解决类库开发的问题，类库的代码往往需要重构，但是类库的使用者却需要代码保持不变，为此，对于类库中的代码，使用者的访问应该是受限制的，他应该只能访问部分需要公开的代码，比如所谓的API。</p>
<p>因此程序代码需要约定代码的访问权限。在java中，访问权限从大到小为:public, protected, 包访问权限(没有关键词), private. 其中包是库的基本单元。</p>
<h2 id="包-库单元"><a href="#包-库单元" class="headerlink" title="包:库单元"></a>包:库单元</h2><p>包是通过名字空间组织的一组类。</p>
<p>通过包可以很方便的把实现某一功能的类组织起来，在类文件(.java)中通过 <code>package package_name</code> 指定类所在的包，通过<code>import</code> 来导入。比如<code>import java.util.ArrayList</code> 导入<code>java.util</code>包下的<code>ArrayList</code>类。</p>
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<h1 id="接口"><a href="#接口" class="headerlink" title="接口"></a>接口</h1><p><strong>接口和内部类</strong>提供了很好的接口与实现分离的方法。</p>
<h2 id="完全解耦"><a href="#完全解耦" class="headerlink" title="完全解耦"></a>完全解耦</h2><p>在多态中，一个方法操作的是类，那么你可以使用这个类及其子类，代码会有很好的复用性。但是对于不在这个继承结构中的某个类，就没有办法了。</p>
<p>而接口却可以放宽这种限制。</p>
<h2 id="多重继承"><a href="#多重继承" class="headerlink" title="多重继承"></a>多重继承</h2><p>多重继承实际上就是组合多个类的接口。<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// B为抽象类或基类，C1,...为接口</span></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">A</span> <span class="keyword">extends</span> <span class="title">B</span> <span class="keyword">implements</span> <span class="title">C1</span>, <span class="title">C2</span>, <span class="title">C3</span> ...</span></span><br></pre></td></tr></table></figure></p>
<p>则如上A可以向上转型为B, C1, C2 …, 因此可以很方便的复用代码</p>
<h2 id="适配接口"><a href="#适配接口" class="headerlink" title="适配接口"></a>适配接口</h2><p>接口最大的用处就是一个接口可以有很多不同的实现。于是如果一个方法以一个接口类型为参数，则使用者可以通过改变接口的实现方式和传入的对象，来产生不同的行为。</p>
<p>比如Java中的Scanner类，它的构造器接收的是一个Readable接口。如果创建了一个新的类，想让Scanner作用于它，只需实现Readable接口。<br><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><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//: interfaces/RandomWords.java</span></span><br><span class="line"><span class="comment">// Implementing an interface to conform to a method.</span></span><br><span class="line"><span class="keyword">import</span> java.nio.*;</span><br><span class="line"><span class="keyword">import</span> java.util.*;</span><br><span class="line"></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">RandomWords</span> <span class="keyword">implements</span> <span class="title">Readable</span> </span>{</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> Random rand = <span class="keyword">new</span> Random(<span class="number">47</span>);</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">final</span> <span class="keyword">char</span>[] capitals =</span><br><span class="line"> <span class="string">"ABCDEFGHIJKLMNOPQRSTUVWXYZ"</span>.toCharArray();</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">final</span> <span class="keyword">char</span>[] lowers =</span><br><span class="line"> <span class="string">"abcdefghijklmnopqrstuvwxyz"</span>.toCharArray();</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">final</span> <span class="keyword">char</span>[] vowels =</span><br><span class="line"> <span class="string">"aeiou"</span>.toCharArray();</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">int</span> count;</span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="title">RandomWords</span><span class="params">(<span class="keyword">int</span> count)</span> </span>{ <span class="keyword">this</span>.count = count; } </span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">int</span> <span class="title">read</span><span class="params">(CharBuffer cb)</span> </span>{</span><br><span class="line"> <span class="keyword">if</span>(count-- == <span class="number">0</span>)</span><br><span class="line"> <span class="keyword">return</span> -<span class="number">1</span>; <span class="comment">// Indicates end of input</span></span><br><span class="line"> cb.append(capitals[rand.nextInt(capitals.length)]);</span><br><span class="line"> <span class="keyword">for</span>(<span class="keyword">int</span> i = <span class="number">0</span>; i < <span class="number">4</span>; i++) {</span><br><span class="line"> cb.append(vowels[rand.nextInt(vowels.length)]);</span><br><span class="line"> cb.append(lowers[rand.nextInt(lowers.length)]);</span><br><span class="line"> }</span><br><span class="line"> cb.append(<span class="string">" "</span>);</span><br><span class="line"> <span class="keyword">return</span> <span class="number">10</span>; <span class="comment">// Number of characters appended</span></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>{</span><br><span class="line"> Scanner s = <span class="keyword">new</span> Scanner(<span class="keyword">new</span> RandomWords(<span class="number">10</span>));</span><br><span class="line"> <span class="keyword">while</span>(s.hasNext())</span><br><span class="line"> System.out.println(s.next());</span><br><span class="line"> }</span><br><span class="line">} <span class="comment">/* Output:</span></span><br><span class="line"><span class="comment">Yazeruyac</span></span><br><span class="line"><span class="comment">Fowenucor</span></span><br><span class="line"><span class="comment">Goeazimom</span></span><br><span class="line"><span class="comment">Raeuuacio</span></span><br><span class="line"><span class="comment">Nuoadesiw</span></span><br><span class="line"><span class="comment">Hageaikux</span></span><br><span class="line"><span class="comment">Ruqicibui</span></span><br><span class="line"><span class="comment">Numasetih</span></span><br><span class="line"><span class="comment">Kuuuuozog</span></span><br><span class="line"><span class="comment">Waqizeyoy</span></span><br><span class="line"><span class="comment">*/</span><span class="comment">//:~</span></span><br></pre></td></tr></table></figure></p>
<p>如果某个类没有实现Readable接口，那应该怎么办呢？只需extends,并实现Readable接口即可。其实就是一个适配器，适配Readable接口。</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="comment">//: interfaces/RandomDoubles.java</span></span><br><span class="line"><span class="keyword">import</span> java.util.*;</span><br><span class="line"></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">RandomDoubles</span> </span>{</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> Random rand = <span class="keyword">new</span> Random(<span class="number">47</span>);</span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">double</span> <span class="title">next</span><span class="params">()</span> </span>{ <span class="keyword">return</span> rand.nextDouble(); }</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>{</span><br><span class="line"> RandomDoubles rd = <span class="keyword">new</span> RandomDoubles();</span><br><span class="line"> <span class="keyword">for</span>(<span class="keyword">int</span> i = <span class="number">0</span>; i < <span class="number">7</span>; i ++)</span><br><span class="line"> System.out.print(rd.next() + <span class="string">" "</span>);</span><br><span class="line"> }</span><br><span class="line">} <span class="comment">/* Output:</span></span><br><span class="line"><span class="comment">0.7271157860730044 0.5309454508634242 0.16020656493302599 0.18847866977771732 0.5166020801268457 0.2678662084200585 0.2613610344283964</span></span><br><span class="line"><span class="comment">*/</span><span class="comment">//:~</span></span><br></pre></td></tr></table></figure>