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<h1 class="kratos-entry-title text-center">面向对象</h1>
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<i class="fa fa-calendar"></i> 2021-02-03
<i class="fa fa-folder"></i> 分类于 <a class="label-link" href="/categories/Java%E5%AD%A6%E4%B9%A0/">Java学习</a>
<i class="fa fa-user"></i> 作者 yongkj
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<ol class="toc"><li class="toc-item toc-level-3"><a class="toc-link" href="#%E9%9D%A2%E5%90%91%E5%AF%B9%E8%B1%A1%E7%9A%84%E7%89%B9%E5%BE%81"><span class="toc-number">1.</span> <span class="toc-text">面向对象的特征</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#%E6%8A%BD%E8%B1%A1%E7%B1%BB%E4%B8%8E%E6%8E%A5%E5%8F%A3"><span class="toc-number">2.</span> <span class="toc-text">抽象类与接口</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#%E5%86%85%E9%83%A8%E7%B1%BB"><span class="toc-number">3.</span> <span class="toc-text">内部类</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#%E9%87%8D%E8%BD%BD%E4%B8%8E%E9%87%8D%E5%86%99"><span class="toc-number">4.</span> <span class="toc-text">重载与重写</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#hashCode-%E5%92%8Cequals-%E6%96%B9%E6%B3%95%E7%9A%84%E8%81%94%E7%B3%BB"><span class="toc-number">5.</span> <span class="toc-text">hashCode()和equals()方法的联系</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#final-finally-finalize%E7%9A%84%E5%8C%BA%E5%88%AB"><span class="toc-number">6.</span> <span class="toc-text">final, finally, finalize的区别</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#Comparable%E5%92%8CComparator%E6%8E%A5%E5%8F%A3%E7%9A%84%E4%BD%9C%E7%94%A8%E4%BB%A5%E5%8F%8A%E5%8C%BA%E5%88%AB"><span class="toc-number">7.</span> <span class="toc-text">Comparable和Comparator接口的作用以及区别</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#Java%E6%98%AF%E5%90%A6%E6%94%AF%E6%8C%81%E5%A4%9A%E7%BB%A7%E6%89%BF"><span class="toc-number">8.</span> <span class="toc-text">Java是否支持多继承</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#extends-%E5%92%8Csuper-%E6%B3%9B%E5%9E%8B%E9%99%90%E5%AE%9A%E7%AC%A6"><span class="toc-number">9.</span> <span class="toc-text">extends 和super 泛型限定符</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#%E4%BB%80%E4%B9%88%E6%98%AF%E6%B3%9B%E5%9E%8B"><span class="toc-number">10.</span> <span class="toc-text">什么是泛型</span></a></li></ol>
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<hr />
<h3 id="面向对象的特征"><a href="#面向对象的特征" class="headerlink" title="面向对象的特征"></a>面向对象的特征</h3><ol>
<li>java面向对象的三大特征是什么?<br>封装、继承、多态。</li>
<li>封装<br>把属性和方法隐藏起来，只保留一些对外的接口和外部进行交互。</li>
<li>继承<br>子类继承父类的特征和行为，使得子类具有父类的非private属性和方法。</li>
<li>多态<br>多态就是同一个接口，使用不同的实现，而执行不同的操作。</li>
<li>抽象<br>将一类对象的共同特征总结出来构造类的过程，数据抽象指的是属性，行为抽象指的是方法。</li>
</ol>
<h3 id="抽象类与接口"><a href="#抽象类与接口" class="headerlink" title="抽象类与接口"></a>抽象类与接口</h3><ol>
<li>抽象类</li>
</ol>
<p>​ 1）抽象类是用来捕捉子类的通用特性的</p>
<p>​ 2）它不能被实例化，只能被用作子类的超类</p>
<p>​ 3）抽象类是被用来创建继承层级里子类的模板</p>
<ol start="2">
<li>接口</li>
</ol>
<p>​ 1）接口是抽象方法的集合</p>
<p>​ 2）如果一个类实现了某个接口，那么它就继承了这个接口的抽象方法</p>
<p>​ 3）如果实现了这个接口，那么就必须确保使用这些方法</p>
<p>​ 4）接口只是一种形式，接口自身不能做任何事情</p>
<ol start="3">
<li>抽象类和接口的对比</li>
</ol>
<table>
<thead>
<tr>
<th align="left">参数</th>
<th align="left">抽象类</th>
<th>接口</th>
</tr>
</thead>
<tbody><tr>
<td align="left">默认的方法实现</td>
<td align="left">它可以有默认的方法实现</td>
<td>接口完全是抽象的。它根本不存在方法的实现</td>
</tr>
<tr>
<td align="left">实现</td>
<td align="left">子类使用<strong>extends</strong>关键字来继承抽象类。如果子类不是抽象类的话，它需要提供抽象类中所有声明的方法的实现。</td>
<td>子类使用关键字<strong>implements</strong>来实现接口。它需要提供接口中所有声明的方法的实现</td>
</tr>
<tr>
<td align="left">构造器</td>
<td align="left">抽象类可以有构造器</td>
<td>接口不能有构造器</td>
</tr>
<tr>
<td align="left">与正常Java类的区别</td>
<td align="left">除了你不能实例化抽象类之外，它和普通Java类没有任何区别</td>
<td>接口是完全不同的类型</td>
</tr>
<tr>
<td align="left">访问修饰符</td>
<td align="left">抽象方法可以有<strong>public</strong>、<strong>protected</strong>和<strong>default</strong>这些修饰符</td>
<td>接口方法默认修饰符是<strong>public</strong>。你不可以使用其它修饰符。</td>
</tr>
<tr>
<td align="left">main方法</td>
<td align="left">抽象方法可以有main方法并且我们<strong>可以运行</strong>它</td>
<td>接口没有main方法，因此我们<strong>不能运行它</strong>。</td>
</tr>
<tr>
<td align="left">多继承</td>
<td align="left">抽象方法可以继承一个类和实现多个接口</td>
<td>接口只可以继承一个或多个其它接口</td>
</tr>
<tr>
<td align="left">速度</td>
<td align="left">它比接口速度要快</td>
<td>接口是稍微有点慢的，因为它需要时间去寻找在类中实现的方法。</td>
</tr>
<tr>
<td align="left">添加新方法</td>
<td align="left">如果你往抽象类中添加新的方法，你可以给它提供默认的实现。因此你不需要改变你现在的代码。</td>
<td>如果你往接口中添加方法，那么你必须改变实现该接口的类。</td>
</tr>
</tbody></table>
<h3 id="内部类"><a href="#内部类" class="headerlink" title="内部类"></a>内部类</h3><ol>
<li>概述</li>
</ol>
<p>​ 1）把类定义在另一个类的内部，该类就被称为内部类</p>
<p>​ 2）把类Inner定义在类Outer中，类Inner就被称为内部类</p>
<ol start="2">
<li>访问规则</li>
</ol>
<p>​ 1）可以直接访问外部类的成员，包括私有</p>
<p>​ 2）外部类要想访问内部类成员，必须创建对象</p>
<ol start="3">
<li>分类</li>
</ol>
<p>​ 1）<strong>成员内部类</strong>：位于外部类成员位置的类，可以使用外部类中所有的成员变量和成员方法（包括private的）</p>
<p>​ 2）<strong>局部内部类</strong>：定义在一个方法或者一个作用域里面的类，主要是作用域发生了变化，只能在自身所在方法和属性中被使用</p>
<p>​ 3）<strong>静态内部类</strong>：用static修饰的内部类，不能使用外部类的非static成员变量和成员方法</p>
<p>​ 4）<strong>匿名内部类</strong>：一个没有名字的类，是内部类的简化写法，其实是继承该类或者实现接口的子类匿名对象</p>
<h3 id="重载与重写"><a href="#重载与重写" class="headerlink" title="重载与重写"></a>重载与重写</h3><ol>
<li>方法重载</li>
</ol>
<p>​ 1）同一个类中的多个方法具有相同的名字，这些方法具有不同的参数列表</p>
<p>​ 2）参数类型和个数不一样，返回值类型可以相同也可以不相同</p>
<p>​ 3）无法以返回型别作为重载函数的区分标准</p>
<p>​ 4）重载Overloading是一个类中多态性的一种表现。</p>
<ol start="2">
<li>方法重写</li>
</ol>
<p>​ 1）子类定义的方法与父类中的方法具有相同的方法名字，相同的参数表和相同的返回类型</p>
<p>​ 2）子类中不能重写父类中的final方法</p>
<p>​ 3）子类中必须重写父类中的abstract方法</p>
<h3 id="hashCode-和equals-方法的联系"><a href="#hashCode-和equals-方法的联系" class="headerlink" title="hashCode()和equals()方法的联系"></a>hashCode()和equals()方法的联系</h3><ol>
<li><p>相等（相同）的对象必须具有相等的哈希码（或者散列码）。</p>
</li>
<li><p>如果两个对象的hashCode相同，它们并不一定相同。</p>
</li>
</ol>
<h3 id="final-finally-finalize的区别"><a href="#final-finally-finalize的区别" class="headerlink" title="final, finally, finalize的区别"></a>final, finally, finalize的区别</h3><ol>
<li>final 用于声明属性，方法和类，分别表示属性不可变，方法不可覆盖，类不可继承。</li>
<li>finally是异常处理语句结构的一部分，表示总是执行。</li>
<li>finalize是Object类的一个方法，在垃圾收集器执行的时候会调用被回收对象的此方法，可以覆盖此方法提供垃圾收集时的其他资源回收，例如关闭文件等。</li>
</ol>
<h3 id="Comparable和Comparator接口的作用以及区别"><a href="#Comparable和Comparator接口的作用以及区别" class="headerlink" title="Comparable和Comparator接口的作用以及区别"></a>Comparable和Comparator接口的作用以及区别</h3><ol>
<li>Comparable接口包含compareTo()方法。这个方法可以个给两个对象排序。</li>
<li>Comparator接口包含compare()和equals()方法。</li>
<li>compare()方法用来给两个输入参数排序，返回负数，0，正数表明第一个参数是小于，等于，大于第二个参数。</li>
<li>equals()方法需要一个对象作为参数，它用来决定输入参数是否和comparator相等。</li>
<li>只有当输入参数也是一个comparator并且输入参数和当前comparator的排序结果是相同的时候，这个方法才返回true。</li>
</ol>
<h3 id="Java是否支持多继承"><a href="#Java是否支持多继承" class="headerlink" title="Java是否支持多继承"></a>Java是否支持多继承</h3><ol>
<li>Java中类不支持多继承，只支持单继承（即一个类只有一个父类）。 </li>
<li>但是java中的接口支持多继承，，即一个子接口可以有多个父接口。</li>
<li>接口的作用是用来扩展对象的功能，一个子接口继承多个父接口，说明子接口扩展了多个功能。</li>
<li>当类实现接口时，类就扩展了相应的功能。</li>
</ol>
<h3 id="extends-和super-泛型限定符"><a href="#extends-和super-泛型限定符" class="headerlink" title="extends 和super 泛型限定符"></a>extends 和super 泛型限定符</h3><ol>
<li>泛型中上界和下界的定义</li>
</ol>
<p>​ 1）上界&lt;? extend Fruit&gt;</p>
<p>​ 2）下界&lt;? super Apple&gt;</p>
<ol start="2">
<li>上界和下界的特点</li>
</ol>
<p>​ 1）上界的list只能get，不能add（确切地说不能add出除null之外的对象，包括Object）</p>
<p>​ 3）下界的list只能add，不能get</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><span class="line">25</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> java.util.ArrayList;</span><br><span class="line"><span class="keyword">import</span> java.util.List;</span><br><span class="line"> </span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Fruit</span> </span>&#123;&#125;</span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Apple</span> <span class="keyword">extends</span> <span class="title">Fruit</span> </span>&#123;&#125;</span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Jonathan</span> <span class="keyword">extends</span> <span class="title">Apple</span> </span>&#123;&#125;</span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Orange</span> <span class="keyword">extends</span> <span class="title">Fruit</span> </span>&#123;&#125;</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">CovariantArrays</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="comment">//上界</span></span><br><span class="line"> List&lt;? extends Fruit&gt; flistTop = <span class="keyword">new</span> ArrayList&lt;Apple&gt;();</span><br><span class="line"> flistTop.add(<span class="keyword">null</span>);</span><br><span class="line"> <span class="comment">//add Fruit对象会报错</span></span><br><span class="line"> <span class="comment">//flist.add(new Fruit());</span></span><br><span class="line"> Fruit fruit1 = flistTop.get(<span class="number">0</span>);</span><br><span class="line"> </span><br><span class="line"> <span class="comment">//下界</span></span><br><span class="line"> List&lt;? <span class="keyword">super</span> Apple&gt; flistBottem = <span class="keyword">new</span> ArrayList&lt;Apple&gt;();</span><br><span class="line"> flistBottem.add(<span class="keyword">new</span> Apple());</span><br><span class="line"> flistBottem.add(<span class="keyword">new</span> Jonathan());</span><br><span class="line"> <span class="comment">//get Apple对象会报错</span></span><br><span class="line"> <span class="comment">//Apple apple = flistBottem.get(0);</span></span><br><span class="line"> &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<ol start="3">
<li>上界&lt;? extend Fruit&gt; ，表示所有继承Fruit的子类，但是具体是哪个子类，无法确定，所以调用add的时候，要add什么类型，谁也不知道。但是get的时候，不管是什么子类，不管追溯多少辈，肯定有个父类是Fruit，所以，我都可以用最大的父类Fruit接着，也就是把所有的子类向上转型为Fruit。</li>
<li>下界&lt;? super Apple&gt;，表示Apple的所有父类，包括Fruit，一直可以追溯到老祖宗Object 。那么当我add的时候，我不能add Apple的父类，因为不能确定List里面存放的到底是哪个父类。但是我可以add Apple及其子类。因为不管我的子类是什么类型，它都可以向上转型为Apple及其所有的父类甚至转型为Object 。但是当我get的时候，Apple的父类这么多，我用什么接着呢，除了Object，其他的都接不住。</li>
</ol>
<h3 id="什么是泛型"><a href="#什么是泛型" class="headerlink" title="什么是泛型"></a>什么是泛型</h3><ol>
<li>泛型，即“参数化类型”。一提到参数，最熟悉的就是定义方法时有形参，然后调用此方法时传递实参。那么参数化类型怎么理解呢？</li>
<li>顾名思义，就是将类型由原来的具体的类型参数化，类似于方法中的变量参数，此时类型也定义成参数形式（可以称之为类型形参），然后在使用/调用时传入具体的类型（类型实参）。</li>
</ol>
<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></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">GenericTest</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 class="comment">/*</span></span><br><span class="line"><span class="comment"> List list = new ArrayList();</span></span><br><span class="line"><span class="comment"> list.add(&quot;qqyumidi&quot;);</span></span><br><span class="line"><span class="comment"> list.add(&quot;corn&quot;);</span></span><br><span class="line"><span class="comment"> list.add(100);</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"></span><br><span class="line"> List&lt;String&gt; list = <span class="keyword">new</span> ArrayList&lt;String&gt;();</span><br><span class="line"> list.add(<span class="string">&quot;qqyumidi&quot;</span>);</span><br><span class="line"> list.add(<span class="string">&quot;corn&quot;</span>);</span><br><span class="line"> <span class="comment">//list.add(100); // 1 提示编译错误</span></span><br><span class="line"></span><br><span class="line"> <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i &lt; list.size(); i++) &#123;</span><br><span class="line"> String name = list.get(i); <span class="comment">// 2</span></span><br><span class="line"> System.out.println(<span class="string">&quot;name:&quot;</span> + name);</span><br><span class="line"> &#125;</span><br><span class="line"> &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<ol start="3">
<li>采用泛型写法后，在//1处想加入一个Integer类型的对象时会出现编译错误，通过List<String>，直接限定了list集合中只能含有String类型的元素，从而在//2处无须进行强制类型转换，因为此时，集合能够记住元素的类型信息，编译器已经能够确认它是String类型了。</li>
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