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继承性
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<p>面向对象特征二:继承性<br>
方法的<font style='color:red;font-weight:bold;'>重写</font><br>
super关键字<br>
子类对象的实例化过程</p>
<a id="more"></a>
<h2 id="一-继承性">一.继承性</h2>
<p>1.为什么要有继承性:<br>
有一些东西拥有某些公共的特性, 将他们相同的部分抽取出来放在一起, <font style='color:red;font-weight:bold;'>方便代码的复用与修改</font><br>
2.通过"<font style='color:red;font-weight:bold;'>class A extends B</font>"类实现类的继承:<br>
子类: A; 父类(或基类SuperClass): B<br>
3.子类继承父类以后, 父类中声明的属性、方法, 子类就可以获取到.<br>
明确: 当父类中有私有的属性或方法时, 子类同样可以获取得到, 只是由于封装性的设计, 使得<font style='color:red;font-weight:bold;'>子类不可以直接<br>
调用</font>罢了.<br>
子类除了通过继承, 获取父类的结构之外, 还可以定义自己的特有的成分.<br>
extends:子类是对父类功能的“扩展”, 明确子类不是父类的子集.<br>
4.Java中类的继承性<font style='color:red;font-weight:bold;'>只支持单继承</font>: 一个类只能继承一个父类. 反之, 一个父类可以有多个子类.<br>
5.子父类是相对的概念.</p>
<h2 id="二-orverwrite-or-override">二.orverwrite or override</h2>
<h3 id="font-style-color-red-font-weight-bold-方法的重载与重写的区别-font"><font style='color:red;font-weight:bold;'>方法的重载与重写的区别?</font></h3>
<p><strong>重载</strong>: “两同一不同”: 同一个类, 同一个方法名, 不同的参数列表. 注: 方法的重载与方法的返回值无关!<br>
构造器也是可以重载的<br>
<strong>重写</strong>: (前提: 在继承的基础之上, 子类在获取了父类的结构以后, 可以对父类中同名的方法进行“重构”)</p>
<ul>
<li>方法的返回值, 方法名, 形参列表形同;</li>
<li><font style='color:red;font-weight:bold;'>权限修饰符不小于父类的同名方法;</font></li>
<li><font style='color:red;font-weight:bold;'>子类方法的异常类型不大于父类的;</font></li>
<li>两个方法要同为static或同为非static.</li>
</ul>
<h2 id="三-super">三.super</h2>
<p>1.super: 相较于关键字this, 可以修饰属性、方法、构造器<br>
2.super修饰属性、方法: 在子类的方法、构造器中, 通过<font style='color:red;font-weight:bold;'>"super.属性"或者"super.方法"</font>的形式, 显式的调用父类的指定属性或方法. 尤其是, 当子类与父类有同名的属性、或方法时, 调用父类中的结构的话, 一定要用“super.”<br>
3.通过“super(形参列表)”, 显式的在子类的构造器中, 调用父类指定的构造器!<br>
<strong>任何一个类(除Object类)的构造器的首行, 要么显式的调用本类中重载的其它的构造器“this(形参列表)”或显式的调用父类中指定的构造器“super(形参列表)”, 要么默认的调用父类空参的构造器"super()"</strong><br>
<font style='color:red;font-weight:bold;'>"super(形参列表)"或"this(形参列表)"只能出现一个</font><br>
建议在设计类时, <font style='color:red;font-weight:bold;'>提供一个空参的构造器!</font></p>
<h2 id="四-子类对象的实例化">四.子类对象的实例化</h2>
<p><font style='color:red;font-weight:bold;'>先构建(调用父类的构造器构建)父类的内容, 再构建子类的内容</font></p>
<p>为了直观的理解, 可以运行一下代码, 看输出的结果:</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><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><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//子类对象实例化的全过程</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">TestDog</span> </span>{</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"> Dog d = <span class="keyword">new</span> Dog();</span><br><span class="line"> d.setAge(<span class="number">10</span>);</span><br><span class="line"> d.setName(<span class="string">"小明"</span>);</span><br><span class="line"> d.setHostName(<span class="string">"花花"</span>);</span><br><span class="line"> System.out.println(<span class="string">"name:"</span> + d.getName() + <span class="string">" age:"</span> + d.getAge()</span><br><span class="line"> + <span class="string">" hostName:"</span> + d.getHostName()); </span><br><span class="line"> System.out.println(d.toString());</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"><span class="comment">// 生物</span></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Creator</span> </span>{</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">int</span> age;</span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">int</span> <span class="title">getAge</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">return</span> age;</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">setAge</span><span class="params">(<span class="keyword">int</span> age)</span> </span>{</span><br><span class="line"> <span class="keyword">this</span>.age = age;</span><br><span class="line"> }</span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="title">Creator</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">super</span>();</span><br><span class="line"> System.out.println(<span class="string">"this is Creator's constructor"</span>);</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"><span class="comment">// 动物类</span></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Animal</span> <span class="keyword">extends</span> <span class="title">Creator</span> </span>{</span><br><span class="line"> <span class="keyword">private</span> String name;</span><br><span class="line"> <span class="function"><span class="keyword">public</span> String <span class="title">getName</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">return</span> name;</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">setName</span><span class="params">(String name)</span> </span>{</span><br><span class="line"> <span class="keyword">this</span>.name = name;</span><br><span class="line"> }</span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="title">Animal</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">super</span>();</span><br><span class="line"> System.out.println(<span class="string">"this is Animal's constructor"</span>);</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"><span class="comment">// 狗</span></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Dog</span> <span class="keyword">extends</span> <span class="title">Animal</span> </span>{</span><br><span class="line"> <span class="keyword">private</span> String hostName;</span><br><span class="line"> <span class="function"><span class="keyword">public</span> String <span class="title">getHostName</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">return</span> hostName;</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">setHostName</span><span class="params">(String hostName)</span> </span>{</span><br><span class="line"> <span class="keyword">this</span>.hostName = hostName;</span><br><span class="line"> }</span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="title">Dog</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">super</span>();</span><br><span class="line"> System.out.println(<span class="string">"this is Dog's constructor"</span>);</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>输出结果为:</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></pre></td><td class="code"><pre><span class="line"><span class="keyword">this</span> is Creator<span class="string">'s constructor</span></span><br><span class="line"><span class="string">this is Animal'</span>s constructor</span><br><span class="line"><span class="keyword">this</span> is Dog<span class="string">'s constructor</span></span><br><span class="line"><span class="string">name:小明 age:10 hostName:花花</span></span><br><span class="line"><span class="string">Dog类的对象dd在堆空间内的信息</span></span><br></pre></td></tr></table></figure>
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