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面向对象3.1
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<p>发布于：2019年3月20日</p>
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<p>static关键字<br>
main()方法<br>
代码块<br>
final</p>
<a id="more"></a>
<h2 id="一-static">一.static</h2>
<p>static: 静态的, 可以用来修饰<font style='color:red;font-weight:bold;'>属性、方法、*代码块(或初始化块)、*内部类</font></p>
<h3 id="A-static修饰属性-类变量">A.static修饰属性(类变量):</h3>
<ol>
<li>由类创建的所有的对象，都共用这一个属性</li>
<li>当其中一个对象对此属性进行修改, 会导致其他对象对此属性调用时也会更改. vs 实例变量(非static修饰的属性, 各个对象各自拥有一套副本)</li>
<li>类变量随着类的加载而加载的, 而且独一份</li>
<li>静态的变量可以直接通过“类.类变量”的形式来调用</li>
<li>类变量的加载是要早于对象. 所以当有对象以后, 可以“对象.类变量”使用. 但是"类.实例变量"是不行的.</li>
<li>类变量存在于内存的静态域中.</li>
</ol>
<h3 id="B-static修饰方法-类方法">B.static修饰方法(类方法):</h3>
<ol>
<li>随着类的加载而加载, 在内存中也是独一份</li>
<li>可以直接通过“类.类方法”的方式调用</li>
<li><strong>方法内部可以调用静态的属性或静态的方法, 而不能调用非静态的属性或方法</strong>. 反之, 非静态的方法是可以调用静态的属性或静态的方法. <font style='color:red;font-weight:bold;'>静态的方法内是不可以有this或super关键字的!</font></li>
</ol>
<p><font style='color:red;font-weight:bold;'>注意: 静态的结构(static的属性、方法、代码块、内部类)的生命周期要早于非静态的结构，同时被回收也要晚于非静态的结构</font></p>
<p>示例, 静态属性及内存状态:</p>
<details>
<summary>
One More Thing 👇
</summary>
<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></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> Test{</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"> SportsMan s1 = <span class="keyword">new</span> SportsMan(<span class="string">"金龙"</span>,<span class="number">23</span>);</span><br><span class="line"> SportsMan s2 = <span class="keyword">new</span> SportsMan(<span class="string">"银龙"</span>,<span class="number">20</span>);</span><br><span class="line"> s1.setName(<span class="string">"花龙"</span>);</span><br><span class="line"> s1.nation = <span class="string">"China"</span>;</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">SportsMan</span></span>{</span><br><span class="line"> <span class="keyword">private</span> String name;</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">int</span> age;</span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">static</span> String nation;</span><br><span class="line"> SportsMan(){</span><br><span class="line"> }</span><br><span class="line"> SportsMan(String name,<span class="keyword">int</span> age){</span><br><span class="line"> <span class="keyword">this</span>.name = name;</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="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><br></pre></td></tr></table></figure>
</details>
<p><img src="https://cdn.jsdelivr.net/gh/crainyday/blog@master/lazyload.svg" data-original="https://cdn.jsdelivr.net/gh/crainyday/blog@master/imgs/Java/static.png" alt="static"></p>
<h3 id="C-单例模式">C.单例模式</h3>
<p>23种设计模式</p>
<ul>
<li>单例模式(Singleton): 即<font style='color:red;font-weight:bold;'>类只能创建一个对象</font>, Runtime类就是一个单例模式的应用</li>
<li>解决的问题: 如何只让设计的类只能创建一个对象</li>
<li>如何实现: 饿汉式 & 懒汉式</li>
</ul>
<ol>
<li>私有化构造器</li>
<li>在类内部创建类的对象, 同时设置为private的, 通过公共的方法来调用, 体现封装性</li>
<li>一个public static的方法, 获得该(单例)对象</li>
<li>要求该对象也为static的</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><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></pre></td><td class="code"><pre><span class="line"><span class="comment">//饿汉式1</span></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Bank</span></span>{</span><br><span class="line"> <span class="comment">//1.私有化构造器</span></span><br><span class="line"> <span class="function"><span class="keyword">private</span> <span class="title">Bank</span><span class="params">()</span></span>{}</span><br><span class="line"> <span class="comment">//2.创建类的对象，同时设置为private的，通过公共的来调用，体现封装性</span></span><br><span class="line"> <span class="comment">//4.要求此对象也为static的</span></span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> Bank instance = <span class="keyword">new</span> Bank();</span><br><span class="line"> <span class="comment">//3.此公共的方法，必须为static </span></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> Bank <span class="title">getInstance</span><span class="params">()</span></span>{</span><br><span class="line"> <span class="keyword">return</span> instance;</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"><span class="comment">//饿汉式2</span></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Bank</span></span>{</span><br><span class="line"> <span class="comment">//1.私有化构造器</span></span><br><span class="line"> <span class="function"><span class="keyword">private</span> <span class="title">Bank</span><span class="params">()</span></span>{}</span><br><span class="line"> <span class="comment">//2.创建类的对象，同时设置为private的，通过公共的来调用，体现封装性</span></span><br><span class="line"> <span class="comment">//4.要求此对象也为static的</span></span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> Bank instance = <span class="keyword">null</span>;</span><br><span class="line"> <span class="keyword">static</span>{</span><br><span class="line"> instance = <span class="keyword">new</span> Bank(); </span><br><span class="line"> }</span><br><span class="line"> <span class="comment">//3.此公共的方法，必须为static </span></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> Bank <span class="title">getInstance</span><span class="params">()</span></span>{</span><br><span class="line"> <span class="keyword">return</span> instance; </span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="comment">//懒汉式</span></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Bank</span></span>{</span><br><span class="line"> <span class="function"><span class="keyword">private</span> <span class="title">Bank</span><span class="params">()</span></span>{}</span><br><span class="line"> </span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> Bank instance = <span class="keyword">null</span>;</span><br><span class="line"> </span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> Bank <span class="title">getInstance</span><span class="params">()</span></span>{</span><br><span class="line"> <span class="keyword">if</span>(instance == <span class="keyword">null</span>){<span class="comment">//可能存在线程安全问题的!</span></span><br><span class="line"> instance = <span class="keyword">new</span> Bank(); </span><br><span class="line"> } </span><br><span class="line"> <span class="keyword">return</span> instance;</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<h2 id="二-main">二.main()</h2>
<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></pre></td><td class="code"><pre><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 class="keyword">throws</span> Exception</span>{</span><br><span class="line"> <span class="comment">//方法体 </span></span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<ol>
<li>main()是一个方法, 是主方法, 为程序的入口</li>
<li>权限修饰符: public protected 缺省 private —面向对象的封装性</li>
<li>对于方法来讲: static final abstract 可修饰</li>
<li>方法的返回值: void / 具体的返回值类型(基本的数据类型 & 引用数据类型), 方法内部一定要有return</li>
<li>方法名: 命名的规则: xxxYyyZzz. 给方法命名时, 要见名知意</li>
<li>形参列表: 同一个方法名不同的形参列表的诸多个方法间构成重载. 形参 & 实参—方法的参数传递机制: 值传递</li>
<li>抛出异常列表</li>
<li>方法体: 方法定义的是一种功能, 具体的实现由方法体操作.</li>
</ol>
<h2 id="三-代码块">三.代码块</h2>
<ul>
<li>代码块: 是类的第4个成员, 由一对大括号括起来</li>
<li>作用: 用来初始化类的属性</li>
<li>分类: 只能用static来修饰.</li>
</ul>
<h3 id="A-静态代码块">A.静态代码块</h3>
<ol>
<li>里面可以有输出语句</li>
<li>随着类的加载而加载，而且只被加载一次</li>
<li>多个静态代码块之间按照顺序结构执行</li>
<li>静态代码块的执行要早于非静态代码块的执行.</li>
<li>静态的代码块中只能执行静态的结构(类属性, 类方法)</li>
</ol>
<h3 id="B-非静态代码块">B.非静态代码块</h3>
<ol>
<li>可以对类的属性(静态的 & 非静态的)进行初始化操作, 同时也可以调用本类声明的方法(静态的 & 非静态的)</li>
<li>里面可以有输出语句</li>
<li>一个类中可以有多个非静态的代码块, 多个代码块之间按照顺序结构执行</li>
<li>每创建一个类的对象, 非静态代码块就加载一次.</li>
<li>非静态代码块的执行要早于构造器</li>
</ol>
<p><font style='color:red;font-weight:bold;'>关于属性赋值的操作:</font></p>
<p>①默认的初始化<br>
②显式的初始化或代码块初始化(此处两个结构按照顺序执行)<br>
③构造器中;<br>
—————————以上是对象的属性初始化的过程—————————————<br>
④通过方法对对象的相应属性进行修改</p>
<h2 id="四-final">四.final</h2>
<p>final:最终的, 可以用来修饰类、属性、方法</p>
<ol>
<li>final修饰类: 这个类就<font style='color:red;font-weight:bold;'>不能被继承.</font> 如: String类、StringBuffer类、System类</li>
<li>final修饰方法: <font style='color:red;font-weight:bold;'>不能被重写</font>. 如: Object类的getClass()</li>
<li>final修饰属性: 此属性就是一个常量, 一旦初始化后, <font style='color:red;font-weight:bold;'>不可再被赋值</font>. 习惯上, 常量用大写字符表示.<br>
此常量在哪里赋值: ①此常量不能使用默认初始化 ②可以显式的赋值、代码块、构造器.<br>
变量用static final修饰时: 全局常量. 比如: Math 类的PI</li>
</ol>
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abstract: 抽象的, 可以用来修饰类、方法
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Object类
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一.多态性
多态性的表现: ①方法的重载与重写 ②子类对象的多态性
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<ol class="toc"><li class="toc-item toc-level-2"><a class="toc-link" href="#一-static"><span class="toc-text">一.static</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#A-static修饰属性-类变量"><span class="toc-text">A.static修饰属性(类变量):</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#B-static修饰方法-类方法"><span class="toc-text">B.static修饰方法(类方法):</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#C-单例模式"><span class="toc-text">C.单例模式</span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#二-main"><span class="toc-text">二.main()</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#三-代码块"><span class="toc-text">三.代码块</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#A-静态代码块"><span class="toc-text">A.静态代码块</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#B-非静态代码块"><span class="toc-text">B.非静态代码块</span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#四-final"><span class="toc-text">四.final</span></a></li></ol>
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