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<a class="post-title-link" href="/2018/03/28/java-tutorial-learning_interface_summary/" itemprop="url">Java 学习指南_学习Java：接口总结</a></h1>
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<h1 id="接口总结"><a href="#接口总结" class="headerlink" title="接口总结"></a>接口总结</h1><p>接口定义可以包含方法签名，默认方法，静态方法以及常量定义。只有默认方法和静态方法可以有实现。</p>
<p>类如果实现了接口必须实现接口里定义的所有抽象方法。</p>
<p>使用有类型名字的地方都可以使用接口名。</p>
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<a class="post-title-link" href="/2018/03/27/java-tutorial-learning_interface_default_methods/" itemprop="url">Java 学习指南_学习Java：默认方法</a></h1>
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<h1 id="默认方法Default-Methods"><a href="#默认方法Default-Methods" class="headerlink" title="默认方法Default Methods"></a>默认方法Default Methods</h1><p>接口章节 <a href="https://docs.oracle.com/javase/tutorial/java/IandI/createinterface.html" target="_blank" rel="external">Interfaces</a> 描述了一个例子，关于电脑控制汽车生产厂商发布工业标准接口，调用其中的方法可以对他们的汽车进行操控。如果电脑控制汽车厂商为汽车增加了新的功能，例如飞行的功能，会怎么样呢？这些厂商需要明确新的方法是其他的公司（例如电子制导系统公司）调整他们的软件来控制汽车飞行。这些厂商要在哪里声明这些飞行相关的方法呢？如果他们吧方法添加在原有的接口中，那么已经实现了这些接口的开发者就必须重写他们的实现。如果作为静态方法添加，开发者会将其作为工具方法，而不是必要的和兴方法来对待。</p>
<p>默认方法可以让你为库里的接口添加新的功能，并且在重写旧版本接口的代码时确保两部分功能兼容。</p>
<p>考虑下面的接口, <a href="https://docs.oracle.com/javase/tutorial/java/IandI/examples/TimeClient.java" target="_blank" rel="external"><code>TimeClient</code></a>, 在之前的联系中出现的 <a href="https://docs.oracle.com/javase/tutorial/java/IandI/QandE/interfaces-answers.html" target="_blank" rel="external">Answers to Questions and Exercises: Interfaces</a>:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">import</span> java.time.*; </div><div class="line"> </div><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">TimeClient</span> </span>{</div><div class="line"> <span class="function"><span class="keyword">void</span> <span class="title">setTime</span><span class="params">(<span class="keyword">int</span> hour, <span class="keyword">int</span> minute, <span class="keyword">int</span> second)</span></span>;</div><div class="line"> <span class="function"><span class="keyword">void</span> <span class="title">setDate</span><span class="params">(<span class="keyword">int</span> day, <span class="keyword">int</span> month, <span class="keyword">int</span> year)</span></span>;</div><div class="line"> <span class="function"><span class="keyword">void</span> <span class="title">setDateAndTime</span><span class="params">(<span class="keyword">int</span> day, <span class="keyword">int</span> month, <span class="keyword">int</span> year,</span></span></div><div class="line"> <span class="keyword">int</span> hour, <span class="keyword">int</span> minute, <span class="keyword">int</span> second);</div><div class="line"> <span class="function">LocalDateTime <span class="title">getLocalDateTime</span><span class="params">()</span></span>;</div><div class="line">}</div></pre></td></tr></table></figure>
<p>下面的类, <a href="https://docs.oracle.com/javase/tutorial/java/IandI/examples/defaultmethods/SimpleTimeClient.java" target="_blank" rel="external"><code>SimpleTimeClient</code></a>, 实现了上面的接口 <code>TimeClient</code>:</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div><div class="line">27</div><div class="line">28</div><div class="line">29</div><div class="line">30</div><div class="line">31</div><div class="line">32</div><div class="line">33</div><div class="line">34</div><div class="line">35</div><div class="line">36</div><div class="line">37</div><div class="line">38</div><div class="line">39</div><div class="line">40</div><div class="line">41</div><div class="line">42</div><div class="line">43</div><div class="line">44</div><div class="line">45</div><div class="line">46</div></pre></td><td class="code"><pre><div class="line">package defaultmethods;</div><div class="line"></div><div class="line">import java.time.*;</div><div class="line">import java.lang.*;</div><div class="line">import java.util.*;</div><div class="line"></div><div class="line">public class SimpleTimeClient implements TimeClient {</div><div class="line"> </div><div class="line"> private LocalDateTime dateAndTime;</div><div class="line"> </div><div class="line"> public SimpleTimeClient() {</div><div class="line"> dateAndTime = LocalDateTime.now();</div><div class="line"> }</div><div class="line"> </div><div class="line"> public void setTime(int hour, int minute, int second) {</div><div class="line"> LocalDate currentDate = LocalDate.from(dateAndTime);</div><div class="line"> LocalTime timeToSet = LocalTime.of(hour, minute, second);</div><div class="line"> dateAndTime = LocalDateTime.of(currentDate, timeToSet);</div><div class="line"> }</div><div class="line"> </div><div class="line"> public void setDate(int day, int month, int year) {</div><div class="line"> LocalDate dateToSet = LocalDate.of(day, month, year);</div><div class="line"> LocalTime currentTime = LocalTime.from(dateAndTime);</div><div class="line"> dateAndTime = LocalDateTime.of(dateToSet, currentTime);</div><div class="line"> }</div><div class="line"> </div><div class="line"> public void setDateAndTime(int day, int month, int year,</div><div class="line"> int hour, int minute, int second) {</div><div class="line"> LocalDate dateToSet = LocalDate.of(day, month, year);</div><div class="line"> LocalTime timeToSet = LocalTime.of(hour, minute, second); </div><div class="line"> dateAndTime = LocalDateTime.of(dateToSet, timeToSet);</div><div class="line"> }</div><div class="line"> </div><div class="line"> public LocalDateTime getLocalDateTime() {</div><div class="line"> return dateAndTime;</div><div class="line"> }</div><div class="line"> </div><div class="line"> public String toString() {</div><div class="line"> return dateAndTime.toString();</div><div class="line"> }</div><div class="line"> </div><div class="line"> public static void main(String... args) {</div><div class="line"> TimeClient myTimeClient = new SimpleTimeClient();</div><div class="line"> System.out.println(myTimeClient.toString());</div><div class="line"> }</div><div class="line">}</div></pre></td></tr></table></figure>
<p>Suppose that you want to add new functionality to the <code>TimeClient</code> interface, such as the ability to specify a time zone through a <a href="https://docs.oracle.com/javase/8/docs/api/java/time/ZonedDateTime.html" target="_blank" rel="external"><code>ZonedDateTime</code></a> object (which is like a<a href="https://docs.oracle.com/javase/8/docs/api/java/time/LocalDateTime.html" target="_blank" rel="external"><code>LocalDateTime</code></a> object except that it stores time zone information):</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">TimeClient</span> </span>{</div><div class="line"> <span class="function"><span class="keyword">void</span> <span class="title">setTime</span><span class="params">(<span class="keyword">int</span> hour, <span class="keyword">int</span> minute, <span class="keyword">int</span> second)</span></span>;</div><div class="line"> <span class="function"><span class="keyword">void</span> <span class="title">setDate</span><span class="params">(<span class="keyword">int</span> day, <span class="keyword">int</span> month, <span class="keyword">int</span> year)</span></span>;</div><div class="line"> <span class="function"><span class="keyword">void</span> <span class="title">setDateAndTime</span><span class="params">(<span class="keyword">int</span> day, <span class="keyword">int</span> month, <span class="keyword">int</span> year,</span></span></div><div class="line"> <span class="keyword">int</span> hour, <span class="keyword">int</span> minute, <span class="keyword">int</span> second);</div><div class="line"> <span class="function">LocalDateTime <span class="title">getLocalDateTime</span><span class="params">()</span></span>; </div><div class="line"> <span class="function">ZonedDateTime <span class="title">getZonedDateTime</span><span class="params">(String zoneString)</span></span>;</div><div class="line">}</div></pre></td></tr></table></figure>
<p>下面对于接口 <code>TimeClient</code>的修改，你也必须修改 类 <code>SimpleTimeClient</code> 并实现其中新增的方法 <code>getZonedDateTime</code>. 不过, 与其将 <code>getZonedDateTime</code> 作为 <code>abstract</code> 抽象的,不如将它替换为默认方法 <em>default implementation</em>. (记住 <a href="https://docs.oracle.com/javase/tutorial/java/IandI/abstract.html" target="_blank" rel="external">abstract method</a> 是没有实现的方法.)</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">package</span> defaultmethods;</div><div class="line"> </div><div class="line"><span class="keyword">import</span> java.time.*;</div><div class="line"></div><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">TimeClient</span> </span>{</div><div class="line"> <span class="function"><span class="keyword">void</span> <span class="title">setTime</span><span class="params">(<span class="keyword">int</span> hour, <span class="keyword">int</span> minute, <span class="keyword">int</span> second)</span></span>;</div><div class="line"> <span class="function"><span class="keyword">void</span> <span class="title">setDate</span><span class="params">(<span class="keyword">int</span> day, <span class="keyword">int</span> month, <span class="keyword">int</span> year)</span></span>;</div><div class="line"> <span class="function"><span class="keyword">void</span> <span class="title">setDateAndTime</span><span class="params">(<span class="keyword">int</span> day, <span class="keyword">int</span> month, <span class="keyword">int</span> year,</span></span></div><div class="line"> <span class="keyword">int</span> hour, <span class="keyword">int</span> minute, <span class="keyword">int</span> second);</div><div class="line"> <span class="function">LocalDateTime <span class="title">getLocalDateTime</span><span class="params">()</span></span>;</div><div class="line"> </div><div class="line"> <span class="function"><span class="keyword">static</span> ZoneId <span class="title">getZoneId</span> <span class="params">(String zoneString)</span> </span>{</div><div class="line"> <span class="keyword">try</span> {</div><div class="line"> <span class="keyword">return</span> ZoneId.of(zoneString);</div><div class="line"> } <span class="keyword">catch</span> (DateTimeException e) {</div><div class="line"> System.err.println(<span class="string">"Invalid time zone: "</span> + zoneString +</div><div class="line"> <span class="string">"; using default time zone instead."</span>);</div><div class="line"> <span class="keyword">return</span> ZoneId.systemDefault();</div><div class="line"> }</div><div class="line"> }</div><div class="line"> </div><div class="line"> <span class="function"><span class="keyword">default</span> ZonedDateTime <span class="title">getZonedDateTime</span><span class="params">(String zoneString)</span> </span>{</div><div class="line"> <span class="keyword">return</span> ZonedDateTime.of(getLocalDateTime(), getZoneId(zoneString));</div><div class="line"> }</div><div class="line">}</div></pre></td></tr></table></figure>
<p>在方法签名前使用 <code>default</code> 关键字来明确接口中的方法是默认方法。接口中定义的所有方法包括默认方法，都是<code>public</code>的，因此你可以省略<code>public</code>修饰符。</p>
<p>这样定义接口的话，你就不需要修改类 <code>SimpleTimeClient</code>, 这个类 (以及所有实现了接口 <code>TimeClient</code>的类),都会已定义的 <code>getZonedDateTime</code> 方法.下面的例子, <a href="https://docs.oracle.com/javase/tutorial/java/IandI/examples/defaultmethods/TestSimpleTimeClient.java" target="_blank" rel="external"><code>TestSimpleTimeClient</code></a>,调用了<code>SimpleTimeClient</code>一个实例的 <code>getZonedDateTime</code> 的方法:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">package</span> defaultmethods;</div><div class="line"> </div><div class="line"><span class="keyword">import</span> java.time.*;</div><div class="line"><span class="keyword">import</span> java.lang.*;</div><div class="line"><span class="keyword">import</span> java.util.*;</div><div class="line"></div><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">TestSimpleTimeClient</span> </span>{</div><div 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>{</div><div class="line"> TimeClient myTimeClient = <span class="keyword">new</span> SimpleTimeClient();</div><div class="line"> System.out.println(<span class="string">"Current time: "</span> + myTimeClient.toString());</div><div class="line"> System.out.println(<span class="string">"Time in California: "</span> +</div><div class="line"> myTimeClient.getZonedDateTime(<span class="string">"Blah blah"</span>).toString());</div><div class="line"> }</div><div class="line">}</div></pre></td></tr></table></figure>
<h2 id="扩展包含有默认方法的接口Extending-Interfaces-That-Contain-Default-Methods"><a href="#扩展包含有默认方法的接口Extending-Interfaces-That-Contain-Default-Methods" class="headerlink" title="扩展包含有默认方法的接口Extending Interfaces That Contain Default Methods"></a><a href="">扩展包含有默认方法的接口Extending Interfaces That Contain Default Methods</a></h2><p>当你从一个包含有默认方法的接口进行扩展时，你可以有一下做法：</p>
<ul>
<li>不提及默认方法，让扩展的几口继承原有的默认方法</li>
<li>重新定义默认方法，声明为<code>abstract</code></li>
<li>重新定义默认方法，复写它</li>
</ul>
<p>假设你按照下面的写法扩展 <code>TimeClient</code> 接口:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">AnotherTimeClient</span> <span class="keyword">extends</span> <span class="title">TimeClient</span> </span>{ }</div></pre></td></tr></table></figure>
<p>任何实现了 <code>AnotherTimeClient</code> 接口的类都有默认方法 <code>TimeClient.getZonedDateTime</code>的实现.</p>
<p>假设你按照如下写法扩展 `TimeClient接口:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">AbstractZoneTimeClient</span> <span class="keyword">extends</span> <span class="title">TimeClient</span> </span>{</div><div class="line"> <span class="function"><span class="keyword">public</span> ZonedDateTime <span class="title">getZonedDateTime</span><span class="params">(String zoneString)</span></span>;</div><div class="line">}</div></pre></td></tr></table></figure>
<p>任何实现接口 <code>AbstractZoneTimeClient</code>的类都必须重新实现方法 <code>getZonedDateTime</code>;这个方法现在是抽象方法 <code>abstract</code> ，就像接口中其他的非默认和非静态方法一样 。</p>
<p>假设你按照如下写法扩展 `TimeClient接口:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">HandleInvalidTimeZoneClient</span> <span class="keyword">extends</span> <span class="title">TimeClient</span> </span>{</div><div class="line"> <span class="function"><span class="keyword">default</span> <span class="keyword">public</span> ZonedDateTime <span class="title">getZonedDateTime</span><span class="params">(String zoneString)</span> </span>{</div><div class="line"> <span class="keyword">try</span> {</div><div class="line"> <span class="keyword">return</span> ZonedDateTime.of(getLocalDateTime(),ZoneId.of(zoneString)); </div><div class="line"> } <span class="keyword">catch</span> (DateTimeException e) {</div><div class="line"> System.err.println(<span class="string">"Invalid zone ID: "</span> + zoneString +</div><div class="line"> <span class="string">"; using the default time zone instead."</span>);</div><div class="line"> <span class="keyword">return</span> ZonedDateTime.of(getLocalDateTime(),ZoneId.systemDefault());</div><div class="line"> }</div><div class="line"> }</div><div class="line">}</div></pre></td></tr></table></figure>
<p>任何实现 <code>HandleInvalidTimeZoneClient</code> 接口的类都会用其中的 <code>getZonedDateTime</code> 方法实现类替代原有接口 <code>TimeClient</code>中的默认方法实现.</p>
<h2 id="静态方法Static-Methods"><a href="#静态方法Static-Methods" class="headerlink" title="静态方法Static Methods"></a><a href="">静态方法Static Methods</a></h2><p>默认方法之外，你也可以在接口中定义静态方法 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/classvars.html" target="_blank" rel="external">static methods </a>.（静态方法属于类而不是其他的对象。类的每个实例都可以分享类它的静态方法）这样能让你更加容易的组织你库里的辅助方法；你可以将静态方法定义在统一接口中而不是其他的类里面。下面的例子定义了一个静态方法，获取一个 <a href="https://docs.oracle.com/javase/8/docs/api/java/time/ZoneId.html" target="_blank" rel="external"><code>ZoneId</code></a> 对象先关的时区标识符；如果该对象没有给出相关的时区标识符则使用系统默认的时区标识符。（这样你就可以简化方法 <code>getZonedDateTime</code>）:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">TimeClient</span> </span>{</div><div class="line"> <span class="comment">// ...</span></div><div class="line"> <span class="function"><span class="keyword">static</span> <span class="keyword">public</span> ZoneId <span class="title">getZoneId</span> <span class="params">(String zoneString)</span> </span>{</div><div class="line"> <span class="keyword">try</span> {</div><div class="line"> <span class="keyword">return</span> ZoneId.of(zoneString);</div><div class="line"> } <span class="keyword">catch</span> (DateTimeException e) {</div><div class="line"> System.err.println(<span class="string">"Invalid time zone: "</span> + zoneString +</div><div class="line"> <span class="string">"; using default time zone instead."</span>);</div><div class="line"> <span class="keyword">return</span> ZoneId.systemDefault();</div><div class="line"> }</div><div class="line"> }</div><div class="line"></div><div class="line"> <span class="function"><span class="keyword">default</span> <span class="keyword">public</span> ZonedDateTime <span class="title">getZonedDateTime</span><span class="params">(String zoneString)</span> </span>{</div><div class="line"> <span class="keyword">return</span> ZonedDateTime.of(getLocalDateTime(), getZoneId(zoneString));</div><div class="line"> } </div><div class="line">}</div></pre></td></tr></table></figure>
<p>就像类中定义的静态方法一样，在接口中定义静态方法同样是在方法签名前面加上<code>static</code>关键字。因为接口中定义的所有方法都是<code>public</code>的，所以你可以省略<code>public</code>修饰符。</p>
<h2 id="在已有的代码库中融入默认方法Integrating-Default-Methods-into-Existing-Libraries"><a href="#在已有的代码库中融入默认方法Integrating-Default-Methods-into-Existing-Libraries" class="headerlink" title="在已有的代码库中融入默认方法Integrating Default Methods into Existing Libraries"></a><a href="">在已有的代码库中融入默认方法Integrating Default Methods into Existing Libraries</a></h2><p>默认方法可以你是你在已有的接口中添加新的功能，并同时确保兼容老版本接口写的代码。特别的是，默认方法允许你添加接受lambda表达式作为参数。本节演示了如何使用默认方法和静态方法增强 <a href="https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html" target="_blank" rel="external"><code>Comparator</code></a>接口.</p>
<p>考虑 <code>Card</code> 以及 <code>Deck</code> 类，在之前的联系中出现过 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/QandE/creating-questions.html" target="_blank" rel="external">Questions and Exercises: Classes</a>.本例重写了两个类 <a href="https://docs.oracle.com/javase/tutorial/java/IandI/examples/defaultmethods/Card.java" target="_blank" rel="external"><code>Card</code></a> 以及 <a href="https://docs.oracle.com/javase/tutorial/java/IandI/examples/defaultmethods/Deck.java" target="_blank" rel="external"><code>Deck</code></a> 作为接口. <code>Card</code> 接口包含了两个枚举类型 <code>enum</code> (<code>Suit</code> 和 <code>Rank</code>) 以及两个抽象方法(<code>getSuit</code> 和 <code>getRank</code>):</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div><div class="line">27</div><div class="line">28</div><div class="line">29</div><div class="line">30</div><div class="line">31</div><div class="line">32</div><div class="line">33</div><div class="line">34</div><div class="line">35</div><div class="line">36</div><div class="line">37</div><div class="line">38</div><div class="line">39</div><div class="line">40</div><div class="line">41</div><div class="line">42</div><div class="line">43</div><div class="line">44</div><div class="line">45</div><div class="line">46</div><div class="line">47</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">package</span> defaultmethods;</div><div class="line"></div><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">Card</span> <span class="keyword">extends</span> <span class="title">Comparable</span><<span class="title">Card</span>> </span>{</div><div class="line"> </div><div class="line"> <span class="keyword">public</span> <span class="keyword">enum</span> Suit { </div><div class="line"> DIAMONDS (<span class="number">1</span>, <span class="string">"Diamonds"</span>), </div><div class="line"> CLUBS (<span class="number">2</span>, <span class="string">"Clubs"</span> ), </div><div class="line"> HEARTS (<span class="number">3</span>, <span class="string">"Hearts"</span> ), </div><div class="line"> SPADES (<span class="number">4</span>, <span class="string">"Spades"</span> );</div><div class="line"> </div><div class="line"> <span class="keyword">private</span> <span class="keyword">final</span> <span class="keyword">int</span> value;</div><div class="line"> <span class="keyword">private</span> <span class="keyword">final</span> String text;</div><div class="line"> Suit(<span class="keyword">int</span> value, String text) {</div><div class="line"> <span class="keyword">this</span>.value = value;</div><div class="line"> <span class="keyword">this</span>.text = text;</div><div class="line"> }</div><div class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">int</span> <span class="title">value</span><span class="params">()</span> </span>{<span class="keyword">return</span> value;}</div><div class="line"> <span class="function"><span class="keyword">public</span> String <span class="title">text</span><span class="params">()</span> </span>{<span class="keyword">return</span> text;}</div><div class="line"> }</div><div class="line"> </div><div class="line"> <span class="keyword">public</span> <span class="keyword">enum</span> Rank { </div><div class="line"> DEUCE (<span class="number">2</span> , <span class="string">"Two"</span> ),</div><div class="line"> THREE (<span class="number">3</span> , <span class="string">"Three"</span>), </div><div class="line"> FOUR (<span class="number">4</span> , <span class="string">"Four"</span> ), </div><div class="line"> FIVE (<span class="number">5</span> , <span class="string">"Five"</span> ), </div><div class="line"> SIX (<span class="number">6</span> , <span class="string">"Six"</span> ), </div><div class="line"> SEVEN (<span class="number">7</span> , <span class="string">"Seven"</span>),</div><div class="line"> EIGHT (<span class="number">8</span> , <span class="string">"Eight"</span>), </div><div class="line"> NINE (<span class="number">9</span> , <span class="string">"Nine"</span> ), </div><div class="line"> TEN (<span class="number">10</span>, <span class="string">"Ten"</span> ), </div><div class="line"> JACK (<span class="number">11</span>, <span class="string">"Jack"</span> ),</div><div class="line"> QUEEN (<span class="number">12</span>, <span class="string">"Queen"</span>), </div><div class="line"> KING (<span class="number">13</span>, <span class="string">"King"</span> ),</div><div class="line"> ACE (<span class="number">14</span>, <span class="string">"Ace"</span> );</div><div class="line"> <span class="keyword">private</span> <span class="keyword">final</span> <span class="keyword">int</span> value;</div><div class="line"> <span class="keyword">private</span> <span class="keyword">final</span> String text;</div><div class="line"> Rank(<span class="keyword">int</span> value, String text) {</div><div class="line"> <span class="keyword">this</span>.value = value;</div><div class="line"> <span class="keyword">this</span>.text = text;</div><div class="line"> }</div><div class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">int</span> <span class="title">value</span><span class="params">()</span> </span>{<span class="keyword">return</span> value;}</div><div class="line"> <span class="function"><span class="keyword">public</span> String <span class="title">text</span><span class="params">()</span> </span>{<span class="keyword">return</span> text;}</div><div class="line"> }</div><div class="line"> </div><div class="line"> <span class="keyword">public</span> Card.<span class="function">Suit <span class="title">getSuit</span><span class="params">()</span></span>;</div><div class="line"> <span class="keyword">public</span> Card.<span class="function">Rank <span class="title">getRank</span><span class="params">()</span></span>;</div><div class="line">}</div></pre></td></tr></table></figure>
<p> <code>Deck</code> 接口包含了许多操作cards的方法:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">package</span> defaultmethods; </div><div class="line"> </div><div class="line"><span class="keyword">import</span> java.util.*;</div><div class="line"><span class="keyword">import</span> java.util.stream.*;</div><div class="line"><span class="keyword">import</span> java.lang.*;</div><div class="line"> </div><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">Deck</span> </span>{</div><div class="line"> </div><div class="line"> <span class="function">List<Card> <span class="title">getCards</span><span class="params">()</span></span>;</div><div class="line"> <span class="function">Deck <span class="title">deckFactory</span><span class="params">()</span></span>;</div><div class="line"> <span class="function"><span class="keyword">int</span> <span class="title">size</span><span class="params">()</span></span>;</div><div class="line"> <span class="function"><span class="keyword">void</span> <span class="title">addCard</span><span class="params">(Card card)</span></span>;</div><div class="line"> <span class="function"><span class="keyword">void</span> <span class="title">addCards</span><span class="params">(List<Card> cards)</span></span>;</div><div class="line"> <span class="function"><span class="keyword">void</span> <span class="title">addDeck</span><span class="params">(Deck deck)</span></span>;</div><div class="line"> <span class="function"><span class="keyword">void</span> <span class="title">shuffle</span><span class="params">()</span></span>;</div><div class="line"> <span class="function"><span class="keyword">void</span> <span class="title">sort</span><span class="params">()</span></span>;</div><div class="line"> <span class="function"><span class="keyword">void</span> <span class="title">sort</span><span class="params">(Comparator<Card> c)</span></span>;</div><div class="line"> <span class="function">String <span class="title">deckToString</span><span class="params">()</span></span>;</div><div class="line"></div><div class="line"> <span class="function">Map<Integer, Deck> <span class="title">deal</span><span class="params">(<span class="keyword">int</span> players, <span class="keyword">int</span> numberOfCards)</span></span></div><div class="line"> <span class="keyword">throws</span> IllegalArgumentException;</div><div class="line"></div><div class="line">}</div></pre></td></tr></table></figure>
<p>类 <a href="https://docs.oracle.com/javase/tutorial/java/IandI/examples/defaultmethods/PlayingCard.java" target="_blank" rel="external"><code>PlayingCard</code></a> 实现了 接口 <code>Card</code>, 类 <a href="https://docs.oracle.com/javase/tutorial/java/IandI/examples/defaultmethods/StandardDeck.java" target="_blank" rel="external"><code>StandardDeck</code></a> 实现了接口 <code>Deck</code>.</p>
<p>类 <code>StandardDeck</code> 实现了类的 <code>Deck.sort</code> 方法:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">StandardDeck</span> <span class="keyword">implements</span> <span class="title">Deck</span> </span>{</div><div class="line"> </div><div class="line"> <span class="keyword">private</span> List<Card> entireDeck;</div><div class="line"> </div><div class="line"> <span class="comment">// ...</span></div><div class="line"> </div><div class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">sort</span><span class="params">()</span> </span>{</div><div class="line"> Collections.sort(entireDeck);</div><div class="line"> }</div><div class="line"> </div><div class="line"> <span class="comment">// ...</span></div><div class="line">}</div></pre></td></tr></table></figure>
<p> <code>Collections.sort</code> 方法讲一个List实例进行了排序，List实例的元素类型是实现了 <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Comparable.html" target="_blank" rel="external"><code>Comparable</code></a>接口.成员变量 <code>entireDeck</code> 是一个元素为<code>Card</code>的 <code>List</code>实例。 <code>Card</code>实现了<code>Comparable</code> 接口. 类 <code>PlayingCard</code> 实现了 <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Comparable.html#compareTo-T-" target="_blank" rel="external"><code>Comparable.compareTo</code></a> 方法，内容如下:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">int</span> <span class="title">hashCode</span><span class="params">()</span> </span>{</div><div class="line"> <span class="keyword">return</span> ((suit.value()-<span class="number">1</span>)*<span class="number">13</span>)+rank.value();</div><div class="line">}</div><div class="line"></div><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">int</span> <span class="title">compareTo</span><span class="params">(Card o)</span> </span>{</div><div class="line"> <span class="keyword">return</span> <span class="keyword">this</span>.hashCode() - o.hashCode();</div><div class="line">}</div></pre></td></tr></table></figure>
<p> 方法 <code>StandardDeck.sort()</code> 通过 <code>compareTo</code> 方法是扑克先按照花色再按照点数排序。如果你想要让扑克先按照点数，再按照花色排序该怎么做呢？你可能需要实现<code>Comparator</code>接口来明确新的排序规则，并使用方法<a href="https://docs.oracle.com/javase/8/docs/api/java/util/Collections.html#sort-java.util.List-java.util.Comparator-" target="_blank" rel="external"><code>sort(List list, Comparator c)</code></a> ( <code>sort</code>方法包含<code>Comparator</code>参数的版本).你可以在类<code>StandardDeck</code>中定义如下方法:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">sort</span><span class="params">(Comparator<Card> c)</span> </span>{</div><div class="line"> Collections.sort(entireDeck, c);</div><div class="line">}</div></pre></td></tr></table></figure>
<p>有了这个方法你可以，你是可以使用 <code>Collections.sort</code> 明确 <code>Card</code> 类实例的排序规则.一种方法是实现 <code>Comparator</code> 接口来明确扑克排序的规则.下面的类 <a href="https://docs.oracle.com/javase/tutorial/java/IandI/examples/defaultmethods/SortByRankThenSuit.java" target="_blank" rel="external"><code>SortByRankThenSuit</code></a> 进行了示例:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">package</span> defaultmethods;</div><div class="line"></div><div class="line"><span class="keyword">import</span> java.util.*;</div><div class="line"><span class="keyword">import</span> java.util.stream.*;</div><div class="line"><span class="keyword">import</span> java.lang.*;</div><div class="line"></div><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">SortByRankThenSuit</span> <span class="keyword">implements</span> <span class="title">Comparator</span><<span class="title">Card</span>> </span>{</div><div class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">int</span> <span class="title">compare</span><span class="params">(Card firstCard, Card secondCard)</span> </span>{</div><div class="line"> <span class="keyword">int</span> compVal =</div><div class="line"> firstCard.getRank().value() - secondCard.getRank().value();</div><div class="line"> <span class="keyword">if</span> (compVal != <span class="number">0</span>)</div><div class="line"> <span class="keyword">return</span> compVal;</div><div class="line"> <span class="keyword">else</span></div><div class="line"> <span class="keyword">return</span> firstCard.getSuit().value() - secondCard.getSuit().value(); </div><div class="line"> }</div><div class="line">}</div></pre></td></tr></table></figure>
<p>下面的调用时扑克先按照点数再按照花色排序:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div></pre></td><td class="code"><pre><div class="line">StandardDeck myDeck = <span class="keyword">new</span> StandardDeck();</div><div class="line">myDeck.shuffle();</div><div class="line">myDeck.sort(<span class="keyword">new</span> SortByRankThenSuit());</div></pre></td></tr></table></figure>
<p>不过，这种方法太冗长verbose；如果你可以明确按照什么排序而不是怎么排序会更好。假设你是写<code>Compatator</code>接口 开发者。你会在<code>Comparator</code>接口中添加什么默认方法或者静态方法来是其他开发者更加简单的明确排序规则？</p>
<p>一开始，你想要将这些扑克按照点数排序而不管花色。你可以这样调用 `StandardDeck.sort方法:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div></pre></td><td class="code"><pre><div class="line">StandardDeck myDeck = <span class="keyword">new</span> StandardDeck();</div><div class="line">myDeck.shuffle();</div><div class="line">myDeck.sort(</div><div class="line"> (firstCard, secondCard) -></div><div class="line"> firstCard.getRank().value() - secondCard.getRank().value()</div><div class="line">);</div></pre></td></tr></table></figure>
<p>因为接口 <code>Comparator</code> 是一个功能接口 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html#approach6" target="_blank" rel="external">functional interface</a>, 所以你可以使用lambda表达式作为<code>sort</code>方法的参数。本例中，lambda表达式比较了两个整数值。</p>
<p>这样可以是开发者通过只调用<code>Card.getRank</code>方法就简单的创建一个<code>Comparator</code>实例。并且，他可以付诸开发者<code>Comparator</code>实例来比较任何可以通过入住<code>getValue</code> <code>hashCode</code>等方法返回数字值的对象。<code>Comparator</code>接口已经通过静态方法 <a href="https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html#comparing-java.util.function.Function-java.util.Comparator-" target="_blank" rel="external"><code>comparing</code></a>增强而拥有这个能力:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div></pre></td><td class="code"><pre><div class="line">myDeck.sort(Comparator.comparing((card) -> card.getRank()));</div></pre></td></tr></table></figure>
<p>这个例子中，你也可以通过方法引用来代替 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/methodreferences.html" target="_blank" rel="external">method reference</a>:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div></pre></td><td class="code"><pre><div class="line">myDeck.sort(Comparator.comparing(Card::getRank));</div></pre></td></tr></table></figure>
<p>这种调用很好的演示了用什么排序而不是怎么排序.</p>
<p> <code>Comparator</code> 接口同样有其他类似<code>comparing</code>版本的静态方法增强如 <a href="https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html#comparingDouble-java.util.function.ToDoubleFunction-java.util.Comparator-" target="_blank" rel="external"><code>comparingDouble</code></a> 和 <a href="https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html#comparingLong-java.util.function.ToLongFunction-" target="_blank" rel="external"><code>comparingLong</code></a> 可以用来创建通过其他数据类型比较的 <code>Comparator</code> 实例.</p>
<p>假设你的开发者想要创建一个 <code>Comparator</code> 实例，使用不止一个条件比较对象。例如，你如何将扑克先按照点数再按照花色排序？像前面的例子一样，你可以使用lambda表达式来明确这些排序规则:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div></pre></td><td class="code"><pre><div class="line">StandardDeck myDeck = <span class="keyword">new</span> StandardDeck();</div><div class="line">myDeck.shuffle();</div><div class="line">myDeck.sort(</div><div class="line"> (firstCard, secondCard) -> {</div><div class="line"> <span class="keyword">int</span> compare =</div><div class="line"> firstCard.getRank().value() - secondCard.getRank().value();</div><div class="line"> <span class="keyword">if</span> (compare != <span class="number">0</span>)</div><div class="line"> <span class="keyword">return</span> compare;</div><div class="line"> <span class="keyword">else</span></div><div class="line"> <span class="keyword">return</span> firstCard.getSuit().value() - secondCard.getSuit().value();</div><div class="line"> } </div><div class="line">);</div></pre></td></tr></table></figure>
<p>如果开发者可以通过一系列的 <code>Comparator</code> 实例来构造 <code>Comparator</code> 实例，将会更加简单. <code>Comparator</code> 接口已经通过默认方法 <a href="https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html#thenComparing-java.util.Comparator-" target="_blank" rel="external"><code>thenComparing</code></a>增强而拥有了这个能力:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div></pre></td><td class="code"><pre><div class="line">myDeck.sort(</div><div class="line"> Comparator</div><div class="line"> .comparing(Card::getRank)</div><div class="line"> .thenComparing(Comparator.comparing(Card::getSuit)));</div></pre></td></tr></table></figure>
<p> <code>Comparator</code> 接口同样有其他版本的 <code>thenComparing</code>方法 (例如 <a href="https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html#thenComparingDouble-java.util.function.ToDoubleFunction-" target="_blank" rel="external"><code>thenComparingDouble</code></a> 以及 <a href="https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html#thenComparingLong-java.util.function.ToLongFunction-" target="_blank" rel="external"><code>thenComparingLong</code></a>) 这样你可以构造 <code>Comparator</code> 实例来比较其他的数据类型.</p>
<p>假设你的开发者想要创建 <code>Comparator</code> 实例，倒序排列一个集合，例如你想要将扑克按照点数降序排列，从A到2.像之前一样你可以定义lambda表达式来实现。不过，如果可以通过已有的<code>Comparator</code>实例调用方法将会更简单。<code>Comparator</code>接口已经预知了默认方法 <a href="https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html#reversed--" target="_blank" rel="external"><code>reversed</code></a>:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div></pre></td><td class="code"><pre><div class="line">myDeck.sort(</div><div class="line"> Comparator.comparing(Card::getRank)</div><div class="line"> .reversed()</div><div class="line"> .thenComparing(Comparator.comparing(Card::getSuit)));</div></pre></td></tr></table></figure>
<p>这个例子演示了 <code>Comparator</code> 接口是如何通过默认方法，静态方法，lambda表达式，以及方法引用来增强的，这样就创建了更有表现力的库和方法，功能开发者可以快速的推断出该如何调用其中的方法。用同样的构造来创建你自己的库里的接口。</p>
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<a class="post-title-link" href="/2018/03/27/java-tutorial-learning_interface_evolving/" itemprop="url">Java 学习指南_学习Java：进化接口</a></h1>
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<h1 id="进化接口-Interfaces"><a href="#进化接口-Interfaces" class="headerlink" title="进化接口 Interfaces"></a>进化接口 Interfaces</h1><p>考虑一下你之前开发的一个名为 <code>DoIt</code>的接口:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">DoIt</span> </span>{</div><div class="line"> <span class="function"><span class="keyword">void</span> <span class="title">doSomething</span><span class="params">(<span class="keyword">int</span> i, <span class="keyword">double</span> x)</span></span>;</div><div class="line"> <span class="function"><span class="keyword">int</span> <span class="title">doSomethingElse</span><span class="params">(String s)</span></span>;</div><div class="line">}</div></pre></td></tr></table></figure>
<p>假设，过了一段时间以后，你想要在接口中添加一个方法，那么接口会变成下面的样子:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">DoIt</span> </span>{</div><div class="line"></div><div class="line"> <span class="function"><span class="keyword">void</span> <span class="title">doSomething</span><span class="params">(<span class="keyword">int</span> i, <span class="keyword">double</span> x)</span></span>;</div><div class="line"> <span class="function"><span class="keyword">int</span> <span class="title">doSomethingElse</span><span class="params">(String s)</span></span>;</div><div class="line"> <span class="function"><span class="keyword">boolean</span> <span class="title">didItWork</span><span class="params">(<span class="keyword">int</span> i, <span class="keyword">double</span> x, String s)</span></span>;</div><div class="line"> </div><div class="line">}</div></pre></td></tr></table></figure>
<p>如果要做这个改变，那么所有实现了旧的<code>DoIt</code>接口的类都不能再使用了，因为他们没有实现接口的所有方法。依赖这个接口的开发者将会大声抗议。</p>
<p>试着在开始就预见你的借口的所有用处并做完整的定义。如果你想要在接口中添加额外的方法，可以有几种选择。你可以创建一个 <code>DoItPlus</code> 接口来扩展 <code>DoIt</code>:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">DoItPlus</span> <span class="keyword">extends</span> <span class="title">DoIt</span> </span>{</div><div class="line"></div><div class="line"> <span class="function"><span class="keyword">boolean</span> <span class="title">didItWork</span><span class="params">(<span class="keyword">int</span> i, <span class="keyword">double</span> x, String s)</span></span>;</div><div class="line"> </div><div class="line">}</div></pre></td></tr></table></figure>
<p>现在你代码的用户可以选择继续使用旧的接口，或者升级到新的接口。</p>
<p>同样，也可以定义新的方法为默认方法 <a href="https://docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html" target="_blank" rel="external">default methods</a>. 下面的列子定义了一个名为<code>didItWork</code>:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">DoIt</span> </span>{</div><div class="line"></div><div class="line"> <span class="function"><span class="keyword">void</span> <span class="title">doSomething</span><span class="params">(<span class="keyword">int</span> i, <span class="keyword">double</span> x)</span></span>;</div><div class="line"> <span class="function"><span class="keyword">int</span> <span class="title">doSomethingElse</span><span class="params">(String s)</span></span>;</div><div class="line"> <span class="function"><span class="keyword">default</span> <span class="keyword">boolean</span> <span class="title">didItWork</span><span class="params">(<span class="keyword">int</span> i, <span class="keyword">double</span> x, String s)</span> </span>{</div><div class="line"> <span class="comment">// Method body </span></div><div class="line"> }</div><div class="line"> </div><div class="line">}</div></pre></td></tr></table></figure>
<p>注意，你必须为默认方法提供一个实现。你也可以在现存从接口中定义一个新的静态方法 <a href="https://docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html#static" target="_blank" rel="external">static methods</a>。实现了这个接口的用户可以通过默认方法或者静态方法类增强他们的功能，而不需要为适应新增的方法修改和重新编译所有的类。</p>
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<h1 id="使用接口作为一个类型"><a href="#使用接口作为一个类型" class="headerlink" title="使用接口作为一个类型"></a>使用接口作为一个类型</h1><p>当你定义一个新的接口，你定义了一个新的引用数据类型。使用其他引用类型名的地方都可以使用接口名。如果你定义的一个接口的引用类型变量，赋值必须为实现了这个接口的类的实例。</p>
<p>下面的例子，定义了一个找到size比较大的对象的方法，对象可以是任何实现了<code>Relatable</code>接口的类的实例：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">public</span> Object <span class="title">findLargest</span><span class="params">(Object object1, Object object2)</span> </span>{</div><div class="line"> Relatable obj1 = (Relatable)object1;</div><div class="line"> Relatable obj2 = (Relatable)object2;</div><div class="line"> <span class="keyword">if</span> ((obj1).isLargerThan(obj2) > <span class="number">0</span>)</div><div class="line"> <span class="keyword">return</span> object1;</div><div class="line"> <span class="keyword">else</span> </div><div class="line"> <span class="keyword">return</span> object2;</div><div class="line">}</div></pre></td></tr></table></figure>
<p>通过转换 <code>object1</code> 类型为 <code>Relatable</code> 类型, 就可以调用其 <code>isLargerThan</code> 方法.</p>
<p>如果你注意到很多的类都实现了 <code>Relatable</code>接口，任何这些类的实例都可以通过方法 <code>findLargest()</code>进行比较—只要他们是同一个类的对象.同样,他们都可以通过下面的方法进行比较:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">public</span> Object <span class="title">findSmallest</span><span class="params">(Object object1, Object object2)</span> </span>{</div><div class="line"> Relatable obj1 = (Relatable)object1;</div><div class="line"> Relatable obj2 = (Relatable)object2;</div><div class="line"> <span class="keyword">if</span> ((obj1).isLargerThan(obj2) < <span class="number">0</span>)</div><div class="line"> <span class="keyword">return</span> object1;</div><div class="line"> <span class="keyword">else</span> </div><div class="line"> <span class="keyword">return</span> object2;</div><div class="line">}</div><div class="line"></div><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">boolean</span> <span class="title">isEqual</span><span class="params">(Object object1, Object object2)</span> </span>{</div><div class="line"> Relatable obj1 = (Relatable)object1;</div><div class="line"> Relatable obj2 = (Relatable)object2;</div><div class="line"> <span class="keyword">if</span> ( (obj1).isLargerThan(obj2) == <span class="number">0</span>)</div><div class="line"> <span class="keyword">return</span> <span class="keyword">true</span>;</div><div class="line"> <span class="keyword">else</span> </div><div class="line"> <span class="keyword">return</span> <span class="keyword">false</span>;</div><div class="line">}</div></pre></td></tr></table></figure>
<p>这个方法对所有的”relatable”对象都有效,不管他们的继承成了什么，当他们实现了 <code>Relatable</code>接口,他们同时可以是谈么本身（或者其父类）的类型， 也可以是一个 <code>Relatable</code> 类型.这就给了他们多重继承的优势，他们同时可以表现出其父类和接口的行为.</p>
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