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Java8发布带来了不少新特性，如Lambda表达式、Stream流计算、函数式接口等，接下来让我们深入了解一下这些新特性
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<h2 id="Lambda匿名函数表达式"><a href="#Lambda匿名函数表达式" class="headerlink" title="Lambda匿名函数表达式"></a>Lambda匿名函数表达式</h2><ul>
<li>最主要的特点：可以将函数作为参数传入方法</li>
<li><p>实际上，Lambda表达式主要用于实现接口（只有一个待实现方法的接口），比如<code>Runnable</code>接口，内部只有<code>run()</code>方法待实现，那么就可以用Lambda表达式实现<code>Runnable</code>接口：<br> <code>Runnable run = () -> System.out.println("run")</code><br> <code>Thread t = new Thread(() ->System.out.println("run"))</code><br> 的方式来实现Thread，而不是古老的显式Override的方式。</p>
</li>
<li><p>Lambda可以利用编译器做很多自动推理的工作，例如传参、返回值、类型等信息都可以通过编译器自动推理得出，所以在编写的时候可以省略很多东西。例如：</p>
</li>
</ul>
<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">List<Integer> list = <span class="keyword">new</span> ArrayList<>(Arrays.asList(<span class="number">4</span>, <span class="number">2</span>, <span class="number">6</span>, <span class="number">1</span>));</span><br><span class="line"></span><br><span class="line">list.sort(((a, b) -> a - b));</span><br></pre></td></tr></table></figure>
<p>上例中，实际是用<code>(a, b) -> a - b</code>实现了Comparator接口，编译器自动推理出了a和b的类型，以及实现接口的类型，以及省略了return</p>
<h2 id="方法引用"><a href="#方法引用" class="headerlink" title="方法引用"></a>方法引用</h2><p>方法引用，可以理解为Lambda的一种简化版，用法是class::method或object::method，方法引用可以分为4类</p>
<ul>
<li>引用静态方法，ClassName::staticMethod，例如：String::valueOf，对应的Lambda就是(s) -> String.valueOf(s)</li>
<li>引用对象实例的方法，object::method，例如：”a”.toString，对应的Lambda就是 () -> this.toString()</li>
<li>引用特定类型的任意对象的方法，Class::method，例如：String::toString，对应的Lambda就是 (s) -> s.toString</li>
<li>引用构造函数，Class::new，例如：String::new，对应的Lambda是() -> new String()</li>
</ul>
<h3 id="方法引用示例"><a href="#方法引用示例" class="headerlink" title="方法引用示例"></a>方法引用示例</h3><p>假设有个接口：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">private</span> <span class="keyword">static</span> <span class="class"><span class="keyword">interface</span> <span class="title">Inter</span> </span>{</span><br><span class="line"> </span><br><span class="line"> <span class="function">String <span class="title">test</span><span class="params">(String a)</span></span>; </span><br><span class="line"> <span class="comment">// 引用对象实例的方法时，去掉String </span></span><br><span class="line"></span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>静态方法引用：<code>Inter inter = String::valueOf;</code> 等效于：<code>inter inter = (a) -> String.valueOf(a);</code></p>
<p>引用特定类型的任意对象方法：<code>inter inter = String::toString;</code> 等效：<code>inter inter = (a) -> a.toString;</code></p>
<p>引用对象实例的方法：<code>inter inter = “test”::toString</code>; 等效：<code>inter inter = () -> “test”.toString;</code></p>
<p>引用构造方法：<code>inter inter = String::new</code>; 等效：<code>inter inter = (a) -> new String(a);</code></p>
<p>个人认为，先理解了Lambda，再用Lambda来理解方法引用，会更加容易理解</p>
<h2 id="函数式接口"><a href="#函数式接口" class="headerlink" title="函数式接口"></a>函数式接口</h2><p>根据上述描述可知，Lambda表达式以及方法引用，实际上都是为了实现接口的，而能被实现的接口都有一个共同点，就是只有一个待实现方法。在Java8中，这种接口被统一称为函数式接口。</p>
<p>在Java8之前，其实就存在了很多这样的接口，比如Runnable、Comparator、InvocationHandler等，在JDK1.8中还加入了java.util.function包，内含有大量的函数式接口可供使用，而Java已经给JDK中所有的函数式接口都加上了@FunctionalInterface接口，来标注这是一个函数式接口</p>
<h2 id="默认方法"><a href="#默认方法" class="headerlink" title="默认方法"></a>默认方法</h2><p>Java8中，为每个接口提供了一种可以实现默认方法的机制，定义方法时，在前面加上default就可以了，之后实现这个接口的类都会继承这个默认方法（除非重写了此方法）</p>
<p>另外，还提供了一种静态默认方法，即每个接口都可以实现一个静态方法，之后就可以像类一样，通过接口名直接调用该方法。</p>
<p>例如：</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></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">Inter</span> </span>{</span><br><span class="line"> <span class="function">String <span class="title">test</span><span class="params">(String a)</span></span>;</span><br><span class="line"></span><br><span class="line"> <span class="keyword">default</span> <span class="keyword">void</span> {</span><br><span class="line"> System.out.println(<span class="string">"Default"</span>);</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">static</span> <span class="keyword">void</span> <span class="title">test3</span><span class="params">()</span> </span>{</span><br><span class="line"> System.out.println(<span class="string">"Static"</span>);</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<h2 id="Stream"><a href="#Stream" class="headerlink" title="Stream"></a>Stream</h2><p>流是一种新的对象，是Java8的新特性之一</p>
<p>对流的操作可以简化为<code>产生流对象->对流进行操作->收集流对象</code>的流程</p>
<h3 id="产生流对象"><a href="#产生流对象" class="headerlink" title="产生流对象"></a>产生流对象</h3><p>可以通过Stream的静态方法、Collection的stream()、parallelStream()方法、Arrays.stream(数组)等方式产生一个流对象</p>
<h3 id="对流进行操作"><a href="#对流进行操作" class="headerlink" title="对流进行操作"></a>对流进行操作</h3><ul>
<li>filter：设置过滤条件，会返回流，如：stream.filter(a -> a % 2 == 0)</li>
<li>forEach： 迭代流中的每个元素进行操作，如：stream.forEach(System.out::println)</li>
<li>map：对每一个元素做映射，会返回一个流，如：stream.map(i -> i + 1)</li>
<li>limit：截取流的一部分，返回流对象，如：stream.limit(2)</li>
<li>sort：对流进行排序，返回流对象，可以带入Comparator自定义排序方式，如：stream.sorted((a, b) -> b-a).forEach(System.out::println)，这是将流逆序排序后输出</li>
<li>等等等….</li>
</ul>
<h3 id="重新收集流对象"><a href="#重新收集流对象" class="headerlink" title="重新收集流对象"></a>重新收集流对象</h3><p>收集是为了将流转换回正常POJO对象</p>
<ul>
<li>count：顾名思义，即返回流中对象的数量</li>
<li>collect：<strong>特别重要</strong>，多数流对象是由集合转换来的，所以最后多数也要转换回集合类型，即可以用collect将流中的对象收集并转换为集合。如：stream.collect(Collectors.toList)、stream.collect(Collectors.toSet)、stream.collect(Collectors.toMap(i -> i, i -> i * i))……，在Collectors类中有很多静态方式可供使用</li>
</ul>
<p>示例：</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></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>{</span><br><span class="line"> Stream<Integer> stream = Stream.of(<span class="number">3</span>, <span class="number">4</span>, <span class="number">1</span>, <span class="number">9</span>, <span class="number">6</span>, <span class="number">2</span>, <span class="number">8</span>, <span class="number">7</span>, <span class="number">0</span>); <span class="comment">// 生成流对象</span></span><br><span class="line"></span><br><span class="line"> <span class="comment">// 逆序排序后截取前5个并返回列表</span></span><br><span class="line"> List<Integer> list = stream.sorted((a, b) -> b - a).limit(<span class="number">5</span>).collect(Collectors.toList());</span><br><span class="line"></span><br><span class="line"> <span class="comment">// 过滤出所有奇数，并以自己为key，平方为value存入map中</span></span><br><span class="line"> Map<Integer, Integer> map = stream.filter((x) -> x % <span class="number">2</span> == <span class="number">1</span>).collect(Collectors.toMap((i) -> i, (i) -> i * i));</span><br><span class="line"></span><br><span class="line"> <span class="comment">// 返回流中对象的个数</span></span><br><span class="line"> Long count = stream.count();</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<h2 id="Optional"><a href="#Optional" class="headerlink" title="Optional"></a>Optional</h2><p>Java8引入了Optional类主要是为了解决null</p>
<p>Optional对象的生成：</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></pre></td><td class="code"><pre><span class="line"></span><br><span class="line"><span class="comment">// 不能放入null</span></span><br><span class="line">Optional<String> optional = Optional.of(<span class="string">"optional"</span>);</span><br><span class="line"><span class="comment">// 可以放入null</span></span><br><span class="line">optional = Optional.ofNullable(<span class="keyword">null</span>);</span><br><span class="line"><span class="comment">// 只能是null</span></span><br><span class="line">optional = Optional.empty;</span><br></pre></td></tr></table></figure>
<p>Optional对象的操作：</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></pre></td><td class="code"><pre><span class="line"></span><br><span class="line">optional.get(); <span class="comment">// 如果非null则返回对象，如果是null则抛出NoSuchElementException异常</span></span><br><span class="line">optional.isPresent(); <span class="comment">//如果非null返回true，是null则返回false</span></span><br><span class="line"><span class="comment">// 下面两句的效果是一样的</span></span><br><span class="line">optional.ifPresent(System.out::println); </span><br><span class="line"><span class="keyword">if</span>(optional.isPresent()) {</span><br><span class="line"> System.out.println(optional.get());</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line">optional.orElse(<span class="string">""</span>); <span class="comment">//如果为null则返回传入的对象，否则返回内部的对象</span></span><br><span class="line"></span><br><span class="line">optional.orElseGet(String::<span class="keyword">new</span>); <span class="comment">//如果为null则返回supplier.get, supplier则为传入的接口</span></span><br><span class="line"></span><br><span class="line">optional.orElseThrow(exceptionSupplier); <span class="comment">//如果为null则抛出exceptionSupplier.get，这是一个异常生成的接口</span></span><br><span class="line"></span><br><span class="line">optional = optional.map((s) -> s + <span class="string">"!"</span>); <span class="comment">//如果不为null，则对内部的对象做一个映射并返回Optional对象，如果为null则返回空Optional对象</span></span><br><span class="line"></span><br><span class="line">optional = optional.flatMap((s) -> Optional.of(s + <span class="string">"!"</span>)); <span class="comment">//和map不同的一点是，flatMap传入的接口必须返回Optional对象，而map会自动封装</span></span><br><span class="line"></span><br><span class="line">optional = optional.filter((s) -> s.length() > <span class="number">6</span>);<span class="comment">// 有点像Stream的filter函数，但是不同的是，Stream中包含了大量元素，因此多数时候会有剩下的，但是Optional的filter更像是断言，true则原样返回，false则返回空Optional对象</span></span><br></pre></td></tr></table></figure>
<h2 id="其他特性"><a href="#其他特性" class="headerlink" title="其他特性"></a>其他特性</h2><p>Nashorn, JavaScript 引擎<br>新的日期API，不需要使用Date+Calendar对日期操作<br>Java8内置了Base64的编码器和解码器</p>
<p>不再一一赘述</p>
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<div class="post-toc-content"><ol class="nav"><li class="nav-item nav-level-2"><a class="nav-link" href="#Lambda匿名函数表达式"><span class="nav-number">1.</span> <span class="nav-text">Lambda匿名函数表达式</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#方法引用"><span class="nav-number">2.</span> <span class="nav-text">方法引用</span></a><ol class="nav-child"><li class="nav-item nav-level-3"><a class="nav-link" href="#方法引用示例"><span class="nav-number">2.1.</span> <span class="nav-text">方法引用示例</span></a></li></ol></li><li class="nav-item nav-level-2"><a class="nav-link" href="#函数式接口"><span class="nav-number">3.</span> <span class="nav-text">函数式接口</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#默认方法"><span class="nav-number">4.</span> <span class="nav-text">默认方法</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#Stream"><span class="nav-number">5.</span> <span class="nav-text">Stream</span></a><ol class="nav-child"><li class="nav-item nav-level-3"><a class="nav-link" href="#产生流对象"><span class="nav-number">5.1.</span> <span class="nav-text">产生流对象</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#对流进行操作"><span class="nav-number">5.2.</span> <span class="nav-text">对流进行操作</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#重新收集流对象"><span class="nav-number">5.3.</span> <span class="nav-text">重新收集流对象</span></a></li></ol></li><li class="nav-item nav-level-2"><a class="nav-link" href="#Optional"><span class="nav-number">6.</span> <span class="nav-text">Optional</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#其他特性"><span class="nav-number">7.</span> <span class="nav-text">其他特性</span></a></li></ol></div>
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