lambda表達式10個示例——學習筆記

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<div class=" "> <p>摘錄：http://www.importnew.com/16436.html</p> <h2>一、lambda實現Runnable</h2> <div class="cnblogs_code"><div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="複製代碼"><img src="//common.cnblogs.com/images/copycode.gif" alt="複製代碼"></a></span></div> <pre><span style="color: #008000;">//</span><span style="color: #008000;"> Java 8以前：</span> <span style="color: #0000ff;">new</span> Thread(<strong><span style="color: #ff0000;">new Runnable() { @Override public void</span></strong><span style="color: #000000;"><strong><span style="color: #ff0000;"> run</span></strong>() { System.out.println(</span>"Before Java8, too much code for too little to do"<span style="color: #000000;">); } }).start();</span></pre> <div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="複製代碼"><img src="//common.cnblogs.com/images/copycode.gif" alt="複製代碼"></a></span></div></div> <div class="cnblogs_code"> <pre><span style="color: #008000;">//</span><span style="color: #008000;">Java 8方式：<span style="color: #ff0000;">用() -&gt; {}代碼塊替代了整個<span style="color: #ff0000;">匿名類</span></span></span> <span style="color: #0000ff;">new</span> Thread(<span style="color: #ff0000;"><strong> () -&gt;</strong></span> System.out.println("In Java8, Lambda expression rocks !!") ).start();</pre> </div> <p>output:</p> <div class="cnblogs_code"> <pre>too much code, <span style="color: #0000ff;">for</span> too little to <span style="color: #0000ff;">do</span><span style="color: #000000;"> Lambda expression rocks </span>!!</pre> </div> <p>若是你的方法不對參數進行修改、重寫，只是在控制檯打印點東西的話，那麼能夠這樣寫：</p> <div class="cnblogs_code"> <pre>() -&gt; System.out.println("Hello Lambda Expressions");</pre> </div> <p>若是你的方法接收兩個參數，那麼能夠寫成以下這樣：</p> <div class="cnblogs_code"> <pre>若是你的方法接收兩個參數，那麼能夠寫成以下這樣：</pre> </div> <h2>二、使用Java 8 lambda表達式進行事件處理</h2> <p>編寫事件監聽：</p> <div class="cnblogs_code"><div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="複製代碼"><img src="//common.cnblogs.com/images/copycode.gif" alt="複製代碼"></a></span></div> <pre><span style="color: #008000;">//</span><span style="color: #008000;"> Java 8以前：</span> JButton show = <span style="color: #0000ff;">new</span> JButton("Show"<span style="color: #000000;">); show.addActionListener(</span><strong><span style="color: #ff0000;">new ActionListener() { @Override public void</span></strong><span style="color: #000000;"><strong><span style="color: #ff0000;"> actionPerformed</span></strong>(ActionEvent e) { System.out.println(</span>"Event handling without lambda expression is boring"<span style="color: #000000;">); } });</span></pre> <div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="複製代碼"><img src="//common.cnblogs.com/images/copycode.gif" alt="複製代碼"></a></span></div></div> <div class="cnblogs_code"> <pre><span style="color: #008000;">//</span><span style="color: #008000;"> Java 8方式：</span> show.addActionListener(<strong><span style="color: #ff0000;">(e) -&gt;</span></strong><span style="color: #000000;"> { System.out.println(</span>"Light, Camera, Action !! Lambda expressions Rocks"<span style="color: #000000;">); });</span></pre> </div> <p>output：</p> <div class="cnblogs_code"> <pre>too much code, <span style="color: #0000ff;">for</span> too little to <span style="color: #0000ff;">do</span><span style="color: #000000;"> Lambda expression rocks </span>!!</pre> </div> <h2>三、使用lambda表達式對列表進行迭代</h2> <div class="cnblogs_code"> <pre><span style="color: #008000;">//</span><span style="color: #008000;"> Java 8以前：</span> List features = Arrays.asList("Lambdas", "Default Method", "Stream API", "Date and Time API"<span style="color: #000000;">); </span><span style="color: #0000ff;">for</span><span style="color: #000000;"> (String feature : features) { System.out.println(feature); }</span></pre> </div> <div class="cnblogs_code"><div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="複製代碼"><img src="//common.cnblogs.com/images/copycode.gif" alt="複製代碼"></a></span></div> <pre><span style="color: #008000;">//</span><span style="color: #008000;"> Java 8以後：</span> List features = Arrays.asList("Lambdas", "Default Method", "Stream API", "Date and Time API"<span style="color: #000000;">); features.forEach(n </span>-&gt;<span style="color: #000000;"> System.out.println(n));

</span><span style="color: #008000;">//</span><span style="color: #008000;"> 使用Java 8的方法引用更方便，<span style="color: #ff0000;">方法引用由::雙冒號操做符標示， </span></span><span style="color: #008000;">//</span><span style="color: #008000;"> 看起來像C++的做用域解析運算符</span> features.forEach(System.out::println);</pre>

<div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="複製代碼"><img src="//common.cnblogs.com/images/copycode.gif" alt="複製代碼"></a></span></div></div> <p>output：</p> <div class="cnblogs_code"> <pre><span style="color: #000000;">Lambdas Default Method Stream API Date and Time API</span></pre> </div> <h2>四、使用lambda表達式和函數式接口Predicate</h2> <p>&nbsp;java.util.function.Predicate 函數式接口以及lambda表達式，能夠向API方法添加邏輯，用更少的代碼支持更多的動態行爲。</p> <div class="cnblogs_code"><div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="複製代碼"><img src="//common.cnblogs.com/images/copycode.gif" alt="複製代碼"></a></span></div> <pre><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">static</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> main(args[]){ List languages </span>= Arrays.asList("Java", "Scala", "C++", "Haskell", "Lisp"<span style="color: #000000;">);

System.out.println(</span>"Languages which starts with J :"<span style="color: #000000;">);
filter(languages, <strong><span style="color: #ff0000;">(str)</span></strong></span><strong><span style="color: #ff0000;">-&gt;</span></strong>str.startsWith("J"<span style="color: #000000;">));

System.out.println(</span>"Languages which ends with a "<span style="color: #000000;">);
filter(languages, <span style="color: #ff0000;"><strong>(str)</strong></span></span><span style="color: #ff0000;"><strong>-&gt;</strong></span>str.endsWith("a"<span style="color: #000000;">));

System.out.println(</span>"Print all languages :"<span style="color: #000000;">);
filter(languages, <strong><span style="color: #ff0000;">(str)</span></strong></span><strong><span style="color: #ff0000;">-&gt;</span></strong><span style="color: #0000ff;">true</span><span style="color: #000000;">);

System.out.println(</span>"Print no language : "<span style="color: #000000;">);
filter(languages, <strong><span style="color: #ff0000;">(str)</span></strong></span><strong><span style="color: #ff0000;">-&gt;</span></strong><span style="color: #0000ff;">false</span><span style="color: #000000;">);

System.out.println(</span>"Print language whose length greater than 4:"<span style="color: #000000;">);
filter(languages, <span style="color: #ff0000;"><strong>(str)</strong></span></span><span style="color: #ff0000;"><strong>-&gt;</strong></span>str.length() &gt; 4<span style="color: #000000;">);

}

</span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">static</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> filter(List names, Predicate condition) { </span><span style="color: #0000ff;">for</span><span style="color: #000000;">(String name: names) { </span><span style="color: #0000ff;">if</span><span style="color: #000000;">(condition.test(name)) { System.out.println(name </span>+ " "<span style="color: #000000;">); } } }</span></pre>

<div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="複製代碼"><img src="//common.cnblogs.com/images/copycode.gif" alt="複製代碼"></a></span></div></div> <p>output：</p> <div class="cnblogs_code"><div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="複製代碼"><img src="//common.cnblogs.com/images/copycode.gif" alt="複製代碼"></a></span></div> <pre><span style="color: #000000;">Languages which starts with J : Java Languages which ends with a Java Scala Print all languages : Java Scala C</span>++<span style="color: #000000;"> Haskell Lisp Print no language : Print language whose length greater than </span>4<span style="color: #000000;">: Scala Haskell</span></pre> <div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="複製代碼"><img src="//common.cnblogs.com/images/copycode.gif" alt="複製代碼"></a></span></div></div> <p>better way:</p> <div class="cnblogs_code"> <pre><span style="color: #008000;">//</span><span style="color: #008000;"> 更好的辦法</span> <span style="color: #0000ff;">public</span> <span style="color: #0000ff;">static</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> filter(List names, Predicate condition) { names.stream().filter((name) </span>-&gt; (condition.test(name))).forEach((name) -&gt;<span style="color: #000000;"> { System.out.println(name </span>+ " "<span style="color: #000000;">); }); }</span></pre> </div> <h2>五、如何在lambda表達式中加入Predicate</h2> <p>java.util.function.Predicate 容許將兩個或更多的 Predicate 合成一個。它提供相似於邏輯操做符AND和OR的方法，名字叫作and()、or()和xor()，用於將傳入 filter() 方法的條件合併起來。</p> <div class="cnblogs_code"><div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="複製代碼"><img src="//common.cnblogs.com/images/copycode.gif" alt="複製代碼"></a></span></div> <pre><span style="color: #008000;">//</span><span style="color: #008000;"> 甚至能夠用and()、or()和xor()邏輯函數來合併Predicate， </span><span style="color: #008000;">//</span><span style="color: #008000;"> 例如要找到全部以J開始，長度爲四個字母的名字，你能夠合併兩個Predicate並傳入</span> Predicate&lt;String&gt; startsWithJ = (n) -&gt; n.startsWith("J"<span style="color: #000000;">); Predicate</span>&lt;String&gt; fourLetterLong = (n) -&gt; n.length() == 4<span style="color: #000000;">; names<strong><span style="color: #ff0000;">.stream()</span></strong> .filter(startsWithJ<strong><span style="color: #ff0000;">.and(</span></strong>fourLetterLong<strong><span style="color: #ff0000;">))</span></strong> .forEach((n) </span>-&gt; System.out.print("nName, which starts with 'J' and four letter long is : " + n));</pre> <div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="複製代碼"><img src="//common.cnblogs.com/images/copycode.gif" alt="複製代碼"></a></span></div></div> <p>也可使用 or() 和 xor() 方法</p> <h2>六、Java 8中使用lambda表達式的Map和Reduce示例</h2> <p>將 costBeforeTax 列表的每一個元素轉換成爲稅後的值。咱們將 x -&gt; x*x lambda表達式傳到 map() 方法，後者將其應用到流中的每個元素。而後用 forEach() 將列表元素打印出來。</p> <div class="cnblogs_code"><div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="複製代碼"><img src="//common.cnblogs.com/images/copycode.gif" alt="複製代碼"></a></span></div> <pre><span style="color: #008000;">//</span><span style="color: #008000;"> 不使用lambda表達式爲每一個訂單加上12%的稅</span> List costBeforeTax = Arrays.asList(100, 200, 300, 400, 500<span style="color: #000000;">); </span><span style="color: #0000ff;">for</span><span style="color: #000000;"> (Integer cost : costBeforeTax) { </span><span style="color: #0000ff;">double</span> price = cost + .12*<span style="color: #000000;">cost; System.out.println(price); }

</span><span style="color: #008000;">//</span><span style="color: #008000;"> 使用lambda表達式</span> List costBeforeTax = Arrays.asList(100, 200, 300, 400, 500<span style="color: #000000;">); costBeforeTax.stream().map((cost) </span>-> cost + .12*cost).forEach(System.out::println);</pre>

<div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="複製代碼"><img src="//common.cnblogs.com/images/copycode.gif" alt="複製代碼"></a></span></div></div> <div class="cnblogs_code"><div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="複製代碼"><img src="//common.cnblogs.com/images/copycode.gif" alt="複製代碼"></a></span></div> <pre>112.0 224.0 336.0 448.0 560.0 112.0 224.0 336.0 448.0 560.0</pre> <div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="複製代碼"><img src="//common.cnblogs.com/images/copycode.gif" alt="複製代碼"></a></span></div></div> <h2>6.二、Java 8中使用lambda表達式的Map和Reduce示例</h2> <p>Map和Reduce操做是函數式編程的核心操做，由於其功能，reduce 又被稱爲摺疊操做。</p> <p>SQL中相似 sum()、avg() 或者 count() 的彙集函數，實際上就是 reduce 操做，由於它們接收多個值並返回一個值。</p> <p>流API定義的 reduceh() 函數能夠接受lambda表達式，並對全部值進行合併。IntStream這樣的類有相似 <span style="color: #ff0000;">average()、count()、sum() 的內建方法來作 reduce 操做</span>，也有<span style="color: #ff0000;">mapToLong()、mapToDouble() 方法來作轉換</span>。</p> <div class="cnblogs_code"><div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="複製代碼"><img src="//common.cnblogs.com/images/copycode.gif" alt="複製代碼"></a></span></div> <pre><span style="color: #008000;">//</span><span style="color: #008000;"> 爲每一個訂單加上12%的稅 </span><span style="color: #008000;">//</span><span style="color: #008000;"> 老方法：</span> List costBeforeTax = Arrays.asList(100, 200, 300, 400, 500<span style="color: #000000;">); </span><span style="color: #0000ff;">double</span> total = 0<span style="color: #000000;">; </span><span style="color: #0000ff;">for</span><span style="color: #000000;"> (Integer cost : costBeforeTax) { </span><span style="color: #0000ff;">double</span> price = cost + .12*<span style="color: #000000;">cost; total </span>= total +<span style="color: #000000;"> price; } System.out.println(</span>"Total : " +<span style="color: #000000;"> total);

</span><span style="color: #008000;">//</span><span style="color: #008000;"> 新方法：<span style="color: #ff0000;">能夠用內建方法，也能夠本身定義</span></span> List costBeforeTax = Arrays.asList(100, 200, 300, 400, 500<span style="color: #000000;">); </span><span style="color: #0000ff;">double</span> bill = costBeforeTax.stream().map((cost) -> cost + .12*cost)<strong><span style="color: #ff0000;">.reduce((sum, cost) -> sum +</span></strong><span style="color: #000000;"><strong><span style="color: #ff0000;"> cost)</span></strong>.get(); System.out.println(</span>"Total : " + bill);</pre>

<div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="複製代碼"><img src="//common.cnblogs.com/images/copycode.gif" alt="複製代碼"></a></span></div></div> <p>output:</p> <div class="cnblogs_code"> <pre>Total : 1680.0<span style="color: #000000;"> Total : </span>1680.0</pre> </div> <h2>七、經過<span style="color: #ff0000;">過濾</span>建立一個String列表</h2> <div class="cnblogs_code"> <pre><span style="color: #008000;">//</span><span style="color: #008000;"> 建立一個字符串列表，每一個字符串長度大於2</span> List&lt;String&gt; filtered = strList.stream()<strong><span style="color: #ff0000;">.filter(x -&gt; x.length()&gt; 2</span></strong><span style="color: #000000;"><strong><span style="color: #ff0000;">).</span></strong>collect(Collectors.toList()); System.out.printf(</span>"Original List : %s, filtered list : %s %n", strList, filtered);</pre> </div> <p>output:</p> <div class="cnblogs_code"> <pre>Original List : [abc, , bcd, , defg, jk], filtered list : [abc, bcd, defg]</pre> </div> <h2>八、對列表的<span style="color: #ff0000;">每一個元素應用函數</span></h2> <div class="cnblogs_code"> <pre><span style="color: #008000;">//</span><span style="color: #008000;"> 將字符串換成大寫並用逗號連接起來</span> List&lt;String&gt; G7 = Arrays.asList("USA", "Japan", "France", "Germany", "Italy", "U.K.","Canada"<span style="color: #000000;">); String G7Countries </span>= G7.stream()<strong><span style="color: #ff0000;">.map(x -&gt; x.toUpperCase())</span></strong>.collect(Collectors.joining(", "<span style="color: #000000;">)); System.out.println(G7Countries);</span></pre> </div> <h2>九、複製不一樣的值，建立一個子列表</h2> <div class="cnblogs_code"> <pre><span style="color: #008000;">//</span><span style="color: #008000;"> 用全部不一樣的數字建立一個正方形列表</span> List&lt;Integer&gt; numbers = Arrays.asList(9, 10, 3, 4, 7, 3, 4<span style="color: #000000;">); List</span>&lt;Integer&gt; distinct = numbers.stream()<span style="color: #ff0000;"><strong>.map( i -&gt; i*</strong></span><span style="color: #000000;"><span style="color: #ff0000;"><strong>i)</strong></span>.distinct().collect(Collectors.toList()); System.out.printf(</span>"Original List : %s, Square Without duplicates : %s %n", numbers, distinct);</pre> </div> <p>output:</p> <div class="cnblogs_code"> <pre>Original List : [9, 10, 3, 4, 7, 3, 4], Square Without duplicates : [81, 100, 9, 16, 49]</pre> </div> <h2>十、計算集合元素的最大值、最小值、總和以及平均值</h2> <p>ntStream、LongStream 和 DoubleStream 等流的類中，有個很是有用的方法叫作 summaryStatistics() 。能夠返回 IntSummaryStatistics、LongSummaryStatistics 或者 DoubleSummaryStatistic s，描述流中元素的各類摘要數據。</p> <p>最大值　　最小值　　全部元素的總和　　平均值</p> <div class="cnblogs_code"><div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="複製代碼"><img src="//common.cnblogs.com/images/copycode.gif" alt="複製代碼"></a></span></div> <pre><span style="color: #008000;">//</span><span style="color: #008000;">獲取數字的個數、最小值、最大值、總和以及平均值</span> List&lt;Integer&gt; primes = Arrays.asList(2, 3, 5, 7, 11, 13, 17, 19, 23, 29<span style="color: #000000;">); IntSummaryStatistics stats </span>= primes.stream().mapToInt((x) -&gt;<span style="color: #000000;"> x).summaryStatistics(); System.out.println(</span>"Highest prime number in List : " +<span style="color: #000000;"> stats.getMax()); System.out.println(</span>"Lowest prime number in List : " +<span style="color: #000000;"> stats.getMin()); System.out.println(</span>"Sum of all prime numbers : " +<span style="color: #000000;"> stats.getSum()); System.out.println(</span>"Average of all prime numbers : " + stats.getAverage());</pre> <div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="複製代碼"><img src="//common.cnblogs.com/images/copycode.gif" alt="複製代碼"></a></span></div></div> <p>輸出：</p> <div class="cnblogs_code"> <pre>Highest prime number in List : 29<span style="color: #000000;"> Lowest prime number in List : </span>2<span style="color: #000000;"> Sum of all prime numbers : </span>129<span style="color: #000000;"> Average of all prime numbers : </span>12.9</pre> </div> <h2>Lambda表達式 vs 匿名類</h2> <p>既然lambda表達式即將正式取代Java代碼中的匿名內部類，那麼有必要對兩者作一個比較分析。</p> <p><span style="color: #ff0000;">一個關鍵的不一樣點就是關鍵字 this:</span></p> <p>匿名類的 this 關鍵字指向匿名類，而lambda表達式的 this 關鍵字指向包圍lambda表達式的類。</p> <p><span style="color: #ff0000;">另外一個不一樣點是兩者的編譯方式:</span></p> <p>Java編譯器將lambda表達式編譯成類的私有方法。使用了Java 7的 invokedynamic 字節碼指令來動態綁定這個方法</p> <h1>注意：</h1> <p>1）lambda表達式僅能放入以下代碼：<span style="color: #ff0000;">預約義使用了 @Functional 註釋的函數式接口，自帶一個抽象函數的方法</span>，或者SAM（Single Abstract Method 單個抽象方法）類型。這些稱爲lambda表達式的目標類型，能夠用做返回類型，或lambda目標代碼的參數。例如，若一個方法接收Runnable、Comparable或者 Callable 接口，都有單個抽象方法，能夠傳入lambda表達式。相似的，若是一個方法接受聲明於 java.util.function 包內的接口，例如 Predicate、Function、Consumer 或 Supplier，那麼能夠向其傳lambda表達式。</p> <p>2）lambda表達式內可使用方法引用，僅當該方法不修改lambda表達式提供的參數。本例中的lambda表達式能夠換爲方法引用，由於這僅是一個參數相同的簡單方法調用。</p> <div class="cnblogs_code"> <pre>list.forEach(n -&gt;<span style="color: #000000;"> System.out.println(n)); list.forEach(System.out::println); </span><span style="color: #008000;">//</span><span style="color: #008000;"> 使用方法引用</span></pre> </div> <div>&nbsp;</div> <p>然而，若對參數有任何修改，則不能使用方法引用，而需鍵入完整地lambda表達式，以下所示：</p> <div> <div id="highlighter_697471" class="syntaxhighlighter notranslate java"> <div class="cnblogs_code"> <pre>list.forEach((String s) -&gt; System.out.println("*" + s + "*"));</pre> </div> </div> </div> <p>事實上，能夠省略這裏的lambda參數的類型聲明，<strong><span style="color: #ff0000;">編譯器能夠從列表的類屬性推測出來。</span></strong></p> <p>3）lambda內部可使用<span style="color: #ff0000;">靜態、非靜態和局部變量</span>，這稱爲lambda內的變量捕獲。</p> <p>4）Lambda表達式在Java中又稱爲閉包或匿名函數，因此若是有同事把它叫閉包的時候，不用驚訝。</p> <p>5）<span style="color: #ff0000;">Lambda方法在編譯器內部被翻譯成私有方法，並派發 invokedynamic 字節碼指令來進行調用。</span>可使用JDK中的 javap 工具來反編譯class文件。使用 javap -p 或 javap -c -v 命令來看一看lambda表達式生成的字節碼。大體應該長這樣：</p> <div> <div id="highlighter_135526" class="syntaxhighlighter notranslate java"> <div class="cnblogs_code"> <pre><span style="color: #0000ff;">private</span> <span style="color: #0000ff;">static</span> java.lang.Object lambda$0(java.lang.String);</pre> </div> </div> </div> <p>6）lambda表達式有個限制，那就是隻能引用 final 或 final 局部變量，<span style="color: #ff0000;">這就是說不能在lambda內部修改定義在域外的變量。</span></p> <div class="cnblogs_code"> <pre>List&lt;Integer&gt; primes = Arrays.asList(<span style="color: #0000ff;">new</span> Integer[]{2, 3,5,7<span style="color: #000000;">}); </span><span style="color: #0000ff;">int</span> factor = 2<span style="color: #000000;">; primes.forEach(element </span>-&gt; { factor++; });</pre> </div> <div class="cnblogs_code"> <pre><span style="color: #ff0000;">Compile time error : "local variables referenced from a lambda expression must be final or effectively final"</span></pre> </div> <div>&nbsp;</div> <p>另外，只是訪問它而不做修改是能夠的，以下所示：</p> <div> <div id="highlighter_719558" class="syntaxhighlighter notranslate java"> <div class="cnblogs_code"> <pre>List&lt;Integer&gt; primes = Arrays.asList(<span style="color: #0000ff;">new</span> Integer[]{2, 3,5,7<span style="color: #000000;">}); </span><span style="color: #0000ff;">int</span> factor = 2<span style="color: #000000;">; primes.forEach(element </span>-&gt; { System.out.println(factor*element); })</pre> </div> </div> </div> <p>輸出：</p> <div> <div id="highlighter_773319" class="syntaxhighlighter notranslate shell"> <div class="cnblogs_code"> <pre>4 6 10 14</pre> </div> <p>&nbsp;</p> </div> </div> </div> <div id="MySignature"></div> <div class="clear"></div> <div id="blog_post_info_block"><div id="BlogPostCategory"> 分類: <a href="https://www.cnblogs.com/L-a-u-r-a/category/1221830.html" target="_blank">JDK8新特性</a></div> <div id="EntryTag"> 標籤: <a href="https://www.cnblogs.com/L-a-u-r-a/tag/JDK8/">JDK8</a>, <a href="https://www.cnblogs.com/L-a-u-r-a/tag/Java/">Java</a></div>

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<div> <p style="text-align: center;"><strong><span style="font-size: 14pt;">Java 8 Lambda表達式和理解</span></strong></p> <p style="text-align: right;">說明：部分資料來源於網絡</p> <p style="text-align: right;">時間：20190704</p> <p>Lambda 表達式，也可稱爲閉包，它是推進 Java 8 發佈的最重要新特性。<strong>Lambda 容許把函數做爲一個方法的參數（函數做爲參數傳遞進方法中）</strong><span style="font-family: Helvetica;">。使用</span> Lambda 表達式可使代碼變的更加簡潔緊湊。</p> <p>&nbsp;</p> <p style="text-align: center;"><span style="background-color: #c0c0c0;"><strong><span style="font-size: 18px;">1、語法</span></strong></span></p> <p class="p">一、lambda 表達式的語法格式以下：</p> <p class="p"><strong><span style="font-family: 宋體;">　　（左邊）</span></strong><strong><span style="font-family: 宋體;">輸入參數</span></strong><strong>-&gt;（右邊）</strong><strong>lambda主體</strong></p> <div class="cnblogs_code"> <pre>(parameters) -&gt; expression;</pre> </div> <p><span style="font-family: Consolas;">　　或</span></p> <div class="cnblogs_code"> <pre>(parameters) -&gt;{ statements; }</pre> </div> <p>&nbsp;</p> <p class="p"><span style="font-family: Helvetica;">二、如下是</span>lambda表達式的重要特徵:</p> <p class="p"><span style="font-family: 宋體;">（1）輸入參數：</span></p> <p><strong><span style="font-family: Helvetica;">　　可選類型聲明：</span></strong><strong><span style="font-family: Helvetica;">不須要聲明參數類型</span></strong><span style="font-family: Helvetica;">，編譯器能夠統一識別參數值。</span></p> <p><strong><span style="font-family: Helvetica;">　　可選的參數圓括號：</span></strong></p> <p>　　　　a、<strong><span style="font-family: Helvetica;">一個參數無需定義圓括號，但多個參數須要定義圓括號。</span></strong></p> <p>　　　　b、<strong><span style="font-family: 宋體;">若是申明瞭參數類型，則必定須要圓括號。</span></strong></p> <p>（2）lambda主體<span style="font-family: 宋體;">：</span></p> <p><strong><span style="font-family: Helvetica;">　　可選的大括號：</span></strong><span style="font-family: Helvetica;">若是主體包含了一個語句，就不須要使用大括號。</span></p> <p><strong><span style="font-family: Helvetica;">　　可選的返回關鍵字：</span></strong><span style="font-family: Helvetica;">若是主體只有一個表達式返回值則編譯器會自動返回值，</span><strong><span style="font-family: Helvetica;">大括號須要指定明表達式返回了一個數值</span></strong><span style="font-family: Helvetica;">。</span></p> <p>&nbsp;</p> <p><span style="font-family: 宋體;">三、按照上面的格式，</span>lambda<span style="font-family: 宋體;">不一樣參數的表達式寫法</span></p> <p>（1）<span style="font-family: 宋體;">沒有參數的表達式：</span></p> <div class="cnblogs_code"> <pre>() -&gt; System.out.println("this is no parameter Lambda expression");</pre> </div> <p>（2）只有一個參數的表達式：</p> <div class="cnblogs_code"> <pre>(x) -&gt; System.out.println("this is only one parameter Lambda expression");</pre> </div> <p>　　或者</p> <div class="cnblogs_code"> <pre>(X x) -&gt; System.out.println("this is only one parameter Lambda expression"<span style="color: #000000;">);</span></pre> </div> <p><span style="font-family: 宋體;">　　和</span></p> <div class="cnblogs_code"> <pre>(x) -&gt;<span style="color: #000000;"> { x </span>= x*2<span style="color: #000000;">; System.out.println(</span>"this is only one parameter Lambda expression"<span style="color: #000000;">); System.out.println(</span>"the function is double input value"<span style="color: #000000;">); </span><span style="color: #0000ff;">return</span><span style="color: #000000;"> x; };</span></pre> </div> <p><span style="font-family: 宋體;">　　說明：一個參數的能夠不用使用</span>()<span style="font-family: 宋體;">，若是參數聲明瞭參數類型則必需要加</span><span style="font-family: Calibri;">()</span><span style="font-family: 宋體;">；</span><span style="font-family: Calibri;">Lambda</span><span style="font-family: 宋體;">主體是語句塊的話須要使用</span><span style="font-family: Calibri;">{}</span><span style="font-family: 宋體;">。</span></p> <p>（3）有兩個或者多個參數的表達式：</p> <div class="cnblogs_code"> <pre>(x,y) -&gt; System.out.println("these are several parameters Lambda expression");</pre> </div> <p>　　或者</p> <div class="cnblogs_code"> <pre>(X x,Y y) -&gt;<span style="color: #000000;"> { 　　System.out.println(</span>"the function is add two input values"<span style="color: #000000;">); 　　System.out.println(</span>"these are several parameters Lambda expression"<span style="color: #000000;">); 　　</span><span style="color: #0000ff;">return</span> x+<span style="color: #000000;">y; };</span></pre> </div> <p><span style="font-family: 宋體;">　　說明：有多個參數的</span>lambda<span style="font-family: 宋體;">表達式，參數使用</span><span style="font-family: Calibri;">,</span><span style="font-family: 宋體;">隔開。</span><span style="font-family: Calibri;">Lambda</span><span style="font-family: 宋體;">主體是語句塊的話須要使用</span><span style="font-family: Calibri;">{}</span><span style="font-family: 宋體;">。</span></p> <p>&nbsp;</p> <p style="text-align: center;"><span style="background-color: #c0c0c0;"><strong><span style="font-size: 18px;">2、Lambda 表達式實例</span></strong></span></p> <p>lambda示例1：</p> <div class="cnblogs_code"><div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="複製代碼"><img src="//common.cnblogs.com/images/copycode.gif" alt="複製代碼"></a></span></div> <pre><span style="color: #008000;">/*</span><span style="color: #008000;"> * Predicate&lt;T&gt;接口中boolean test(T t)方法只接收一個參數，返回值爲boolean類型， * 故lambda表達式的輸入參數只有一個參數:x，lambda的主體返回值爲boolean類型 </span><span style="color: #008000;">*/</span><span style="color: #000000;"> Predicate</span>&lt;Integer&gt; atLeast5 = x -&gt; x &gt;= 5<span style="color: #000000;">; System.out.println(</span>"傳入參數是否大於5：" + atLeast5.test(6<span style="color: #000000;">));

</span><span style="color: #008000;">/*</span><span style="color: #008000;">

• BinaryOperator<T>接口中R apply(T t, U u)方法接收兩個泛型參數，返回值爲也爲泛型
• 故lambda表達式輸入Long參數有兩個：x,y，lambda的主體返回值爲Long類型 </span><span style="color: #008000;">*/</span><span style="color: #000000;"> BinaryOperator</span><Long> addLongs = (x, y) -><span style="color: #000000;"> { 　　Long z </span>= x +<span style="color: #000000;"> y; 　　</span><span style="color: #0000ff;">return</span><span style="color: #000000;"> z; }; System.out.println(</span>"計算傳入兩個Long參數的和：" + addLongs.apply(5l, 6l<span style="color: #000000;">));

</span><span style="color: #008000;">/*</span><span style="color: #008000;">

• Runnable接口中run()方法沒有參數，有沒有返回值，故lambda表達式沒有參數，lambda主體也沒有返回值 </span><span style="color: #008000;">*/</span><span style="color: #000000;"> Runnable run1 </span>= () -> System.out.println("這個方法就是run裏面的方法"<span style="color: #000000;">); run1.run();

</span><span style="color: #0000ff;">try</span><span style="color: #000000;"> { 　　</span><span style="color: #0000ff;">final</span> Integer value = 9<span style="color: #000000;">; 　　</span><span style="color: #008000;">/</span><span style="color: #008000;"> 　　 * 一、Callable<V>接口中V call() throws Exception方法沒有參數，故lambda表達式也不能有輸入參數, 　　 * 返回類型爲泛型，故lambda主體須要返回指定Integer類型 　　 * 二、由於.call()方法拋出異常，因此須要拋出或者捕獲異常 　　 * 三、lambda表達式使用外部參數，須要是final類型，及時外部參數沒有定義爲final類型， 　　 * 也會隱式的指定爲final，故值或者引用地址不能修改。 　　 </span><span style="color: #008000;">/</span><span style="color: #000000;"> 　　Callable</span><Integer> call = () -><span style="color: #000000;"> value; 　　System.out.println(</span>"無參的方法，因此lambda的參數列表中不能傳遞參數：" +<span style="color: #000000;"> call.call()); } </span><span style="color: #0000ff;">catch</span><span style="color: #000000;"> (Exception e) { 　　e.printStackTrace(); }</span></pre>

<div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="複製代碼"><img src="//common.cnblogs.com/images/copycode.gif" alt="複製代碼"></a></span></div></div> <p>lambda示例二：</p> <div class="cnblogs_code"><div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="複製代碼"><img src="//common.cnblogs.com/images/copycode.gif" alt="複製代碼"></a></span></div> <pre><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">class</span><span style="color: #000000;"> Java8Tester {

　　</span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">static</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> main(String args[]){ <br> 　　　　Java8Tester tester </span>= <span style="color: #0000ff;">new</span><span style="color: #000000;"> Java8Tester(); 　　　　</span><span style="color: #008000;">//</span><span style="color: #008000;"> 類型聲明 </span> 　　　　MathOperation addition = (<span style="color: #0000ff;">int</span> a, <span style="color: #0000ff;">int</span> b) -> a +<span style="color: #000000;"> b; 　　　　</span><span style="color: #008000;">//</span><span style="color: #008000;"> 不用類型聲明 </span> 　　　　MathOperation subtraction = (a, b) -> a -<span style="color: #000000;"> b; 　　　　</span><span style="color: #008000;">//</span><span style="color: #008000;"> 大括號中的返回語句 </span> 　　　　MathOperation multiplication = (<span style="color: #0000ff;">int</span> a, <span style="color: #0000ff;">int</span> b) -> { <span style="color: #0000ff;">return</span> a *<span style="color: #000000;"> b; }; 　　　　</span><span style="color: #008000;">//</span><span style="color: #008000;"> 沒有大括號及返回語句 </span> 　　　　MathOperation division = (<span style="color: #0000ff;">int</span> a, <span style="color: #0000ff;">int</span> b) -> a /<span style="color: #000000;"> b;

　　　　System.out.println(</span>"10 + 5 = " + tester.operate(10, 5<span style="color: #000000;">, addition)); 　　　　System.out.println(</span>"10 - 5 = " + tester.operate(10, 5<span style="color: #000000;">, subtraction)); 　　　　System.out.println(</span>"10 x 5 = " + tester.operate(10, 5<span style="color: #000000;">, multiplication)); 　　　　System.out.println(</span>"10 / 5 = " + tester.operate(10, 5<span style="color: #000000;">, division));

　　　　</span><span style="color: #008000;">//</span><span style="color: #008000;"> 不用括號 </span> 　　　　GreetingService greetService1 = message -> System.out.println("Hello " +<span style="color: #000000;"> message); 　　　　</span><span style="color: #008000;">//</span><span style="color: #008000;"> 用括號 </span> 　　　　GreetingService greetService2 = (message) -> System.out.println("Hello " +<span style="color: #000000;"> message);

　　　　greetService1.sayMessage(</span>"Runoob"<span style="color: #000000;">); 　　　　greetService2.sayMessage(</span>"Google"<span style="color: #000000;">); 　　}<br><br>　　// 定義接口和接口中爲實現的方法 　　</span><span style="color: #0000ff;">interface</span> MathOperation {<br><span style="color: #0000ff;">　　　　int</span> operation(<span style="color: #0000ff;">int</span> a, <span style="color: #0000ff;">int</span><span style="color: #000000;"> b);<br>　　}<br><br></span></pre>

<pre><span>　　// 定義接口和接口中爲實現的方法<br></span><span style="color: #0000ff;">　　interface</span> GreetingService {<br><span style="color: #0000ff;">　　　　void</span><span> sayMessage(String message);<br></span><em id="__mceDel"><em id="__mceDel"><em id="__mceDel"><span>　　} </span></em></em></em></pre>

<pre><em id="__mceDel"><em id="__mceDel"><em id="__mceDel"><span style="color: #000000;"><br>　　// 定義方法，方法經過lambda表達式實現了接口中的方法 　　</span><span style="color: #0000ff;">private</span> <span style="color: #0000ff;">int</span> operate(<span style="color: #0000ff;">int</span> a, <span style="color: #0000ff;">int</span><span style="color: #000000;"> b, MathOperation mathOperation){ 　　　　</span><span style="color: #0000ff;">return</span><span style="color: #000000;"> mathOperation.operation(a, b); 　　} }</span></em></em></em></pre>

<div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="複製代碼"><img src="//common.cnblogs.com/images/copycode.gif" alt="複製代碼"></a></span></div></div> <p><span style="font-family: Helvetica;">執行以上代碼，輸出結果爲：</span></p> <div class="cnblogs_code"> <pre>10 + 5 = 15 10 - 5 = 5 10 x 5 = 50 10 / 5 = 2<span style="color: #000000;"> Hello Runoob Hello Google</span></pre> </div> <p class="p"><span style="font-family: Helvetica;">使用</span> Lambda 表達式須要注意如下兩點：</p> <p><strong>Lambda表達式主要用來定義行內執行的方法類型接口</strong><span style="font-family: Helvetica;">，例如，一個簡單方法接口。在上面例子中，咱們使用各類類型的</span>Lambda表達式來定義MathOperation接口的方法。而後咱們定義了sayMessage的執行。</p> <p>Lambda 表達式免去了使用匿名方法的麻煩，而且給予Java簡單可是強大的函數化的編程能力。</p> <p>Lambda表達式經常使用的場景是：函數式接口。函數式接口是指的只有一個抽象方法的接口，咱們經常使用的函數式接口有：</p> <div class="cnblogs_code"> <pre>Runnable、Callable、PrivilegedAction、Comparator、FileFilter、PathMatcher、InvocationHandler、PropertyChangeListener、ActionListener、ChangeListener、Function、Predicate、BinaryOperator</pre> </div> <p>&nbsp;</p> <p style="text-align: center;"><span style="background-color: #c0c0c0;"><strong><span style="font-size: 18px;">3、lambda表達式中的變量做用域</span></strong></span></p> <p class="p">lambda表達式只能引用標記了<strong>final的外層局部變量</strong><span style="font-family: Helvetica;">，這就是說</span><strong><span style="font-family: Helvetica;">不能在</span>lambda內部修改定義在域外的局部變量</strong><span style="font-family: Helvetica;">，不然會編譯錯誤。</span></p> <p class="p"><span style="font-family: Helvetica;">在</span> Java8Tester.java 文件輸入如下代碼：</p> <div class="cnblogs_code"><div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="複製代碼"><img src="//common.cnblogs.com/images/copycode.gif" alt="複製代碼"></a></span></div> <pre><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">class</span><span style="color: #000000;"> Java8Tester {

</span><span style="color: #0000ff;">　　final</span> <span style="color: #0000ff;">static</span> String salutation = "Hello! "<span style="color: #000000;">;

</span><span style="color: #0000ff;">　　public</span> <span style="color: #0000ff;">static</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> main(String args[]){<br> 　　　　GreetingService greetService1 </span>= message -> System.out.println(salutation +<span style="color: #000000;"> message); 　　　　greetService1.sayMessage(</span>"Runoob"<span style="color: #000000;">);

　　　　</span><span style="color: #008000;">//</span><span style="color: #008000;">====================至關於下面==============================</span> 　　　　GreetingService g = <span style="color: #0000ff;">new</span><span style="color: #000000;"> GreetingService() { 　　　　　　@Override </span><span style="color: #0000ff;">　　　　　　public</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> sayMessage(String message) { 　　　　　　　　System.out.println(salutation </span>+<span style="color: #000000;"> message); 　　　　　　} 　　　　}; 　　　　g.sayMessage(</span>"jack"<span style="color: #000000;">);<br> 　　}

　　</span><span style="color: #0000ff;">interface</span><span style="color: #000000;"> GreetingService { 　　　　</span><span style="color: #0000ff;">void</span><span style="color: #000000;"> sayMessage(String message); 　　} } </span></pre>

<div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="複製代碼"><img src="//common.cnblogs.com/images/copycode.gif" alt="複製代碼"></a></span></div></div> <p><span style="font-family: Helvetica;">執行以上腳本，輸出結果爲：</span></p> <div class="cnblogs_code"> <pre><span style="color: #000000;">$ javac Java8Tester.java $ java Java8Tester Hello</span>!<span style="color: #000000;"> Runoob Hello</span>! jack</pre> </div> <p><span style="font-family: Helvetica;">咱們也能夠直接在</span> lambda 表達式中訪問外層的局部變量：</p> <div class="cnblogs_code"><div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="複製代碼"><img src="//common.cnblogs.com/images/copycode.gif" alt="複製代碼"></a></span></div> <pre><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">class</span><span style="color: #000000;"> Java8Tester {

　　</span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">static</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> main(String args[]) { 　　　　</span><span style="color: #0000ff;">final</span> <span style="color: #0000ff;">int</span> num = 1<span style="color: #000000;">; 　　　　Converter</span><Integer, String> s = (param) -> System.out.println(String.valueOf(param +<span style="color: #000000;"> num)); 　　　　s.convert(</span>2); <span style="color: #008000;">//</span><span style="color: #008000;"> 輸出結果爲 3 </span> 　　<span style="color: #000000;">}

　　</span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">interface</span> Converter<T1, T2><span style="color: #000000;"> { 　　　　</span><span style="color: #0000ff;">void</span> convert(<span style="color: #0000ff;">int</span><span style="color: #000000;"> i); 　　} }</span></pre>

<div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="複製代碼"><img src="//common.cnblogs.com/images/copycode.gif" alt="複製代碼"></a></span></div></div> <p class="p">lambda表達式的局部變量能夠不聲明爲final，可是不可被後面的代碼修改（即隱性的具備final 的語義）</p> <div class="cnblogs_code"> <pre><span style="color: #0000ff;">int</span> num = 1<span style="color: #000000;">; Converter</span>&lt;Integer, String&gt; s = (param) -&gt; System.out.println(String.valueOf(param +<span style="color: #000000;"> num)); s.convert(</span>2<span style="color: #000000;">); num </span>= 5; <span style="color: #008000;">//</span><span style="color: #008000;">報錯信息：Local variable num defined in an enclosing scope must be final or effectively final</span></pre> </div> <p class="p"><span style="font-family: Helvetica;">在</span> Lambda 表達式當中不容許聲明一個與局部變量同名的參數或者局部變量。</p> <div class="cnblogs_code"> <pre>String first = ""<span style="color: #000000;">; Comparator</span>&lt;String&gt; comparator = (first, second) -&gt; Integer.compare(first.length(), second.length()); <span style="color: #008000;">//</span><span style="color: #008000;">編譯會出錯</span></pre> </div> <p>&nbsp;</p> </div>