JAVA8之lambda表达式详解，及stream中的lambda使用
2016-07-13 21:00
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lambda表达式详解
一.问题
1.什么是lambda表达式？ 2.lambda表达式用来干什么的？
3.lambda表达式的优缺点？
4.lambda表达式的使用场景？
5.lambda只是一个语法糖吗？
二.概念
lambda表达式是JAVA8中提供的一种新的特性，它支持JAVA也能进行简单的“函数式编程”。 它是一个匿名函数，Lambda表达式基于数学中的λ演算得名，直接对应于其中的lambda抽象(lambda abstraction)，是一个匿名函数，即没有函数名的函数。
三.先看看效果
先看几个例子： 1.使用lambda表达式实现Runnable
<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">package</span> com.lambda;
<span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**
* 使用lambda表达式替换Runnable匿名内部类
*<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @author</span> MingChenchen
*
*/</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">RunableTest</span> {</span>
<span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**
* 普通的Runnable
*/</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">runSomeThing</span>(){
Runnable runnable = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Runnable() {
<span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">run</span>() {
System.out.println(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"I am running"</span>);
}
};
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Thread(runnable).start();
}
<span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**
* 使用lambda后的
*/</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">runSomeThingByLambda</span>(){
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Thread(() -> System.out.println(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"I am running"</span>)).start();
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">main</span>(String[] args) {
runSomeThing();
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// runSomeThingByLambda();</span>
}
}
上述代码中：
() -> System.out.println(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"I am running"</span>)就是一个lambda表达式，
可以看出，它是替代了<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Runnable(){}这个匿名内部类。
</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li></ul>2.使用lambda表达式实现Comparator
<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">package</span> com.lambda;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> java.util.Arrays;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> java.util.Collections;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> java.util.Comparator;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> java.util.List;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">SortList</span> {</span>
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//给入一个List</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> List<String> list =
Arrays.asList(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"my"</span>,<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"name"</span>,<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"is"</span>,<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"uber"</span>,<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"and"</span>,<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"uc"</span>);
<span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**
* 对一个String的list进行排序 - 使用老方法
*/</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">oldSort</span>(){
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//排序</span>
Collections.sort(list,<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Comparator<String>() {
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//使用新的排序规则 根据第二个字符进行逆序排</span>
<span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> <span class="hljs-title" style="box-sizing: border-box;">compare</span>(String a,String b){
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (a.charAt(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>) <= b.charAt(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>)) {
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>;
}<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span>{
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> -<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>;
}
}
});
}
<span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**
* 新的排序方法 - 使用lambda表达式实现
*/</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">newSort</span>(){
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//lambda会自动推断出 a,b 的类型</span>
Collections.sort(list, (a, b) -> a.charAt(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>) < b.charAt(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>) ? <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>:-<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>);
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">main</span>(String[] args) {
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// oldSort();</span>
newSort();
}
}
</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li></ul>3.使用lambda表达式实现ActionListener
<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">package</span> com.lambda;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> java.awt.event.ActionEvent;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> java.awt.event.ActionListener;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> javax.swing.JButton;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">ActionEventDemo</span> {</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> JButton button = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> JButton();
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">bindEvent</span>(){
button.addActionListener(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> ActionListener() {
<span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">actionPerformed</span>(ActionEvent e) {
System.out.println(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"你好！"</span> );
}
});
}
<span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**
* 使用Lambda表达式 为button添加ActionListener
*/</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">bindEventByLambda</span>(){
button.addActionListener(e -> System.out.println(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"你好！"</span>));
}
}
</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li></ul>
四.来由
好了，通过上述的几个例子，大家差不多也能明白了lambda是用来干什么以及好处了。 显而易见的，好处就是代码量大大减少了！程序逻辑也很清晰明了。
它的用处浅显来说就是替代“内部匿名类”、可以对集合或者数组进行循环操作。
以前：
面向对象式编程就应该纯粹的面向对象，于是经常看到这样的写法：
如果你想写一个方法，那么就必须把它放到一个类里面，然后new出来对象，对象调用这个方法。
匿名类型最大的问题就在于其冗余的语法。
有人戏称匿名类型导致了“高度问题”（height problem）：
比如大多匿名内部类的多行代码中仅有一行在做实际工作。
因此JAVA8中就提供了这种“函数式编程”的方法 —— lambda表达式，供我们来更加简明扼要的实现内部匿名类的功能。
五.什么时候可以使用它？
先说一个名词的概念函数式接口：Functional Interface.
定义的一个接口，接口里面必须 有且只有一个抽象方法 ，这样的接口就成为函数式接口。
在可以使用lambda表达式的地方，方法声明时必须包含一个函数式的接口。
（JAVA8的接口可以有多个default方法）
任何函数式接口都可以使用lambda表达式替换。
例如：ActionListener、Comparator、Runnable
lambda表达式只能出现在目标类型为函数式接口的上下文中。
注意：
此处是只能！！！
意味着如果我们提供的这个接口包含一个以上的Abstract Method，那么使用lambda表达式则会报错。
这点已经验证过了。
场景：
这种场景其实很常见：
你在某处就真的只需要一个能做一件事情的函数而已，连它叫什么名字都无关紧要。
Lambda 表达式就可以用来做这件事。
六.写法、规则
基本语法: (parameters) -> expression 或 (parameters) ->{ statements; }
即: 参数 -> 带返回值的表达式/无返回值的陈述
<code class="hljs coffeescript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-regexp" style="color: rgb(0, 136, 0); box-sizing: border-box;">//</span><span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1.</span> 接收<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>个int型整数,返回他们的和 <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(int x, int y)</span> -></span> x + y; <span class="hljs-regexp" style="color: rgb(0, 136, 0); box-sizing: border-box;">//</span><span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2.</span> 接受一个 string 对象,并在控制台打印,不返回任何值(看起来像是返回<span class="hljs-reserved" style="box-sizing: border-box;">void</span>) <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(String s)</span> -></span> System.out.<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">print</span>(s);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>
七.几个特性
1. 类型推导 编译器负责推导lambda表达式的类型。它利用lambda表达式所在上下文所期待的类型进行推导，
这个被期待的类型被称为目标类型。就是说我们传入的参数可以无需写类型了！
2.变量捕获
在Java SE 7中，编译器对内部类中引用的外部变量（即捕获的变量）要求非常严格：
如果捕获的变量没有被声明为final就会产生一个编译错误。
我们现在放宽了这个限制——对于lambda表达式和内部类，
我们允许在其中捕获那些符合有效只读（Effectively final）的局部变量。
简单的说，如果一个局部变量在初始化后从未被修改过，那么它就符合有效只读的要求，
换句话说，加上final后也不会导致编译错误的局部变量就是有效只读变量。
注意：此处和final关键字一样，指的是引用不可改！（感觉没多大意义，还不是用的final）
3.方法引用
如果我们想要调用的方法拥有一个名字，我们就可以通过它的名字直接调用它。
Comparator byName = Comparator.comparing(Person::getName);
此处无需再传入参数，lambda会自动装配成Person类型进来然后执行getName()方法，而后返回getName()的String
方法引用有很多种，它们的语法如下：
静态方法引用：ClassName::methodName
实例上的实例方法引用：instanceReference::methodName
超类上的实例方法引用：super::methodName
类型上的实例方法引用：ClassName::methodName
构造方法引用：Class::new
数组构造方法引用：TypeName[]::new
4.JAVA提供给我们的SAM接口
Java SE 8中增加了一个新的包：java.util.function，它里面包含了常用的函数式接口，例如：
<code class="hljs r has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">Predicate<<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">T</span>>——接收<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">T</span>对象并返回boolean Consumer<<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">T</span>>——接收<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">T</span>对象，不返回值 Function<<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">T</span>, R>——接收<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">T</span>对象，返回R对象 Supplier<<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">T</span>>——提供<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">T</span>对象（例如工厂），不接收值 UnaryOperator<<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">T</span>>——接收<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">T</span>对象，返回<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">T</span>对象 BinaryOperator<<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">T</span>>——接收两个<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">T</span>对象，返回<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">T</span>对象</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li></ul>
那么在参数为这些接口的地方，我们就可以直接使用lambda表达式了！
八.更多的例子
1.自定义SAM接口，从而使用lambda表达式<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">package</span> com.lambda.myaction;
<span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**
* 自定义一个函数式接口
*<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @author</span> MingChenchen
*
*/</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">interface</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">MyActionInterface</span> {</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">saySomeThing</span>(String str);
<span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**
* Java8引入的新特性 接口中可以定义一个default方法的实现 (不是abstract)
*/</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">default</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> say(){
System.out.println(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"default say"</span>);
}
}
</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li></ul><code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">package</span> com.lambda.myaction;
<span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**
* 在我们自定义的函数式接口的地方使用lambda表达式
*<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @author</span> MingChenchen
*
*/</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">WantSay</span> {</span>
<span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**
* 执行接口中的saySomeThing方法
*<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @param</span> action
*<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @param</span> thing
*/</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">excuteSay</span>(MyActionInterface action,String thing){
action.saySomeThing(thing);
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">main</span>(String[] args) {
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/*
excuteSay(new MyActionInterface(){
@Override
public void saySomeThing(String str) {
System.out.println(str);
}
},"Hello World");
*/</span>
excuteSay((String s) -> System.out.println(s),<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Hello world new"</span>);
}
}
</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li></ul>2.使用方法引用( ClassName::Method,无括号)
<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">package</span> com.lambda.usebean;
<span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**
* 实体类Person
*<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @author</span> MingChenchen
*
*/</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Person</span> {</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> String name; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//姓名</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> String location; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//地址</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> String <span class="hljs-title" style="box-sizing: border-box;">getName</span>() {
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> name;
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">setName</span>(String name) {
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.name = name;
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> String <span class="hljs-title" style="box-sizing: border-box;">getLocation</span>() {
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> location;
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">setLocation</span>(String location) {
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.location = location;
}
<span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> String <span class="hljs-title" style="box-sizing: border-box;">toString</span>() {
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// TODO Auto-generated method stub</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Person:"</span> + name + <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">","</span> + location;
}
}
</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li></ul><code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//使用String默认的排序规则，比较的是Person的name字段</span> Comparator<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><</span>Person<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">></span> byName <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> Comparator<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>comparing(p <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">-> </span>p<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>getName()); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//不用写传入参数,传入的用Person来声明</span> Comparator<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><</span>Person<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">></span> byName2 <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> Comparator<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>comparing(Person<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::getName</span>);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>
3.使用lambda表达式完成for-each循环操作
<code class="hljs php has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//原本的for-each循环写做法</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">List</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">list</span> = Arrays.asList(....);
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> (int i = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>; i < <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">list</span>.size(); i++) {
System.out.println(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">list</span>.get(i));
}
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//使用lambda表达式后的写法</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">list</span>.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">forEach</span>(str -> System.out.println(str));</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul>list.forEach()是JAVA8的新方法，支持函数式编程，此处使用的参数就是JAVA提供给我们的函数式接口：Consumer< T>
<code class="hljs axapta has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">interface</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">List</span><<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">E</span>> <span class="hljs-inheritance" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">extends</span></span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Collection</span><<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">E</span>>
<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">interface</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Collection</span><<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">E</span>> <span class="hljs-inheritance" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">extends</span></span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Iterable</span><<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">E</span>>
<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">public</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">interface</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Iterable</span><<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">T</span>> {</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">default</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> forEach(Consumer<? <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">super</span> T> action) {
Objects.requireNonNull(action);
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> (T t : <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>) {
action.accept(t);
}
}
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li></ul>4.一个完整的例子
<code class="hljs r has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">//普通写法：
List<Person> people = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">...</span>
Collections.sort(people, new Comparator<Person>() {
public int compare(Person x, Person y) {
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> x.getLastName().compareTo(y.getLastName());
}
})
//使用lambda表达式写法：
people.sort(comparing(Person::getLastName));</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li></ul>化简流程：
<code class="hljs coffeescript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">第一步：去掉冗余的匿名类 Collections.sort<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(people,(Person x, Person y) -> x.getLastName().compareTo(y.getLastName()))</span>; 第二步：使用<span class="hljs-title" style="box-sizing: border-box;">Comparator</span>里的<span class="hljs-title" style="box-sizing: border-box;">comparing</span>方法 <span class="hljs-title" style="box-sizing: border-box;">Collections</span>.<span class="hljs-title" style="box-sizing: border-box;">sort</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(people, Comparator.comparing((Person p) -> p.getLastName()))</span>; 第三步：类型推导和静态导入 <span class="hljs-title" style="box-sizing: border-box;">Collections</span>.<span class="hljs-title" style="box-sizing: border-box;">sort</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(people, comparing(p -> p.getLastName()))</span>; 第四步：方法引用 <span class="hljs-title" style="box-sizing: border-box;">Collections</span>.<span class="hljs-title" style="box-sizing: border-box;">sort</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(people, comparing(Person::getLastName))</span>; 第五步：使用<span class="hljs-title" style="box-sizing: border-box;">List</span>本身的<span class="hljs-title" style="box-sizing: border-box;">sort</span>更优 <span class="hljs-title" style="box-sizing: border-box;">people</span>.<span class="hljs-title" style="box-sizing: border-box;">sort</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(comparing(Person::getLastName))</span>;;</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li></ul>
九.优缺点
优点： 1.极大的简化代码。去除了很多无用的Java代码，使得代码更为简洁明了。
2.比匿名内部类更加高效（不确定）。编译器会生成专门的lambda方法，可以使用javap -p查看编译过后的代码
缺点：
1.可读性差。在代码简洁的情况下，另一方面又让大多程序员很难读懂。因为很少程序员接触使用它。
(不过这个缺点不是本身缺点，而且源于程序员较少使用)
十.它是一个语法糖吗？
答： 就我自身的理解来说，lambda表达式不算是一个语法糖。
语法糖就是说只是帮助我们程序员轻松的少写一些代码，之后编译器帮我们把那部分代码生成出来。
但是从编译过后的结果来说，并不是自动帮我们生成一个内部匿名类，而是生成了一个lambda$X方法。
第二个就是lambda其实表达的是目前流行的“函数式编程”这种思维。区别于我们面向对象的思维方法。
这点我认为很有意义，即我们要从各种思维来对待事情。而不是说，面向对象的这种方法就是最NB的。
但是论坛基本都认为这是一个语法糖，也没错。毕竟它提倡的只是一种思想，而且jdk底层为lambda生成了新的高效的代码这个事儿并不确定。
接下来介绍 lambda的 好哥们:stream.
stream的方法里面大多都使用了lambda表达式
stream概要
一.什么是stream？
官方解释：<code class="hljs oxygene has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">A <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">sequence</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">of</span> elements supporting sequential <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">and</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">parallel</span> aggregate operations.</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>
简单来讲，stream就是JAVA8提供给我们的对于元素集合统一、快速、并行操作的一种方式。
它能充分运用多核的优势，以及配合lambda表达式、链式结构对集合等进行许多有用的操作。
概念：
stream:可以支持顺序和并行对元素操作的元素集合。
作用：
提供了一种操作大数据接口，让数据操作更容易和更快
使用stream，我们能够对collection的元素进行过滤、映射、排序、去重等许多操作。
中间方法和终点方法：
它具有过滤、映射以及减少遍历数等方法，这些方法分两种：中间方法和终端方法，
“流”抽象天生就该是持续的，中间方法永远返回的是Stream，因此如果我们要获取最终结果的话，
必须使用终点操作才能收集流产生的最终结果。区分这两个方法是看他的返回值，
如果是Stream则是中间方法，否则是终点方法
二.如何使用stream?
1.通过Stream接口的静态工厂方法（注意：Java8里接口可以带静态方法）； 2.通过Collection接口的默认方法（默认方法：Default method，也是Java8中的一个新特性，就是接口中的一个带有实现的方法）–stream()，把一个Collection对象转换成Stream
一般情况下，我们都使用Collection接口的 .stream()方法得到stream.
三.常见的几个中间方法
中间方法即是一些列对元素进行的操作。譬如过滤、去重、截断等。1.Filter（过滤）
<code class="hljs avrasm has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">//过滤<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">18</span>岁以上的人 List persons = … Stream personsOver18 = persons<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.stream</span>()<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.filter</span>(p -> p<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.getAge</span>() > <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">18</span>)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">; </span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>
2.Map(对元素进行操作)
<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//把person转成Adult</span> Stream <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">map</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> persons<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>stream()<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>filter(p <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">-> </span>p<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>getAge() <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">></span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">18</span>)<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">map</span>(person <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">-> </span><span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">new</span> Adult(person)); </code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>
3.limit(截断)
对一个Stream进行截断操作，获取其前N个元素，如果原Stream中包含的元素个数小于N，那就获取其所有的元素
4.distinct(去重)
对于Stream中包含的元素进行去重操作（去重逻辑依赖元素的equals方法），新生成的Stream中没有重复的元素
四.常用的终点方法
通过中间方法，我们对stream的元素进行了统一的操作，但是中间方法得到还是一个stream。要想把它转换为新的集合、或者是统计等。我们需要使用终点方法。1.count(统计)
count方法是一个流的终点方法，可使流的结果最终统计，返回int，比如我们计算一下满足18岁的总人数
<code class="hljs avrasm has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">int countOfAdult=persons<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.stream</span>() <span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.filter</span>(p -> p<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.getAge</span>() > <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">18</span>) <span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.map</span>(person -> new Adult(person)) <span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.count</span>()<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>
2.Collect(收集流的结果)
collect方法也是一个流的终点方法，可收集最终的结果
<code class="hljs avrasm has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">List adultList= persons<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.stream</span>() <span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.filter</span>(p -> p<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.getAge</span>() > <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">18</span>) <span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.map</span>(person -> new Adult(person)) <span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.collect</span>(Collectors<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.toList</span>())<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>
五.顺序流和并行流
每个Stream都有两种模式：顺序执行和并行执行。<code class="hljs avrasm has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">//顺序流： List <Person> people = list<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.getStream</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.collect</span>(Collectors<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.toList</span>())<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span> //并行流： List <Person> people = list<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.getStream</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.parallel</span>()<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.collect</span>(Collectors<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.toList</span>())<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span> //可以看出，要使用并行流，只需要<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.parallel</span>()即可</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul><div class="save_code tracking-ad" data-mod="popu_249" style="box-sizing: border-box; position: absolute; width: 105px; height: 60px; right: 30px; top: 5px; color: rgb(255, 255, 255); cursor: pointer; z-index: 2;"><a target=_blank target="_blank" style="color: rgb(255, 153, 0); box-sizing: border-box;"><img src="http://static.blog.csdn.net/images/save_snippets.png" style="border: none; box-sizing: border-box; max-width: 100%;" alt="" /></a></div>
顾名思义，当使用顺序方式去遍历时，每个item读完后再读下一个item。
而使用并行去遍历时，数组会被分成多个段，其中每一个都在不同的线程中处理，然后将结果一起输出。
并行流原理：
List originalList = someData;
split1 = originalList(0, mid);//将数据分小部分
split2 = originalList(mid,end);
new Runnable(split1.process());//小部分执行操作
new Runnable(split2.process());
List revisedList = split1 + split2;//将结果合并
性能：如果是多核机器，理论上并行流则会比顺序流快上一倍。
以下是借用他人的一个测试两者性能的Demo.
<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">package com.lambda.stream;
import java.util.stream.IntStream;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> TestPerformance {
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">main</span>(String[] args) {
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">long</span> t0 = System.nanoTime();
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//初始化一个范围100万整数流,求能被2整除的数字，toArray（）是终点方法</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> a[]=IntStream.range(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>_000_000).filter(p -> p % <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>==<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>).toArray();
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">long</span> t1 = System.nanoTime();
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//和上面功能一样，这里是用并行流来计算</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> b[]=IntStream.range(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>_000_000).parallel().filter(p -> p % <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>==<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>).toArray();
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">long</span> t2 = System.nanoTime();
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//我本机的结果是serial: 0.06s, parallel 0.02s，证明并行流确实比顺序流快</span>
System.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">out</span>.printf(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"serial: %.2fs, parallel %.2fs%n"</span>, (t1 - t0) * <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1e-9</span>, (t2 - t1) * <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1e-9</span>);
}
}
</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li></ul>运行结果:
<code class="hljs avrasm has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-label" style="box-sizing: border-box;">serial:</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0.07</span>s parallel <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0.02</span>s</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>
可以看出，并行流的效率确实提高了3.5倍(我本机是4核,电脑较差.)
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