Java8新特性-Stream - 图1
先贴上几个案例，水平高超的同学可以挑战一下：

从员工集合中筛选出salary大于8000的员工，并放置到新的集合里。
统计员工的最高薪资、平均薪资、薪资之和。
将员工按薪资从高到低排序，同样薪资者年龄小者在前。
将员工按性别分类，将员工按性别和地区分类，将员工按薪资是否高于8000分为两部分。

用传统的迭代处理也不是很难，但代码就显得冗余了，跟Stream相比高下立判。

Stream概述

Java 8 是一个非常成功的版本，这个版本新增的Stream，配合同版本出现的 Lambda ，给我们操作集合（Collection）提供了极大的便利。
那么什么是Stream？

Stream将要处理的元素集合看作一种流，在流的过程中，借助Stream API对流中的元素进行操作，比如：筛选、排序、聚合等。

Stream可以由数组或集合创建，对流的操作分为两种：

中间操作，每次返回一个新的流，可以有多个。
终端操作，每个流只能进行一次终端操作，终端操作结束后流无法再次使用。终端操作会产生一个新的集合或值。

Java8新特性-Stream - 图2
另外，Stream有几个特性：

stream不存储数据，而是按照特定的规则对数据进行计算，一般会输出结果。
stream不会改变数据源，通常情况下会产生一个新的集合或一个值。
stream具有延迟执行特性，只有调用终端操作时，中间操作才会执行。
Stream的创建
Stream可以通过集合数组创建。
（1）通过java.util.Collection下的 stream() 方法和 parallelStream() 用集合创建流: ```java List list = new ArrayList<>(); Stream stream = list.stream(); //获取一个顺序流 Stream parallelStream = list.parallelStream(); //获取一个并行流


（2）使用`java.util.Arrays.stream(T[] array)`方法用数组创建流
```java
Integer[] nums = new Integer[10];
Stream<Integer> stream = Arrays.stream(nums);

（3）使用Stream的静态方法：of()、iterate()、generate()

Stream<Integer> stream = Stream.of(1,2,3,4,5,6);
Stream<Integer> stream2 = Stream.iterate(0, (x) -> x + 2).limit(6);
stream2.forEach(System.out::println); // 0 2 4 6 8 10
Stream<Double> stream3 = Stream.generate(Math::random).limit(2);
stream3.forEach(System.out::println);

（4）使用 BufferedReader.lines() 方法，将每行内容转成流

BufferedReader reader = new BufferedReader(new FileReader("F:\\test_stream.txt"));
Stream<String> lineStream = reader.lines();
lineStream.forEach(System.out::println);

（5）使用 Pattern.splitAsStream() 方法，将字符串分隔成流

Pattern pattern = Pattern.compile(",");
Stream<String> stringStream = pattern.splitAsStream("a,b,c,d");
stringStream.forEach(System.out::println);

stream 和 parallelStream 的区分

stream和parallelStream的简单区分：
stream是顺序流，由主线程按顺序对流执行操作，而parallelStream是并行流，内部以多线程并行执行的方式对流进行操作，但前提是流中的数据处理没有顺序要求。
例如筛选集合中的奇数，两者的处理不同之处：
Java8新特性-Stream - 图3
如果流中的数据量足够大，并行流可以加快处速度。
除了直接创建并行流，还可以通过parallel()把顺序流转换成并行流：

Optional<Integer> findFirst = list.stream()
    .parallel()
    .filter(x->x>6)
    .findFirst();

Stream的使用

流的中间操作

3.1 遍历/匹配（foreach/find/match）

Stream也是支持类似集合的遍历和匹配元素的，只是Stream中的元素是以Optional类型存在的。Stream的遍历、匹配非常简单。
Java8新特性-Stream - 图4
// import已省略，请自行添加，后面代码亦是
public class StreamTest {
public static void main(String[] args) {
List list = Arrays.asList(7, 6, 9, 3, 8, 2, 1);

    <span class="token comment">// 遍历输出符合条件的元素</span>
    list<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">filter</span><span class="token punctuation">(</span>x <span class="token operator">-</span><span class="token operator">&gt;</span> x <span class="token operator">&gt;</span> <span class="token number">6</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span>System<span class="token punctuation">.</span>out<span class="token operator">:</span><span class="token operator">:</span>println<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">// 匹配第一个</span>
    Optional<span class="token generics function"><span class="token punctuation">&lt;</span>Integer<span class="token punctuation">&gt;</span></span> findFirst <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">filter</span><span class="token punctuation">(</span>x <span class="token operator">-</span><span class="token operator">&gt;</span> x <span class="token operator">&gt;</span> <span class="token number">6</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">findFirst</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">// 匹配任意（适用于并行流）</span>
    Optional<span class="token generics function"><span class="token punctuation">&lt;</span>Integer<span class="token punctuation">&gt;</span></span> findAny <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">parallelStream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">filter</span><span class="token punctuation">(</span>x <span class="token operator">-</span><span class="token operator">&gt;</span> x <span class="token operator">&gt;</span> <span class="token number">6</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">findAny</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">// 是否包含符合特定条件的元素</span>
    <span class="token keyword">boolean</span> anyMatch <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">anyMatch</span><span class="token punctuation">(</span>x <span class="token operator">-</span><span class="token operator">&gt;</span> x <span class="token operator">&lt;</span> <span class="token number">6</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"匹配第一个值："</span> <span class="token operator">+</span> findFirst<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"匹配任意一个值："</span> <span class="token operator">+</span> findAny<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"是否存在大于6的值："</span> <span class="token operator">+</span> anyMatch<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}

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3.2 筛选（filter）
筛选，是按照一定的规则校验流中的元素，将符合条件的元素提取到新的流中的操作。

案例一：筛选出Integer集合中大于7的元素，并打印出来
publicclassStreamTest{publicstaticvoidmain([]){=.asList(6,7,3,8,1,2,9);=.stream();.filter(->>7).forEach(.::);}}
```
     String args 
      List list  Arrays      
      Stream stream  list
      streamx  x  Systemoutprintln
```
1
2
3
4
5
6
7

预期结果：

8 9

案例二：筛选员工中工资高于8000的人，并形成新的集合。 形成新集合依赖collect（收集），后文有详细介绍。
publicclassStreamTest{publicstaticvoidmain([]){=newArrayList();.add(newPerson(“Tom”,8900,23,”male”,”New York”));.add(newPerson(“Jack”,7000,25,”male”,”Washington”));.add(newPerson(“Lily”,7800,21,”female”,”Washington”));.add(newPerson(“Anni”,8200,24,”female”,”New York”));.add(newPerson(“Owen”,9500,25,”male”,”New York”));.add(newPerson(“Alisa”,7900,26,”female”,”New York”));

    List<span class="token generics function"><span class="token punctuation">&lt;</span>String<span class="token punctuation">&gt;</span></span> fiterList <span class="token operator">=</span> personList<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">filter</span><span class="token punctuation">(</span>x <span class="token operator">-</span><span class="token operator">&gt;</span> x<span class="token punctuation">.</span><span class="token function">getSalary</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">&gt;</span> <span class="token number">8000</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span>Person<span class="token operator">:</span><span class="token operator">:</span>getName<span class="token punctuation">)</span>
            <span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span>Collectors<span class="token punctuation">.</span><span class="token function">toList</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">print</span><span class="token punctuation">(</span><span class="token string">"高于8000的员工姓名："</span> <span class="token operator">+</span> fiterList<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}


       String args 
        List personList   
        personList     
        personList     
        personList     
        personList     
        personList     
        personList

运行结果：

高于8000的员工姓名：[Tom, Anni, Owen]

3.3 聚合（max/min/count)

max、min、count这些字眼你一定不陌生，没错，在mysql中我们常用它们进行数据统计。Java stream中也引入了这些概念和用法，极大地方便了我们对集合、数组的数据统计工作。
Java8新特性-Stream - 图6
案例一：获取String集合中最长的元素。
publicclassStreamTest{publicstaticvoidmain([]){=.asList(“adnm”,”admmt”,”pot”,”xbangd”,”weoujgsd”);

    Optional<span class="token generics function"><span class="token punctuation">&lt;</span>String<span class="token punctuation">&gt;</span></span> max <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">max</span><span class="token punctuation">(</span>Comparator<span class="token punctuation">.</span><span class="token function">comparing</span><span class="token punctuation">(</span>String<span class="token operator">:</span><span class="token operator">:</span>length<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"最长的字符串："</span> <span class="token operator">+</span> max<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}


       String args 
        List list  Arrays

输出结果：

最长的字符串：weoujgsd

案例二：获取Integer集合中的最大值。
publicclassStreamTest{publicstaticvoidmain([]){=.asList(7,6,9,4,11,6);

    <span class="token comment">// 自然排序</span>
    Optional<span class="token generics function"><span class="token punctuation">&lt;</span>Integer<span class="token punctuation">&gt;</span></span> max <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">max</span><span class="token punctuation">(</span>Integer<span class="token operator">:</span><span class="token operator">:</span>compareTo<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">// 自定义排序</span>
    Optional<span class="token generics function"><span class="token punctuation">&lt;</span>Integer<span class="token punctuation">&gt;</span></span> max2 <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">max</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Comparator</span><span class="token generics function"><span class="token punctuation">&lt;</span>Integer<span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
        <span class="token annotation punctuation">@Override</span>
        <span class="token keyword">public</span> <span class="token keyword">int</span> <span class="token function">compare</span><span class="token punctuation">(</span>Integer o1<span class="token punctuation">,</span> Integer o2<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
            <span class="token keyword">return</span> o1<span class="token punctuation">.</span><span class="token function">compareTo</span><span class="token punctuation">(</span>o2<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"自然排序的最大值："</span> <span class="token operator">+</span> max<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"自定义排序的最大值："</span> <span class="token operator">+</span> max2<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}


       String args 
        List list  Arrays

输出结果：

自然排序的最大值：11 自定义排序的最大值：11

案例三：获取员工工资最高的人。
publicclassStreamTest{publicstaticvoidmain([]){=newArrayList();.add(newPerson(“Tom”,8900,23,”male”,”New York”));.add(newPerson(“Jack”,7000,25,”male”,”Washington”));.add(newPerson(“Lily”,7800,21,”female”,”Washington”));.add(newPerson(“Anni”,8200,24,”female”,”New York”));.add(newPerson(“Owen”,9500,25,”male”,”New York”));.add(newPerson(“Alisa”,7900,26,”female”,”New York”));

    Optional<span class="token generics function"><span class="token punctuation">&lt;</span>Person<span class="token punctuation">&gt;</span></span> max <span class="token operator">=</span> personList<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">max</span><span class="token punctuation">(</span>Comparator<span class="token punctuation">.</span><span class="token function">comparingInt</span><span class="token punctuation">(</span>Person<span class="token operator">:</span><span class="token operator">:</span>getSalary<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"员工工资最大值："</span> <span class="token operator">+</span> max<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">getSalary</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}


       String args 
        List personList   
        personList     
        personList     
        personList     
        personList     
        personList     
        personList

输出结果：

员工工资最大值：9500

案例四：计算Integer集合中大于6的元素的个数。
import..;import..;
public class StreamTest {
public static void main(String[] args) {
List list = Arrays.asList(7, 6, 4, 8, 2, 11, 9);

    <span class="token keyword">long</span> count <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">filter</span><span class="token punctuation">(</span>x <span class="token operator">-</span><span class="token operator">&gt;</span> x <span class="token operator">&gt;</span> <span class="token number">6</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">count</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"list中大于6的元素个数："</span> <span class="token operator">+</span> count<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}

 javautilArrays
 javautilList

输出结果：

list中大于6的元素个数：4

3.4 映射(map/flatMap)

映射，可以将一个流的元素按照一定的映射规则映射到另一个流中。分为map和flatMap：

map：接收一个函数作为参数，该函数会被应用到每个元素上，并将其映射成一个新的元素。
flatMap：接收一个函数作为参数，将流中的每个值都换成另一个流，然后把所有流连接成一个流。

Java8新特性-Stream - 图7
Java8新特性-Stream - 图8
案例一：英文字符串数组的元素全部改为大写。整数数组每个元素+3。
publicclassStreamTest{publicstaticvoidmain([]){[]={“abcd”,”bcdd”,”defde”,”fTr”};=.stream().map(::).collect(.toList());

    List<span class="token generics function"><span class="token punctuation">&lt;</span>Integer<span class="token punctuation">&gt;</span></span> intList <span class="token operator">=</span> Arrays<span class="token punctuation">.</span><span class="token function">asList</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">,</span> <span class="token number">7</span><span class="token punctuation">,</span> <span class="token number">9</span><span class="token punctuation">,</span> <span class="token number">11</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    List<span class="token generics function"><span class="token punctuation">&lt;</span>Integer<span class="token punctuation">&gt;</span></span> intListNew <span class="token operator">=</span> intList<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span>x <span class="token operator">-</span><span class="token operator">&gt;</span> x <span class="token operator">+</span> <span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span>Collectors<span class="token punctuation">.</span><span class="token function">toList</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"每个元素大写："</span> <span class="token operator">+</span> strList<span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"每个元素+3："</span> <span class="token operator">+</span> intListNew<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}


       String args 
        String strArr       
        List strList  ArraysstrArrStringtoUpperCaseCollectors

输出结果：

每个元素大写：[ABCD, BCDD, DEFDE, FTR] 每个元素+3：[4, 6, 8, 10, 12, 14]

案例二：将员工的薪资全部增加1000。
publicclassStreamTest{publicstaticvoidmain([]){=newArrayList();.add(newPerson(“Tom”,8900,23,”male”,”New York”));.add(newPerson(“Jack”,7000,25,”male”,”Washington”));.add(newPerson(“Lily”,7800,21,”female”,”Washington”));.add(newPerson(“Anni”,8200,24,”female”,”New York”));.add(newPerson(“Owen”,9500,25,”male”,”New York”));.add(newPerson(“Alisa”,7900,26,”female”,”New York”));

    <span class="token comment">// 不改变原来员工集合的方式</span>
    List<span class="token generics function"><span class="token punctuation">&lt;</span>Person<span class="token punctuation">&gt;</span></span> personListNew <span class="token operator">=</span> personList<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span>person <span class="token operator">-</span><span class="token operator">&gt;</span> <span class="token punctuation">{<!-- --></span>
        Person personNew <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Person</span><span class="token punctuation">(</span>person<span class="token punctuation">.</span><span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> null<span class="token punctuation">,</span> null<span class="token punctuation">)</span><span class="token punctuation">;</span>
        personNew<span class="token punctuation">.</span><span class="token function">setSalary</span><span class="token punctuation">(</span>person<span class="token punctuation">.</span><span class="token function">getSalary</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token number">10000</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">return</span> personNew<span class="token punctuation">;</span>
    <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span>Collectors<span class="token punctuation">.</span><span class="token function">toList</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"一次改动前："</span> <span class="token operator">+</span> personList<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token string">"--&gt;"</span> <span class="token operator">+</span> personList<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">getSalary</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"一次改动后："</span> <span class="token operator">+</span> personListNew<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token string">"--&gt;"</span> <span class="token operator">+</span> personListNew<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">getSalary</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">// 改变原来员工集合的方式</span>
    List<span class="token generics function"><span class="token punctuation">&lt;</span>Person<span class="token punctuation">&gt;</span></span> personListNew2 <span class="token operator">=</span> personList<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span>person <span class="token operator">-</span><span class="token operator">&gt;</span> <span class="token punctuation">{<!-- --></span>
        person<span class="token punctuation">.</span><span class="token function">setSalary</span><span class="token punctuation">(</span>person<span class="token punctuation">.</span><span class="token function">getSalary</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token number">10000</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">return</span> person<span class="token punctuation">;</span>
    <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span>Collectors<span class="token punctuation">.</span><span class="token function">toList</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"二次改动前："</span> <span class="token operator">+</span> personList<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token string">"--&gt;"</span> <span class="token operator">+</span> personListNew<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">getSalary</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"二次改动后："</span> <span class="token operator">+</span> personListNew2<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token string">"--&gt;"</span> <span class="token operator">+</span> personListNew<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">getSalary</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}


       String args 
        List personList   
        personList     
        personList     
        personList     
        personList     
        personList     
        personList

输出结果：

一次改动前：Tom–>8900 一次改动后：Tom–>18900 二次改动前：Tom–>18900 二次改动后：Tom–>18900

案例三：将两个字符数组合并成一个新的字符数组。
publicclassStreamTest{publicstaticvoidmain([]){=.asList(“m,k,l,a”,”1,3,5,7”);=.stream().flatMap(->{// 将每个元素转换成一个stream[]=.split(“,”);=.stream();return;}).collect(.toList());

    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"处理前的集合："</span> <span class="token operator">+</span> list<span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"处理后的集合："</span> <span class="token operator">+</span> listNew<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}


       String args 
        List list  Arrays 
        List listNew  lists  
            String split  s
            Stream s2  Arrayssplit
             s2
        Collectors

输出结果：

处理前的集合：[m-k-l-a, 1-3-5] 处理后的集合：[m, k, l, a, 1, 3, 5]

3.5 归约(reduce)

归约，也称缩减，顾名思义，是把一个流缩减成一个值，能实现对集合求和、求乘积和求最值操作。
Java8新特性-Stream - 图9
案例一：求Integer集合的元素之和、乘积和最大值。
publicclassStreamTest{publicstaticvoidmain([]){=.asList(1,3,2,8,11,4);// 求和方式1=.stream().reduce((,)->+);// 求和方式2=.stream().reduce(::);// 求和方式3=.stream().reduce(0,::);

    <span class="token comment">// 求乘积</span>
    Optional<span class="token generics function"><span class="token punctuation">&lt;</span>Integer<span class="token punctuation">&gt;</span></span> product <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">reduce</span><span class="token punctuation">(</span><span class="token punctuation">(</span>x<span class="token punctuation">,</span> y<span class="token punctuation">)</span> <span class="token operator">-</span><span class="token operator">&gt;</span> x <span class="token operator">*</span> y<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">// 求最大值方式1</span>
    Optional<span class="token generics function"><span class="token punctuation">&lt;</span>Integer<span class="token punctuation">&gt;</span></span> max <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">reduce</span><span class="token punctuation">(</span><span class="token punctuation">(</span>x<span class="token punctuation">,</span> y<span class="token punctuation">)</span> <span class="token operator">-</span><span class="token operator">&gt;</span> x <span class="token operator">&gt;</span> y <span class="token operator">?</span> x <span class="token operator">:</span> y<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">// 求最大值写法2</span>
    Integer max2 <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">reduce</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span> Integer<span class="token operator">:</span><span class="token operator">:</span>max<span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"list求和："</span> <span class="token operator">+</span> sum<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token string">","</span> <span class="token operator">+</span> sum2<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token string">","</span> <span class="token operator">+</span> sum3<span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"list求积："</span> <span class="token operator">+</span> product<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"list求和："</span> <span class="token operator">+</span> max<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token string">","</span> <span class="token operator">+</span> max2<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}


       String args 
        List list  Arrays     
        Optional sum  listx y  x  y
        Optional sum2  listIntegersum
        Integer sum3  list Integersum

输出结果：

list求和：29,29,29 list求积：2112 list求和：11,11

案例二：求所有员工的工资之和和最高工资。
publicclassStreamTest{publicstaticvoidmain([]){=newArrayList();.add(newPerson(“Tom”,8900,23,”male”,”New York”));.add(newPerson(“Jack”,7000,25,”male”,”Washington”));.add(newPerson(“Lily”,7800,21,”female”,”Washington”));.add(newPerson(“Anni”,8200,24,”female”,”New York”));.add(newPerson(“Owen”,9500,25,”male”,”New York”));.add(newPerson(“Alisa”,7900,26,”female”,”New York”));

    <span class="token comment">// 求工资之和方式1：</span>
    Optional<span class="token generics function"><span class="token punctuation">&lt;</span>Integer<span class="token punctuation">&gt;</span></span> sumSalary <span class="token operator">=</span> personList<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span>Person<span class="token operator">:</span><span class="token operator">:</span>getSalary<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">reduce</span><span class="token punctuation">(</span>Integer<span class="token operator">:</span><span class="token operator">:</span>sum<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">// 求工资之和方式2：</span>
    Integer sumSalary2 <span class="token operator">=</span> personList<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">reduce</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token punctuation">(</span>sum<span class="token punctuation">,</span> p<span class="token punctuation">)</span> <span class="token operator">-</span><span class="token operator">&gt;</span> sum <span class="token operator">+=</span> p<span class="token punctuation">.</span><span class="token function">getSalary</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
            <span class="token punctuation">(</span>sum1<span class="token punctuation">,</span> sum2<span class="token punctuation">)</span> <span class="token operator">-</span><span class="token operator">&gt;</span> sum1 <span class="token operator">+</span> sum2<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">// 求工资之和方式3：</span>
    Integer sumSalary3 <span class="token operator">=</span> personList<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">reduce</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token punctuation">(</span>sum<span class="token punctuation">,</span> p<span class="token punctuation">)</span> <span class="token operator">-</span><span class="token operator">&gt;</span> sum <span class="token operator">+=</span> p<span class="token punctuation">.</span><span class="token function">getSalary</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> Integer<span class="token operator">:</span><span class="token operator">:</span>sum<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">// 求最高工资方式1：</span>
    Integer maxSalary <span class="token operator">=</span> personList<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">reduce</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token punctuation">(</span>max<span class="token punctuation">,</span> p<span class="token punctuation">)</span> <span class="token operator">-</span><span class="token operator">&gt;</span> max <span class="token operator">&gt;</span> p<span class="token punctuation">.</span><span class="token function">getSalary</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">?</span> max <span class="token operator">:</span> p<span class="token punctuation">.</span><span class="token function">getSalary</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
            Integer<span class="token operator">:</span><span class="token operator">:</span>max<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">// 求最高工资方式2：</span>
    Integer maxSalary2 <span class="token operator">=</span> personList<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">reduce</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token punctuation">(</span>max<span class="token punctuation">,</span> p<span class="token punctuation">)</span> <span class="token operator">-</span><span class="token operator">&gt;</span> max <span class="token operator">&gt;</span> p<span class="token punctuation">.</span><span class="token function">getSalary</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">?</span> max <span class="token operator">:</span> p<span class="token punctuation">.</span><span class="token function">getSalary</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
            <span class="token punctuation">(</span>max1<span class="token punctuation">,</span> max2<span class="token punctuation">)</span> <span class="token operator">-</span><span class="token operator">&gt;</span> max1 <span class="token operator">&gt;</span> max2 <span class="token operator">?</span> max1 <span class="token operator">:</span> max2<span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"工资之和："</span> <span class="token operator">+</span> sumSalary<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token string">","</span> <span class="token operator">+</span> sumSalary2 <span class="token operator">+</span> <span class="token string">","</span> <span class="token operator">+</span> sumSalary3<span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"最高工资："</span> <span class="token operator">+</span> maxSalary <span class="token operator">+</span> <span class="token string">","</span> <span class="token operator">+</span> maxSalary2<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}


       String args 
        List personList   
        personList     
        personList     
        personList     
        personList     
        personList     
        personList

输出结果：

工资之和：49300,49300,49300 最高工资：9500,9500

3.6 收集(collect)

collect，收集，可以说是内容最繁多、功能最丰富的部分了。从字面上去理解，就是把一个流收集起来，最终可以是收集成一个值也可以收集成一个新的集合。

collect主要依赖java.util.stream.Collectors类内置的静态方法。

3.6.1 归集(toList/toSet/toMap)

因为流不存储数据，那么在流中的数据完成处理后，需要将流中的数据重新归集到新的集合里。toList、toSet和toMap比较常用，另外还有toCollection、toConcurrentMap等复杂一些的用法。
下面用一个案例演示toList、toSet和toMap：
publicclassStreamTest{publicstaticvoidmain([]){=.asList(1,6,3,4,6,7,9,6,20);=.stream().filter(->%2==0).collect(.toList());=.stream().filter(->%2==0).collect(.toSet());

    List<span class="token generics function"><span class="token punctuation">&lt;</span>Person<span class="token punctuation">&gt;</span></span> personList <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics function"><span class="token punctuation">&lt;</span>Person<span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    personList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Person</span><span class="token punctuation">(</span><span class="token string">"Tom"</span><span class="token punctuation">,</span> <span class="token number">8900</span><span class="token punctuation">,</span> <span class="token number">23</span><span class="token punctuation">,</span> <span class="token string">"male"</span><span class="token punctuation">,</span> <span class="token string">"New York"</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    personList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Person</span><span class="token punctuation">(</span><span class="token string">"Jack"</span><span class="token punctuation">,</span> <span class="token number">7000</span><span class="token punctuation">,</span> <span class="token number">25</span><span class="token punctuation">,</span> <span class="token string">"male"</span><span class="token punctuation">,</span> <span class="token string">"Washington"</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    personList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Person</span><span class="token punctuation">(</span><span class="token string">"Lily"</span><span class="token punctuation">,</span> <span class="token number">7800</span><span class="token punctuation">,</span> <span class="token number">21</span><span class="token punctuation">,</span> <span class="token string">"female"</span><span class="token punctuation">,</span> <span class="token string">"Washington"</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    personList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Person</span><span class="token punctuation">(</span><span class="token string">"Anni"</span><span class="token punctuation">,</span> <span class="token number">8200</span><span class="token punctuation">,</span> <span class="token number">24</span><span class="token punctuation">,</span> <span class="token string">"female"</span><span class="token punctuation">,</span> <span class="token string">"New York"</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    Map<span class="token operator">&lt;</span><span class="token operator">?</span><span class="token punctuation">,</span> Person<span class="token operator">&gt;</span> map <span class="token operator">=</span> personList<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">filter</span><span class="token punctuation">(</span>p <span class="token operator">-</span><span class="token operator">&gt;</span> p<span class="token punctuation">.</span><span class="token function">getSalary</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">&gt;</span> <span class="token number">8000</span><span class="token punctuation">)</span>
            <span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span>Collectors<span class="token punctuation">.</span><span class="token function">toMap</span><span class="token punctuation">(</span>Person<span class="token operator">:</span><span class="token operator">:</span>getName<span class="token punctuation">,</span> p <span class="token operator">-</span><span class="token operator">&gt;</span> p<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"toList:"</span> <span class="token operator">+</span> listNew<span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"toSet:"</span> <span class="token operator">+</span> set<span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"toMap:"</span> <span class="token operator">+</span> map<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}


       String args 
        List list  Arrays        
        List listNew  listx  x    Collectors
        Set set  listx  x    Collectors

运行结果：

toList：[6, 4, 6, 6, 20] toSet：[4, 20, 6] toMap：{Tom=mutest.Person@5fd0d5ae, Anni=mutest.Person@2d98a335}

3.6.2 统计(count/averaging)

Collectors提供了一系列用于数据统计的静态方法：

计数：count
平均值：averagingInt、averagingLong、averagingDouble
最值：maxBy、minBy
求和：summingInt、summingLong、summingDouble
统计以上所有：summarizingInt、summarizingLong、summarizingDouble

案例：统计员工人数、平均工资、工资总额、最高工资。
publicclassStreamTest{publicstaticvoidmain([]){=newArrayList();.add(newPerson(“Tom”,8900,23,”male”,”New York”));.add(newPerson(“Jack”,7000,25,”male”,”Washington”));.add(newPerson(“Lily”,7800,21,”female”,”Washington”));

    <span class="token comment">// 求总数</span>
    Long count <span class="token operator">=</span> personList<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span>Collectors<span class="token punctuation">.</span><span class="token function">counting</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">// 求平均工资</span>
    Double average <span class="token operator">=</span> personList<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span>Collectors<span class="token punctuation">.</span><span class="token function">averagingDouble</span><span class="token punctuation">(</span>Person<span class="token operator">:</span><span class="token operator">:</span>getSalary<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">// 求最高工资</span>
    Optional<span class="token generics function"><span class="token punctuation">&lt;</span>Integer<span class="token punctuation">&gt;</span></span> max <span class="token operator">=</span> personList<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span>Person<span class="token operator">:</span><span class="token operator">:</span>getSalary<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span>Collectors<span class="token punctuation">.</span><span class="token function">maxBy</span><span class="token punctuation">(</span>Integer<span class="token operator">:</span><span class="token operator">:</span>compare<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">// 求工资之和</span>
    Integer sum <span class="token operator">=</span> personList<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span>Collectors<span class="token punctuation">.</span><span class="token function">summingInt</span><span class="token punctuation">(</span>Person<span class="token operator">:</span><span class="token operator">:</span>getSalary<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">// 一次性统计所有信息</span>
    DoubleSummaryStatistics collect <span class="token operator">=</span> personList<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span>Collectors<span class="token punctuation">.</span><span class="token function">summarizingDouble</span><span class="token punctuation">(</span>Person<span class="token operator">:</span><span class="token operator">:</span>getSalary<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"员工总数："</span> <span class="token operator">+</span> count<span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"员工平均工资："</span> <span class="token operator">+</span> average<span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"员工工资总和："</span> <span class="token operator">+</span> sum<span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"员工工资所有统计："</span> <span class="token operator">+</span> collect<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}


       String args 
        List personList   
        personList     
        personList     
        personList

运行结果：

员工总数：3 员工平均工资：7900.0 员工工资总和：23700 员工工资所有统计：DoubleSummaryStatistics{count=3, sum=23700.000000,min=7000.000000, average=7900.000000, max=8900.000000}

3.6.3 分组(partitioningBy/groupingBy)

分区：将stream按条件分为两个Map，比如员工按薪资是否高于8000分为两部分。
分组：将集合分为多个Map，比如员工按性别分组。有单级分组和多级分组。

Java8新特性-Stream - 图10
案例：将员工按薪资是否高于8000分为两部分；将员工按性别和地区分组
publicclassStreamTest{publicstaticvoidmain([]){=newArrayList();.add(newPerson(“Tom”,8900,”male”,”New York”));.add(newPerson(“Jack”,7000,”male”,”Washington”));.add(newPerson(“Lily”,7800,”female”,”Washington”));.add(newPerson(“Anni”,8200,”female”,”New York”));.add(newPerson(“Owen”,9500,”male”,”New York”));.add(newPerson(“Alisa”,7900,”female”,”New York”));

    <span class="token comment">// 将员工按薪资是否高于8000分组</span>
    Map<span class="token operator">&lt;</span>Boolean<span class="token punctuation">,</span> List<span class="token generics function"><span class="token punctuation">&lt;</span>Person<span class="token punctuation">&gt;</span></span><span class="token operator">&gt;</span> part <span class="token operator">=</span> personList<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span>Collectors<span class="token punctuation">.</span><span class="token function">partitioningBy</span><span class="token punctuation">(</span>x <span class="token operator">-</span><span class="token operator">&gt;</span> x<span class="token punctuation">.</span><span class="token function">getSalary</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">&gt;</span> <span class="token number">8000</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">// 将员工按性别分组</span>
    Map<span class="token operator">&lt;</span>String<span class="token punctuation">,</span> List<span class="token generics function"><span class="token punctuation">&lt;</span>Person<span class="token punctuation">&gt;</span></span><span class="token operator">&gt;</span> group <span class="token operator">=</span> personList<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span>Collectors<span class="token punctuation">.</span><span class="token function">groupingBy</span><span class="token punctuation">(</span>Person<span class="token operator">:</span><span class="token operator">:</span>getSex<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">// 将员工先按性别分组，再按地区分组</span>
    Map<span class="token operator">&lt;</span>String<span class="token punctuation">,</span> Map<span class="token operator">&lt;</span>String<span class="token punctuation">,</span> List<span class="token generics function"><span class="token punctuation">&lt;</span>Person<span class="token punctuation">&gt;</span></span><span class="token operator">&gt;&gt;</span> group2 <span class="token operator">=</span> personList<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span>Collectors<span class="token punctuation">.</span><span class="token function">groupingBy</span><span class="token punctuation">(</span>Person<span class="token operator">:</span><span class="token operator">:</span>getSex<span class="token punctuation">,</span> Collectors<span class="token punctuation">.</span><span class="token function">groupingBy</span><span class="token punctuation">(</span>Person<span class="token operator">:</span><span class="token operator">:</span>getArea<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"员工按薪资是否大于8000分组情况："</span> <span class="token operator">+</span> part<span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"员工按性别分组情况："</span> <span class="token operator">+</span> group<span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"员工按性别、地区："</span> <span class="token operator">+</span> group2<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}


       String args 
        List personList   
        personList    
        personList    
        personList    
        personList    
        personList    
        personList

输出结果：
8000{false=[.@2d98a335,.@16b98e56,.@7ef20235],true=[.@27d6c5e0,.@4f3f5b24,.@15aeb7ab]}{=[.@16b98e56,.@4f3f5b24,.@7ef20235],=[.@27d6c5e0,.@2d98a335,.@15aeb7ab]}{={York=[.@4f3f5b24,.@7ef20235],=[.@16b98e56]},={York=[.@27d6c5e0,.@15aeb7ab],=[.@2d98a335]}}

员工按薪资是否大于分组情况：mutestPerson mutestPerson mutestPerson mutestPerson mutestPerson mutestPerson
员工按性别分组情况：femalemutestPerson mutestPerson mutestPerson malemutestPerson mutestPerson mutestPerson
员工按性别、地区：femaleNew mutestPerson mutestPerson WashingtonmutestPerson maleNew mutestPerson mutestPerson WashingtonmutestPerson

3.6.4 接合(joining)

joining可以将stream中的元素用特定的连接符（没有的话，则直接连接）连接成一个字符串。
publicclassStreamTest{publicstaticvoidmain([]){=newArrayList();.add(newPerson(“Tom”,8900,23,”male”,”New York”));.add(newPerson(“Jack”,7000,25,”male”,”Washington”));.add(newPerson(“Lily”,7800,21,”female”,”Washington”));

  String names <span class="token operator">=</span> personList<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span>p <span class="token operator">-</span><span class="token operator">&gt;</span> p<span class="token punctuation">.</span><span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span>Collectors<span class="token punctuation">.</span><span class="token function">joining</span><span class="token punctuation">(</span><span class="token string">","</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"所有员工的姓名："</span> <span class="token operator">+</span> names<span class="token punctuation">)</span><span class="token punctuation">;</span>
  List<span class="token generics function"><span class="token punctuation">&lt;</span>String<span class="token punctuation">&gt;</span></span> list <span class="token operator">=</span> Arrays<span class="token punctuation">.</span><span class="token function">asList</span><span class="token punctuation">(</span><span class="token string">"A"</span><span class="token punctuation">,</span> <span class="token string">"B"</span><span class="token punctuation">,</span> <span class="token string">"C"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  String string <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span>Collectors<span class="token punctuation">.</span><span class="token function">joining</span><span class="token punctuation">(</span><span class="token string">"-"</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"拼接后的字符串："</span> <span class="token operator">+</span> string<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}


     String args 
      List personList   
      personList     
      personList     
      personList

运行结果：

所有员工的姓名：Tom,Jack,Lily 拼接后的字符串：A-B-C

3.6.5 归约(reducing)

Collectors类提供的reducing方法，相比于stream本身的reduce方法，增加了对自定义归约的支持。
publicclassStreamTest{publicstaticvoidmain([]){=newArrayList();.add(newPerson(“Tom”,8900,23,”male”,”New York”));.add(newPerson(“Jack”,7000,25,”male”,”Washington”));.add(newPerson(“Lily”,7800,21,”female”,”Washington”));

    <span class="token comment">// 每个员工减去起征点后的薪资之和（这个例子并不严谨，但一时没想到好的例子）</span>
    Integer sum <span class="token operator">=</span> personList<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span>Collectors<span class="token punctuation">.</span><span class="token function">reducing</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> Person<span class="token operator">:</span><span class="token operator">:</span>getSalary<span class="token punctuation">,</span> <span class="token punctuation">(</span>i<span class="token punctuation">,</span> j<span class="token punctuation">)</span> <span class="token operator">-</span><span class="token operator">&gt;</span> <span class="token punctuation">(</span>i <span class="token operator">+</span> j <span class="token operator">-</span> <span class="token number">5000</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"员工扣税薪资总和："</span> <span class="token operator">+</span> sum<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">// stream的reduce</span>
    Optional<span class="token generics function"><span class="token punctuation">&lt;</span>Integer<span class="token punctuation">&gt;</span></span> sum2 <span class="token operator">=</span> personList<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span>Person<span class="token operator">:</span><span class="token operator">:</span>getSalary<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">reduce</span><span class="token punctuation">(</span>Integer<span class="token operator">:</span><span class="token operator">:</span>sum<span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"员工薪资总和："</span> <span class="token operator">+</span> sum2<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}


       String args 
        List personList   
        personList     
        personList     
        personList

运行结果：

员工扣税薪资总和：8700 员工薪资总和：23700

3.7 排序(sorted)

sorted，中间操作。有两种排序：

sorted()：自然排序，流中元素需实现Comparable接口
sorted(Comparator com)：Comparator排序器自定义排序

案例：将员工按工资由高到低（工资一样则按年龄由大到小）排序
publicclassStreamTest{publicstaticvoidmain([]){=newArrayList();

    personList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Person</span><span class="token punctuation">(</span><span class="token string">"Sherry"</span><span class="token punctuation">,</span> <span class="token number">9000</span><span class="token punctuation">,</span> <span class="token number">24</span><span class="token punctuation">,</span> <span class="token string">"female"</span><span class="token punctuation">,</span> <span class="token string">"New York"</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    personList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Person</span><span class="token punctuation">(</span><span class="token string">"Tom"</span><span class="token punctuation">,</span> <span class="token number">8900</span><span class="token punctuation">,</span> <span class="token number">22</span><span class="token punctuation">,</span> <span class="token string">"male"</span><span class="token punctuation">,</span> <span class="token string">"Washington"</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    personList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Person</span><span class="token punctuation">(</span><span class="token string">"Jack"</span><span class="token punctuation">,</span> <span class="token number">9000</span><span class="token punctuation">,</span> <span class="token number">25</span><span class="token punctuation">,</span> <span class="token string">"male"</span><span class="token punctuation">,</span> <span class="token string">"Washington"</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    personList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Person</span><span class="token punctuation">(</span><span class="token string">"Lily"</span><span class="token punctuation">,</span> <span class="token number">8800</span><span class="token punctuation">,</span> <span class="token number">26</span><span class="token punctuation">,</span> <span class="token string">"male"</span><span class="token punctuation">,</span> <span class="token string">"New York"</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    personList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Person</span><span class="token punctuation">(</span><span class="token string">"Alisa"</span><span class="token punctuation">,</span> <span class="token number">9000</span><span class="token punctuation">,</span> <span class="token number">26</span><span class="token punctuation">,</span> <span class="token string">"female"</span><span class="token punctuation">,</span> <span class="token string">"New York"</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">// 按工资升序排序（自然排序）</span>
    List<span class="token generics function"><span class="token punctuation">&lt;</span>String<span class="token punctuation">&gt;</span></span> newList <span class="token operator">=</span> personList<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">sorted</span><span class="token punctuation">(</span>Comparator<span class="token punctuation">.</span><span class="token function">comparing</span><span class="token punctuation">(</span>Person<span class="token operator">:</span><span class="token operator">:</span>getSalary<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span>Person<span class="token operator">:</span><span class="token operator">:</span>getName<span class="token punctuation">)</span>
            <span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span>Collectors<span class="token punctuation">.</span><span class="token function">toList</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">// 按工资倒序排序</span>
    List<span class="token generics function"><span class="token punctuation">&lt;</span>String<span class="token punctuation">&gt;</span></span> newList2 <span class="token operator">=</span> personList<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">sorted</span><span class="token punctuation">(</span>Comparator<span class="token punctuation">.</span><span class="token function">comparing</span><span class="token punctuation">(</span>Person<span class="token operator">:</span><span class="token operator">:</span>getSalary<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">reversed</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
            <span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span>Person<span class="token operator">:</span><span class="token operator">:</span>getName<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span>Collectors<span class="token punctuation">.</span><span class="token function">toList</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">// 先按工资再按年龄升序排序</span>
    List<span class="token generics function"><span class="token punctuation">&lt;</span>String<span class="token punctuation">&gt;</span></span> newList3 <span class="token operator">=</span> personList<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
            <span class="token punctuation">.</span><span class="token function">sorted</span><span class="token punctuation">(</span>Comparator<span class="token punctuation">.</span><span class="token function">comparing</span><span class="token punctuation">(</span>Person<span class="token operator">:</span><span class="token operator">:</span>getSalary<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">thenComparing</span><span class="token punctuation">(</span>Person<span class="token operator">:</span><span class="token operator">:</span>getAge<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span>Person<span class="token operator">:</span><span class="token operator">:</span>getName<span class="token punctuation">)</span>
            <span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span>Collectors<span class="token punctuation">.</span><span class="token function">toList</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">// 先按工资再按年龄自定义排序（降序）</span>
    List<span class="token generics function"><span class="token punctuation">&lt;</span>String<span class="token punctuation">&gt;</span></span> newList4 <span class="token operator">=</span> personList<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">sorted</span><span class="token punctuation">(</span><span class="token punctuation">(</span>p1<span class="token punctuation">,</span> p2<span class="token punctuation">)</span> <span class="token operator">-</span><span class="token operator">&gt;</span> <span class="token punctuation">{<!-- --></span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span>p1<span class="token punctuation">.</span><span class="token function">getSalary</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">==</span> p2<span class="token punctuation">.</span><span class="token function">getSalary</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
            <span class="token keyword">return</span> p2<span class="token punctuation">.</span><span class="token function">getAge</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-</span> p1<span class="token punctuation">.</span><span class="token function">getAge</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{<!-- --></span>
            <span class="token keyword">return</span> p2<span class="token punctuation">.</span><span class="token function">getSalary</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-</span> p1<span class="token punctuation">.</span><span class="token function">getSalary</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span>Person<span class="token operator">:</span><span class="token operator">:</span>getName<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span>Collectors<span class="token punctuation">.</span><span class="token function">toList</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"按工资升序排序："</span> <span class="token operator">+</span> newList<span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"按工资降序排序："</span> <span class="token operator">+</span> newList2<span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"先按工资再按年龄升序排序："</span> <span class="token operator">+</span> newList3<span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"先按工资再按年龄自定义降序排序："</span> <span class="token operator">+</span> newList4<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}


       String args 
        List personList

运行结果：

按工资自然排序：[Lily, Tom, Sherry, Jack, Alisa] 按工资降序排序：[Sherry, Jack, Alisa,Tom, Lily] 先按工资再按年龄自然排序：[Sherry, Jack, Alisa, Tom, Lily] 先按工资再按年龄自定义降序排序：[Alisa, Jack, Sherry, Tom, Lily]

3.8 提取/组合

流也可以进行合并、去重、限制、跳过等操作。
Java8新特性-Stream - 图11
Java8新特性-Stream - 图12
Java8新特性-Stream - 图13
publicclassStreamTest{publicstaticvoidmain([]){[]={“a”,”b”,”c”,”d”};[]={“d”,”e”,”f”,”g”};

    Stream<span class="token generics function"><span class="token punctuation">&lt;</span>String<span class="token punctuation">&gt;</span></span> stream1 <span class="token operator">=</span> Stream<span class="token punctuation">.</span><span class="token function">of</span><span class="token punctuation">(</span>arr1<span class="token punctuation">)</span><span class="token punctuation">;</span>
    Stream<span class="token generics function"><span class="token punctuation">&lt;</span>String<span class="token punctuation">&gt;</span></span> stream2 <span class="token operator">=</span> Stream<span class="token punctuation">.</span><span class="token function">of</span><span class="token punctuation">(</span>arr2<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">// concat:合并两个流 distinct：去重</span>
    List<span class="token generics function"><span class="token punctuation">&lt;</span>String<span class="token punctuation">&gt;</span></span> newList <span class="token operator">=</span> Stream<span class="token punctuation">.</span><span class="token function">concat</span><span class="token punctuation">(</span>stream1<span class="token punctuation">,</span> stream2<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">distinct</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span>Collectors<span class="token punctuation">.</span><span class="token function">toList</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">// limit：限制从流中获得前n个数据</span>
    List<span class="token generics function"><span class="token punctuation">&lt;</span>Integer<span class="token punctuation">&gt;</span></span> collect <span class="token operator">=</span> Stream<span class="token punctuation">.</span><span class="token function">iterate</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span> x <span class="token operator">-</span><span class="token operator">&gt;</span> x <span class="token operator">+</span> <span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">limit</span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span>Collectors<span class="token punctuation">.</span><span class="token function">toList</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">// skip：跳过前n个数据</span>
    List<span class="token generics function"><span class="token punctuation">&lt;</span>Integer<span class="token punctuation">&gt;</span></span> collect2 <span class="token operator">=</span> Stream<span class="token punctuation">.</span><span class="token function">iterate</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span> x <span class="token operator">-</span><span class="token operator">&gt;</span> x <span class="token operator">+</span> <span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">skip</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">limit</span><span class="token punctuation">(</span><span class="token number">5</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span>Collectors<span class="token punctuation">.</span><span class="token function">toList</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"流合并："</span> <span class="token operator">+</span> newList<span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"limit："</span> <span class="token operator">+</span> collect<span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"skip："</span> <span class="token operator">+</span> collect2<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}


       String args 
        String arr1       
        String arr2

运行结果：

流合并：[a, b, c, d, e, f, g] limit：[1, 3, 5, 7, 9, 11, 13, 15, 17, 19] skip：[3, 5, 7, 9, 11]

4 Stream源码解读

这部分等有时间慢慢分解吧。
好，以上就是全部内容，能坚持看到这里，你一定很有收获，那么动一动拿offer的小手，点个赞再走吧。

Java8新特性-Stream

Stream概述

Stream的创建

stream 和 parallelStream 的区分

Stream的使用

流的中间操作

3.1 遍历/匹配（foreach/find/match）

3.2 筛选（filter）

3.3 聚合（max/min/count)

3.4 映射(map/flatMap)

3.5 归约(reduce)

3.6 收集(collect)

3.6.1 归集(toList/toSet/toMap)

3.6.2 统计(count/averaging)

3.6.3 分组(partitioningBy/groupingBy)

3.6.4 接合(joining)

3.6.5 归约(reducing)

3.7 排序(sorted)

3.8 提取/组合

4 Stream源码解读