1. 什么是stream？
   Stream流和传统的IO流虽然都叫做流，但是他们是两种完全不同的东西。
   流可以简单的说是处理数据集合的东西，可以使用流式API来处理集合，而不是写一个逻辑来实现。
   stream位于java.util.stream.Stream

2. 流的分类
   2.1 顺序流
   顺序流即每个指令按顺序执行。

    List<Integer> nums = Arrays.asList(1,2,3,4);
    nums.stream().forEach(n->System.out.println(n));
   

   总是按顺序输出1234

   2.2 并行流
   集合中的操作并行执行。

   List<Integer> nums = Arrays.asList(1,2,3,4);
   nums.parallelStream().forEach(n->System.out.println(n));
   

   不一定是按顺序输出1234，乱序的，每次结果不一定。
   并行流使用了ForkJoinPool。

3. 创建流
   3.1 Stream.of() 可变参数

     Stream<String> stream1 = Stream.of("A","B","C");
     System.out.println("stream1:"+ stream1.collect(Collectors.joining()));
   
   

   3.2 Stream.of() 数组

       String strings[] = new String[]{"A","B","C"};
       Stream<String> stream1 = Stream.of(strings);
       System.out.println("stream1:"+ stream1.collect(Collectors.joining()));
   
   

   3.3 Arrays.stream()

        String strings[] = new String[]{"A","B","C"};
        Stream<String> stream1 = Arrays.stream(strings);
        System.out.println("stream1:"+ stream1.collect(Collectors.joining()));
    
   

   3.4 List

        List<String> list = Arrays.asList("A","B","C");
        Stream<String> stream1 = list.stream();
        System.out.println("stream1:"+ stream1.collect(Collectors.joining()));
    
   

   3.5 Set

        Set<String> set = new HashSet<>(Arrays.asList("A","B","C"));
        Stream<String> stream1 = set.stream();
        System.out.println("stream1:"+ stream1.collect(Collectors.joining()));
    
   

   3.6 Map

        Map<String,String> map = new HashMap<>();
        map.put("A","A");
        map.put("B","B");
        map.put("C","C");
        Stream<String> stream1 = map.values().stream();
        System.out.println("stream1:"+ stream1.collect(Collectors.joining()));
    
   

   3.7 Stream.iterate()

        Stream<String> stream1 = Stream.iterate("A",e->String.valueOf((char) (e.charAt(0)+1))).limit(3);
        System.out.println("stream1:"+ stream1.collect(Collectors.joining()));
    
   

   3.8 Pattern

        String string = "A,B,C";
        Stream<String> stream1 = Pattern.compile("\\W").splitAsStream(string);
        System.out.println("stream1:"+ stream1.collect(Collectors.joining()));
    
   

   3.9 Files.lines()

        Stream<String> stream1 = Files.lines(Paths.get("文件路径"));
        System.out.println("stream1:"+ stream1.collect(Collectors.joining()));
    
   

   3.10 Stream.generate()

        Stream<String> stream1 = Stream.generate(()->"A").limit(3);
        System.out.println("stream1:"+ stream1.collect(Collectors.joining()));
    
   

4. Stream中map和flatMap的区别
   4.1 map
   对流中的每个元素进行转换。例如 对List<String>集合每个元素后面都加上一个","；List<String>转List<Long>等。

        List<String> strings = Arrays.asList("A","B","C");
        List<String> mapList = strings.stream().map(e->e+",").collect(Collectors.toList());
        mapList.forEach(System.out::println);
        
        List<String> strings2 = Arrays.asList("11","12","13");
        List<Long> longs = strings2.stream().map(Long::valueOf).collect(Collectors.toList());
        longs.forEach(System.out::println);

4.2 flatMap
对流中每个元素进行平铺，形成多个流合在一起。
例如：合并三个字符串数组
普通情况：

String arr1[] = {"A","B","C"};
        String arr2[] = {"D","E","F"};
        String arr3[] = {"G","H","I"};

        List<String[]> list = Stream.of(arr1,arr2,arr3).collect(Collectors.toList());

flatMap：

   List<String> list2 = list.stream().flatMap(Arrays::stream).collect(Collectors.toList());

可以看出：flatMap把数组流中的元素全部合并到一个流中了。