//1、通过Collection系列提供的stream()（串行） 或parallelStream()（并行）获取
    List<String> list = new ArrayList<>();
    Stream<String> stream1 = list.stream();//串行流
    Stream<String> stream2 = list.parallelStream();//并行流
  //2、通过Arrays中的静态方法stream() 获取数据流
    User[] u = new User[2];
    Stream<User> stream3 = Arrays.stream(u);
  //3、通过Stream；类中的静态方法of()
    Stream<String> stream4 = Stream.of("11","2");

/**
   * 打印年龄大于18的前4位用户信息（不重复）
   * 并跳过第1个用户
   */
@Test
  public void test1(){
    list.stream()
    .filter((x)->x.getAge()>18)
    .distinct()
    .limit(4)
    .skip(1).forEach(System.out::println);
  }

 @Test
  public void test2(){
    ///map
    list.stream().map(User::getName)
    .forEach(System.out::println);
    //flatMap
    List<List<User>> list1 = new ArrayList<>();
    list1.add(list);
    list1.stream().flatMap(Stream::getNames)
    .forEach(System.out::println);
  }
  public static Stream<String> getNames(List<User> list){
    List<String> list1 = new ArrayList<String>();
    for (User user : list) {
      list1.add(user.getName());
    }
    return list1.stream();
  }

@Test
  public void test3(){
    List<String> list =Arrays.asList("aa","bb","cc","dd");
    list.stream().sorted()
    .forEach(System.out::println);
    //
    list.stream().sorted((x,y) -> {
      if(x.equals(y)){
        return 1;
      }else{
        return -1;
      }
    } ).forEach(System.out::println);
  }

@Test
  public void test4(){
    boolean b = list.stream().
        noneMatch((e) ->
        e.getName().equals("zhao"));
    System.out.println(b);
    Optional<User> op = list.parallelStream()
        .filter((x) -> x.getAge() == 18)
        .findAny();
    System.out.println(op.get());
  }

@Test
  public void test5(){
    //转List
    List<String> list1 =list.stream()
        .map(User::getName)
        .collect(Collectors.toList()) ;
    list1.forEach(System.out::println);
    //转HashSet
    HashSet<String> set = list.stream().
        map(User::getName)
        .collect(Collectors.toCollection(HashSet::new));
    set.forEach(System.out::println);
    //总数
    Long count = list.stream()
        .collect(Collectors.counting());
    System.out.println(count);
    //平均年龄
    double avAge = list.stream()
        .collect(Collectors.averagingInt(User::getAge));
    System.out.println(avAge); 
    //总年龄
    int toAge = list.stream()
        .collect(Collectors.summingInt(User::getAge));
    System.out.println(toAge);
    //最大值
    Optional<User> u = list.stream()
        .collect(Collectors.maxBy((e1,e2) 
            -> Integer.compare(e1.getAge(),e2.getAge() )));
    System.out.println(u);
    //平均年龄
    IntSummaryStatistics collect = list.stream()
        .collect(Collectors.summarizingInt(User::getAge));
    System.out.println(collect.getAverage());
    //分组
    Map<Integer, List<User>> l= list.stream()
        .collect(Collectors.groupingBy(User::getAge));
    System.out.println(l);
    //多级分组
    Map<Integer,Map<String,List<User>> > ls= list.stream()
        .collect(Collectors.groupingBy(
            User::getAge,Collectors.groupingBy(User::getSex)));
    System.out.println(ls);
    //分区
    Map<Boolean,List<User>> map= list.stream()
        .collect(Collectors.partitioningBy((x) 
            -> x.getAge()>18));
    System.out.println(map);
    //连接字符串
    String str = list.stream().map(User::getName)
        .collect(Collectors.joining(",","-","-"));
    System.out.println(str);
  }