Stream方法

1、filter(T-> boolean) 过滤

    public static void filter(){
        List<User> list = users();
        List<User> newlist = list.stream().filter(user -> user.getAge() > 20)
                .collect(Collectors.toList());
        for (User user : newlist) {
            System.out.println(user.getName()+" --> "+ user.getAge());
        }
    }
 /*   ---结果---
    袁天罡 --> 99
   陆佑劫 --> 45
   张天师 --> 48*/

2、distinct 去重

    数据源中复制new User("李星云", 18, 0, "渝州",new BigDecimal(1000)) 并粘贴两个
    public static void distinct(){
        List<User> list = users();
        List<User> newlist = list.stream().distinct().collect(Collectors.toList().reversed());
        //List<User> newlist = list.stream().distinct().collect(Collectors.toList());
        for (User user : newlist) {
            System.out.println(user.getName()+" --> "+ user.getAge());
        }
    }
/*    ---结果---
   李星云 --> 18
   陆林轩 --> 16
   姬如雪 --> 17
   袁天罡 --> 99
   张子凡 --> 19
   陆佑劫 --> 45
   张天师 --> 48
   蚩梦 --> 18*/
       

3、sorted 排序

    public static void sorted(){
        List<User> list = users();
        List<User> newlist = list.stream()
                .sorted(Comparator.comparingInt(User::getAge))
                .collect(Collectors.toList());
        for (User user : newlist) {
            System.out.println(user.getName()+" --> "+ user.getAge());
        }
    }
//倒叙
    list.stream().sorted(Comparator.comparing(Student::getAge).reversed())
/*    ---结果---
    陆林轩 --> 16
   姬如雪 --> 17
   李星云 --> 18
   蚩梦 --> 18
   张子凡 --> 19
   陆佑劫 --> 45
   张天师 --> 48
   袁天罡 --> 9*/

4、limit返回前n个元素

    public static void limit(){
        List<User> list = users();
        List<User> newlist = list.stream()
                .sorted(Comparator.comparingInt(User::getAge))
                .limit(2)
                .collect(Collectors.toList());
        for (User user : newlist) {
            System.out.println(user.getName()+" --> "+ user.getAge());
        }
    }
/*    ---结果---
   陆林轩 --> 16
   姬如雪 --> 17*/

5、skip去除前n个元素

    public static void skip(){
        List<User> list = users();
        List<User> newlist = list.stream()
                .sorted(Comparator.comparingInt(User::getAge))
                .skip(2)
                .collect(Collectors.toList());
        for (User user : newlist) {
            System.out.println(user.getName()+" --> "+ user.getAge());
        }
    }

/*    ---结果---
   李星云 --> 18
   蚩梦 --> 18
   张子凡 --> 19
   陆佑劫 --> 45
   张天师 --> 48
   袁天罡 --> 99*/

6、map(T->R) 输出查询的内容

 public static void map(){
        List<User> list = users();
        List<String> newlist = list.stream()
                .map(User::getName).distinct().collect(Collectors.toList());
        for (String add : newlist) {
            System.out.println(add);
        }
    }
 /*   ---结果---
   李星云
   陆林轩
   姬如雪
   袁天罡
   张子凡
   陆佑劫
   张天师
   蚩梦*/

7、flatMap(T -> Stream)

    public static void flatmap(){
        List<String> flatmap = new ArrayList<>();
        flatmap.add("常宣灵,常昊灵");
        flatmap.add("孟婆,判官红,判官蓝");
        /*
            这里原集合中的数据由逗号分割，使用split进行拆分后，得到的是Stream<String[]>，
            字符串数组组成的流，要使用flatMap的Arrays::stream
            将Stream<String[]>转为Stream<String>,然后把流相连接
   、     */
        flatmap = flatmap.stream()
                .map(s -> s.split(","))
                .flatMap(Arrays::stream)
                .collect(Collectors.toList());
        for (String name : flatmap) {
            System.out.println(name);
        }
    }
   /* ---结果---
    常宣灵
   常昊灵
   孟婆
   判官红
   判官蓝*/

8、allMatch（T->boolean）检测是否全部满足参数行为

    public static void allMatch(){
        List<User> list = users();
        boolean flag = list.stream()
                .allMatch(user -> user.getAge() >= 17);
        System.out.println(flag);
    }
/* ---结果---
false/*

9、anyMatch（T->boolean）检测是否有任意元素满足给定的条件

    public static void anyMatch(){
        List<User> list = users();
        boolean flag = list.stream()
                .anyMatch(user -> user.getSex() == 1);
        System.out.println(flag);
    }
 /*  ---结果---
   true*/

10、noneMatchT->boolean）流中是否有元素匹配给定的 T -> boolean条件

    public static void noneMatch(){
        List<User> list = users();
        boolean flag = list.stream()
                .noneMatch(user -> user.getAddress().contains("郑州"));
        System.out.println(flag);
    }
/*   ---结果---
   true*/

11、findFirst( ):找到第一个元素

    public static void findfirst(){
        List<User> list = users();
        Optional<User> optionalUser = list.stream()
                .sorted(Comparator.comparingInt(User::getAge))
                .findFirst();
        System.out.println(optionalUser.toString());
    }
 /*  ---结果---
   Optional[User{name='陆林轩', age=16, sex=1, money=500, address='渝州'}]*/

12、findAny( ):找到任意一个元素

    public static void findAny(){
        List<User> list = users();
//        Optional<User> optionalUser = list.stream()
                .findAny();
        Optional<User> optionalUser = list.stream()
                .findAny();
        System.out.println(optionalUser.toString());
    }
 /*  ---结果---
   Optional[User{name='李星云', age=18, sex=0, money=1000, address='渝州'}]*/

13、计算总数

​```java
    public static void count(){
        List<User> list = users();
        long count = list.stream().count();
        System.out.println(count);
    }
 /*   ---结果---
   8*/

14、最大值 最小值

    public static void max_min(){
        List<User> list = users();
        Optional<User> max = list.stream()
                .collect(
                        Collectors.maxBy(
                                Comparator.comparing(User::getAge)
                        )
                );
        Optional<User> min = list.stream()
                .collect(
                        Collectors.minBy(
                                Comparator.comparing(User::getAge)
                  )
                );
        System.out.println("max--> " + max+"  min--> "+ min);
    }
/*   ---结果---
   max--> Optional[User{name='袁天罡', age=99, sex=0, money=100000, address='藏兵谷'}]  min--> Optional[User{name='陆林轩', age=16, sex=1, money=500, address='渝州'}]*/

15、求和_平均值

    public static void sum_avg(){
        List<User>list = users();
        int totalAge = list.stream()
                .collect(Collectors.summingInt(User::getAge));
        System.out.println("totalAge--> "+ totalAge);

        /*获得列表对象金额， 使用reduce聚合函数,实现累加器*/
        BigDecimal totalMpney = list.stream()
                .map(User::getMoney)
                .reduce(BigDecimal.ZERO, BigDecimal::add);
        System.out.println("totalMpney--> " + totalMpney);

        double avgAge = list.stream()
                .collect(Collectors.averagingInt(User::getAge));
        System.out.println("avgAge--> " + avgAge);
    }
/*   ---结果---
   totalAge--> 280
   totalMpney--> 105700
   avgAge--> 35.0*/

16、一次性得到元素的个数、总和、最大值、最小值

    public static void allVlaue(){
        List<User> list = users();
        IntSummaryStatistics statistics = list.stream()
                .collect(Collectors.summarizingInt(User::getAge));
        System.out.println(statistics);
    }
 /*  ---结果---
   IntSummaryStatistics{count=8, sum=280, min=16, average=35.000000, max=99}*/

17、拼接

    public static void join(){
        List<User> list = users();
        String names = list.stream()
                .map(User::getName)
                .collect(Collectors.joining(", "));
        System.out.println(names);
    }
  /* ---结果---
   李星云, 陆林轩, 姬如雪, 袁天罡, 张子凡, 陆佑劫, 张天师, 蚩梦*/

18、分组

     public static void group(){
        Map<Integer, List<User>> map = users().stream()
                .collect(Collectors.groupingBy(User::getSex));
        System.out.println(new Gson().toJson(map));
        System.out.println();
        Map<Integer, Map<Integer,List<User>>> map2 = users().stream()
                .collect(Collectors.groupingBy(User::getSex,
                        Collectors.groupingBy(User::getAge)));
        System.out.println(new Gson().toJson(map2));
    }
 /*  ---结果---
{"0":[{"name":"李星云","age":18,"sex":0,"address":"渝州","money":1000},{"name":"袁天罡","age":99,"sex":0,"address":"藏兵谷","money":100000},{"name":"张子凡","age":19,"sex":0,"address":"天师府","money":900},{"name":"陆佑劫","age":45,"sex":0,"address":"不良人","money":600},{"name":"张天师","age":48,"sex":0,"address":"天师府","money":1100}],"1":[{"name":"陆林轩","age":16,"sex":1,"address":"渝州","money":500},{"name":"姬如雪","age":17,"sex":1,"address":"幻音坊","money":800},{"name":"蚩梦","age":18,"sex":1,"address":"万毒窟","money":800}]}

{"0":{"48":[{"name":"张天师","age":48,"sex":0,"address":"天师府","money":1100}],"18":[{"name":"李星云","age":18,"sex":0,"address":"渝州","money":1000}],"19":[{"name":"张子凡","age":19,"sex":0,"address":"天师府","money":900}],"99":[{"name":"袁天罡","age":99,"sex":0,"address":"藏兵谷","money":100000}],"45":[{"name":"陆佑劫","age":45,"sex":0,"address":"不良人","money":600}]},"1":{"16":[{"name":"陆林轩","age":16,"sex":1,"address":"渝州","money":500}],"17":[{"name":"姬如雪","age":17,"sex":1,"address":"幻音坊","money":800}],"18":[{"name":"蚩梦","age":18,"sex":1,"address":"万毒窟","money":800}]}}*/

19、分组合计

   
    public static void groupCount(){
        Map<Integer, Long> num = users().stream()
                .collect(Collectors.groupingBy(User::getSex, Collectors.counting()));
        System.out.println(num);
        Map<Integer, Long> num2 = users().stream()
                .filter(user -> user.getAge()>=18)
                .collect(Collectors.groupingBy(User::getSex, Collectors.counting()));
        System.out.println(num2);
    }
  /* ---结果---
   {0=5, 1=3}
   {0=5, 1=1}*/

20、分区

  public static void partitioningBy(){
        List<User> list = users();
        Map<Boolean, List<User>> part = list.stream()
                .collect(Collectors.partitioningBy(user -> user.getAge() <= 30));
        System.out.println(new Gson().toJson(part));
    }
 /*  ---结果---
    {"false":[{"name":"袁天罡","age":99,"sex":0,"address":"藏兵谷","money":100000},{"name":"陆佑劫","age":45,"sex":0,"address":"不良人","money":600},{"name":"张天师","age":48,"sex":0,"address":"天师府","money":1100}],"true":[{"name":"李星云","age":18,"sex":0,"address":"渝州","money":1000},{"name":"陆林轩","age":16,"sex":1,"address":"渝州","money":500},{"name":"姬如雪","age":17,"sex":1,"address":"幻音坊","money":800},{"name":"张子凡","age":19,"sex":0,"address":"天师府","money":900},{"name":"蚩梦","age":18,"sex":1,"address":"万毒窟","money":800}]}*/

21、计算Stream流长度

  public static void partitioningBy(){
        List<User> list = users();
        long size;
        size= list.stream().count();
        System.out.println(size);
    }