java8 stream中的Collectors的使用

public static void main(String[] args) {
    @AllArgsConstructor
    @Data
    class User{
        private String name;
        private Integer score;
    }

    User user1 = new User("zhangsan", 60);
    User user2 = new User("lisi", 80);
    User user3 = new User("zhangsan", 80);
    User user4 = new User("wangwu", 50);
    User user5 = new User("wangwu2", 50);

    List<User> userList = Arrays.asList(user1, user2, user3, user4, user5);

    //算出分数最小的那个并输出
    userList.stream().collect(Collectors.minBy(Comparator.comparingInt(User::getScore))).ifPresent(System.out::println);

    //算出分数最大的那个并输出（无法做到多个并列的时候求值）
    Optional optional = userList.stream().collect(Collectors.maxBy(Comparator.comparingInt(User::getScore)));
    //optional.isPresent(System.out::println);//isPresent是判断是否存在，不能接受参数
    optional.ifPresent(System.out::println);//直接使用时ifPresent

    //算出分数平均值并输出
    double averagint = userList.stream().collect(Collectors.averagingInt(User::getScore));
    System.out.println("averagint = " + averagint);

    //算出分数总和并输出
    int summingInt = userList.stream().collect(Collectors.summingInt(User::getScore));
    System.out.println("summingInt = " + summingInt);

    //算出汇总信息
    IntSummaryStatistics intSummaryStatistics = userList.stream().collect(Collectors.summarizingInt(User::getScore));
    System.out.println("intSummaryStatistics = " + intSummaryStatistics);

    //拼接名字
    String nameStrs = userList.stream().map(User::getName).collect(Collectors.joining());
    System.out.println("nameStrs = " + nameStrs);

    //拼接名字，调用另外一个方法，可以加前缀和后缀
    String nameStrs2 = userList.stream().map(User::getName).collect(Collectors.joining(", ", "[", "]"));
    System.out.println("nameStrs2 = " + nameStrs2);

    //分组：按照分数（返回的map的key是根据分组的条件来决定的，score是int，那么key就是Integer）
    Map<Integer, List<User>> scoreUsers = userList.stream().collect(Collectors.groupingBy(User::getScore));
    System.out.println("scoreUsers = " + scoreUsers);

    //二级分组：线按照分数分组，返回一个Map<Integer, List<User>>, 在根据用户名分组
    Map<Integer, Map<String, List<User>>> scoreNameUsers = userList.stream().collect(Collectors.groupingBy(User::getScore, Collectors.groupingBy(User::getName)));
    System.out.println("scoreNameUsers = " + scoreNameUsers);

    //分区,是否及格
    Map<Boolean, List<User>> jigeUsers = userList.stream().collect(Collectors.partitioningBy(user -> user.getScore() >= 60));
    System.out.println("jigeUsers = " + jigeUsers);

    //二级分区,是否及格,及格里面是否大于80
    Map<Boolean, Map<Boolean, List<User>>> youxiuUsers = userList.stream().collect(Collectors.partitioningBy(user -> user.getScore() >= 60, Collectors.partitioningBy(user -> user.getScore() >= 80)));
    System.out.println("youxiuUsers = " + youxiuUsers);

    //分区,是否及格,算出及格的个数
    Map<Boolean, Long> jigeUserCount = userList.stream().collect(Collectors.partitioningBy(user -> user.getScore() >= 60, Collectors.counting()));
    System.out.println("jigeUserCount = " + jigeUserCount);

    //先按照名字分组,获取每个分组分数最小的
    Map<String, User> UserCount = userList.stream().collect(Collectors.groupingBy(User::getName, Collectors.collectingAndThen(Collectors.minBy(Comparator.comparingInt(User::getScore)), Optional::get)));
    System.out.println("UserCount = " + UserCount);


}

转载修改自：https://blog.csdn.net/lengguoxing/article/details/78415008