Java8- Lambda如何实现条件去重distinct List,如何实现条件分组groupBy List

电玩女神 2023-06-17 03:54 63阅读 0赞

转载自：[https://www.wanaright.com/2016/12/17/java8-lambda%E5%A6%82%E4%BD%95%E5%AE%9E%E7%8E%B0%E6%9D%A1%E4%BB%B6%E5%8E%BB%E9%87%8Ddistinct-list%E5%A6%82%E4%BD%95%E5%AE%9E%E7%8E%B0%E6%9D%A1%E4%BB%B6%E5%88%86%E7%BB%84groupby-list/][https_www.wanaright.com_2016_12_17_java8-lambda_E5_A6_82_E4_BD_95_E5_AE_9E_E7_8E_B0_E6_9D_A1_E4_BB_B6_E5_8E_BB_E9_87_8Ddistinct-list_E5_A6_82_E4_BD_95_E5_AE_9E_E7_8E_B0_E6_9D_A1_E4_BB_B6_E5_88_86_E7_BB_84groupby-list]

## 叙述 ##

我们知道, Java8 lambda自带的去重为 distinct 方法, 但是只能过滤整体对象, 不能实现对象里的某个值进行判定去重。

## 解决方案 ##

### 去重 ###

比如:

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 5, 5, 5, 6, 7);
     
    List<Integer> distinctNumbers = numbers.stream()
    .distinct()
    .collect(Collectors.toList());
    System.out.println(distinctNumbers);//1, 2, 3, 4, 5, 6, 7

但是, 如果我们有一个 List<User> 类似这样的对象, 要对 User 的 name 进行条件去重怎么办?

我们想要的效果是这样的:

List<User> distinctUsers = users.stream()
    .distinct(User::getName)
    .collect(Collectors.toList());

但是很遗憾, distinct()方法并不能设置条件. 解决方案如下:

首先定义一个过滤器:

public static <T> Predicate<T> distinctByKey(Function<? super T, Object> keyExtractor) {
        Map<Object, Boolean> seen = new ConcurrentHashMap<>();
        return object -> seen.putIfAbsent(keyExtractor.apply(object), Boolean.TRUE) == null;
    }

然后就可以进行条件去重啦:

users.add(new User("Jim"));
    users.add(new User("Jim"));
    users.add(new User("Tom"));
    users.add(new User("Leo"));
     
    List<User> distinctUsers = users.stream()
            .filter(distinctByKey(User::getName))
            .collect(Collectors.toList());
     
    System.out.println(distinctUsers);//[Jim, Tom, Leo]

### 分组 ###

还是一样的例子, 我们有一个 List<User>, User 有 name 和 age. 现在我们想把这个 List 按年龄分成三组:0<age<=20, 20<age<=40, 40<age.

直接上代码:

List<User> users = new LinkedList<>();
    users.add(new User("Jim", 12));
    users.add(new User("John", 18));
    users.add(new User("Tom", 21));
    users.add(new User("Leo", 30));
    users.add(new User("Kate", 44));
    users.add(new User("Lio", 50));
     
    Map<String, List<User>> tripleUsers = users.stream()
            .collect(Collectors.groupingBy((Function<User, String>) user -> {
        String key;
        if (user.getAge() <= 20) {
            key = "less20";
        } else if (user.getAge() <= 40) {
            key = "less40";
        } else {
            key = "more40";
        }
        return key;
    }, Collectors.toList()));
     
    System.out.println(tripleUsers);
    //{more40=[Kate, Lio], less40=[Tom, Leo], less20=[Jim, John]}

[https_www.wanaright.com_2016_12_17_java8-lambda_E5_A6_82_E4_BD_95_E5_AE_9E_E7_8E_B0_E6_9D_A1_E4_BB_B6_E5_8E_BB_E9_87_8Ddistinct-list_E5_A6_82_E4_BD_95_E5_AE_9E_E7_8E_B0_E6_9D_A1_E4_BB_B6_E5_88_86_E7_BB_84groupby-list]: https://www.wanaright.com/2016/12/17/java8-lambda%E5%A6%82%E4%BD%95%E5%AE%9E%E7%8E%B0%E6%9D%A1%E4%BB%B6%E5%8E%BB%E9%87%8Ddistinct-list%E5%A6%82%E4%BD%95%E5%AE%9E%E7%8E%B0%E6%9D%A1%E4%BB%B6%E5%88%86%E7%BB%84groupby-list/