如何使用Java 8流获取每个不同键的第一个值? [英] How to get the first value for each distinct keys using Java 8 streams?

问题描述

例如:我有一个带有电子邮件的用户对象列表.我想收集具有不同电子邮件的用户列表(因为使用同一电子邮件的两个用户可能是同一个人，因此我不在乎这些差异).换句话说，对于每个不同的键(电子邮件)，我想获取通过该键找到的第一个值(用户)，而忽略其余的值.

For example: I have a list of user objects with e-mails. I would like to collect the list of users with distinct e-mails (because two users with the same e-mail would be probably the same person, so I do not care about the differences). In other words, for each distinct key (e-mail), I want to grab the first value (user) found with that key and ignore the rest.

这是我想出的代码:

public List<User> getUsersToInvite(List<User> users) {
    return users
            .stream()
            // group users by their e-mail
            // the resulting map structure: email -> [user]
            .collect(Collectors.groupingBy(User::getEmail, LinkedHashMap::new, Collectors.toList()))
            // browse each map entry
            .entrySet()
            .stream()
            // get the first user object for each entry
            .map(entry -> entry.getValue().get(0))
            // collect such first entries into a list
            .collect(Collectors.toList());
}

还有一些更优雅的方式吗?

Is there some more elegant way?

推荐答案

您可以使用 values() .

You can use Collectors.toMap(keyMapper, valueMapper, mergeFunction), use the e-mail as key, user as value and ignore the key conflicts by always returning the first value. From the Map, you can then get the users by calling values().

public List<User> getUsersToInvite(List<User> users) {
    return new ArrayList<>(users.stream()
                                .collect(Collectors.toMap(User::getEmail, 
                                                          Function.identity(), 
                                                          (u1, u2) -> u1))
                                .values());
}

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