Java collect函数给出了循环推理错误 [英] Java collect function gives cyclic inference error

问题描述

当键入以下代码时，我在groupingBy函数的参数上得到循环推理错误：

When typing the following code I get a "cyclic inference" error on the argument for the groupingBy function:

Map<String, User> mapByEmail = users.stream().collect(Collectors.groupingBy(User::getEmail));

我发现这令人困惑，因为以下不会导致任何问题：

I find this confusing because the following does not cause any problem:

users.stream().collect(Collectors.groupingBy(User::getEmail));

且这两者都不是：

List<User> listByEmail = users.stream().collect(Collectors.groupingBy(User::getEmail)).values().stream().reduce(null, (a,b)-> a=b);

所以问题是，什么是循环推理，我该如何避免呢？

So the question is, what is a cyclic inference and how can I avoid it?

编辑感谢您的回答。经过进一步研究后，我发现需要通过以下方式减少我的结果：

EDIT Thanks for the answers. After further research I found out that I need to reduce my result by doing the following:

 Map<String, User> mapByEmail = users.stream().collect(Collectors.groupingBy(User::getEmail, Collectors.reducing(new User(),(a,b)-> a=b)));

推荐答案

问题是结果类型不正确。它应该是 Map< String，List< User>> ：

The problem is that your resulting type is incorrect. It should be Map<String, List<User>>:

Map<String, List<User>> mapByEmail = users.stream()
                                          .collect(Collectors.groupingBy(User::getEmail));

错误消息看起来令人困惑，但代码中确实存在错误。

The error message looks confusing, but there's actual an error in your code.

这篇关于Java collect函数给出了循环推理错误的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！