使用Java 8 Stream API根据ID和日期过滤对象 [英] Use Java 8 Stream API to filter objects based on an ID and Date

问题描述

我有一个联系类，每个实例都有一个唯一的 contactId 。

I have a Contact class, for which each instance has a unique contactId.

public class Contact {
    private Long contactId;

    ... other variables, getters, setters, etc ...
}

一个 Log 类详细说明行动由联系人执行在某个 lastUpdated 日期。

And a Log class that details an action performed by a Contact on a certain lastUpdated date.

public class Log {
    private Contact contact;
    private Date lastUpdated;
    private String action;

    ... other variables, getters, setters, etc ...
}

现在，在我的代码中，我有一个 List< Log> ，它可以包含多个 Log 个实例单个联系。我想根据<$ c $过滤列表，为每个联系只包含一个 Log 实例c> Log 对象中的lastUpdated 变量。结果列表应包含每个联系人的最新日志实例。

Now, in my code I have a List<Log> that can contain multiple Log instances for a single Contact. I would like to filter the list to include only one Log instance for each Contact, based on the lastUpdated variable in the Log object. The resulting list should contain the newest Log instance for each Contact.

我可以通过创建一个 Map< Contact，List< Log>> ，然后循环并获取 Log 每个联系的最大 lastUpdated 变量的实例，但这似乎可以做得更简单使用Java 8 Stream API。

I could do this by creating a Map<Contact, List<Log>>, then looping through and getting the Log instance with max lastUpdated variable for each Contact, but this seems like it could be done much simpler with the Java 8 Stream API.

如何使用Java 8 Stream API实现此目的？

How would one accomplish this using the Java 8 Stream API?

推荐答案

你可以链接几个收藏家来获得你想要的东西：

You can chain several collectors to get what you want:

import static java.util.stream.Collectors.*;

List<Log> list = ...
Map<Contact, Log> logs = list.stream()
    .collect(groupingBy(Log::getContact,
        collectingAndThen(maxBy(Comparator.comparing(Log::getLastUpdated)), Optional::get)));

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