使用Java 8 Streams的收集器根据现有键/值对增加值 [英] Using Java 8 Streams' Collectors to increment value based of existing key/value pair

问题描述

假设有一个List<Object>，并且Object包含两种方法:getUserId和getPoints.

Suppose there is a List<Object> and that Object contains two methods: getUserId and getPoints.

考虑List<Object>包含三个对象，并且它们包含以下数据:

Consider that List<Object> contains three objects, and they contain the following data:

userId A; 3 points
userId A; 5 points
userId B; 1 point

正确收集了此内容后，我希望它具有一个如下所示的Map<String, Integer>:

After collecting this properly, I am expecting to have a Map<String, Integer> that would look like this:

A: 8,
B: 1

我正在尝试使用Java的8个功能接口(流)进行此操作，因为我只需要最终结果，而在它们之间也不需要任何映射.如果没有其他方法，我愿意对此进行修改. 我首先想到了这个:

I am attempting this using Java's 8 functional interfaces, the streams, because I only need the final result and I do not have nor need any of these mappings in between. I am willing to modify this if there is no other way. I first came up with this:

this.service.getListObject()
    .stream()
    .collect(Collectors.toMap(Object::getUserId, Object::getPoints));

但是，这显示不足，因为它将始终用最新值替换密钥，从而产生:

However, this has shown insufficient because it will always replace the key with the newest value, thus producing:

A: 5,
B: 1

如何调整Collectors.toMap()中的最后一位以根据已存储在映射中的值自动增加其值，以便产生上面的结果?

How can I tweak the last bits from the Collectors.toMap() to automatically increment its value according to the value already stored in the map, so that I produce the result above?

推荐答案

使用

Use Collectors.groupingBy together with Collectors.summingInt as a downstream collector:

this.service.getListObject()
    .stream().collect(Collectors.groupingBy(
        Object::getUserId, Collectors.summingInt(Object::getPoints)));

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