流分组方式:减少到列表的第一个元素 [英] Stream groupingBy: reducing to first element of list

问题描述

我有一个List<Valuta>，可以表示(简化)JSON样式:

I have a List<Valuta> which can be represented (simplified) JSON-style:

[{codice = EUR，描述=欧元，比率= 1}，{codice = USD， description =美元，比率= 1.1}]

[ { codice=EUR, description=Euro, ratio=1 }, { codice=USD, description=Dollars, ratio=1.1 } ]

我想像这样在Map<String, Valuta>中对其进行转换:

I want to transform that in a Map<String, Valuta> like this:

{EUR = {codice = EUR，描述=欧元，比率= 1}，USD = {codice = USD， description =美元，比率= 1.1}}

{ EUR={ codice=EUR, description=Euro, ratio=1 }, USD={ codice=USD, description=Dollars, ratio=1.1 }}

我写了这样的一句话:

getValute().stream().collect(Collectors.groupingBy(Valuta::getCodice));

但是这会返回Map<String, List<Valuta>>而不是我所需要的.

but this returns a Map<String, List<Valuta>> instead of what I need.

我想mapping()函数对我有用，但是不知道如何.

I suppose mapping() function would work for me, but don't know how.

推荐答案

实际上，您需要在此处使用Collectors.toMap而不是Collectors.groupingBy:

Actually, you need to use Collectors.toMap here instead of Collectors.groupingBy:

Map<String, Valuta> map = 
    getValute().stream()
               .collect(Collectors.toMap(Valuta::getCodice, Function.identity()));

toMap will collect the elements into a Map where the key is the result of applying a given key mapper and the value is the result of applying a value mapper. Note that toMap, by default, will throw an exception if a duplicate is encountered.

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