循环推理java lambda groupingby [英] cyclic inference java lambda groupingby

问题描述

我是lambdas的新手并且在这行代码中与我做错了相混淆：

I'm new to lambdas and confused with what I'm doing wrong in this line of code:

HashMap<Date, ArrayList<Trade>> groupTrades = allTrades.stream().collect(Collectors.groupingBy(Trade::getTradeDate()));

由于循环推理，IntelliJ无法编译。

IntelliJ won't compile because of a cyclic inference.

推荐答案

经过一段时间的努力我已经解决了，希望这对其他人也有用

After a bit of pain I've worked it out and hopefully this will be useful to others as well

你不能使用HashMap或ArrayList - 只需使用接口Map和List，代码应为：

You mustn't use HashMap or ArrayList - just use the interfaces Map and List, code should read:

Map<Date, List<Trade>> groupTrades = allTrades.stream().collect(Collectors.groupingBy(Trade::getTradeDate));

请注意，当groupingBy中的任何参数不匹配时，可能会导致此相当通用的消息宣言中的预期结果。

Note that this rather generic message could be caused when any of the parameters in the groupingBy don't match what is expected in the declaration of the Map.

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