cyclic inference java lambda groupingby

问题:

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()));

IntelliJ won't compile because of a cyclic inference.

回答1:

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

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));

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.

回答2:

try to remove the () on getTradeDate

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

Here is a nice little overview: http://www.java8.org/