Comparator中的链方法。比较Java 8中的List

Question

I have the following Objects

Class Trade{
   private Integer tradeId;
   private Event  eventId;
   private Integer tradeVersion;
}

Class Event{
  private Integer value;
}

1. For the above scenario I am trying to write something like Comparator.comparing(Trade::getEventId().getValue())
2. The above did not work

3. So for now I went with the usual lambda which is the following

   someList.sort((Trade o1, Trade o2) -> o1.getEventId().getValue().compareTo(o2.getEventId().getValue())) ;

and it worked but curious to know if #1 is possible?

4. Tried Comparator.comparing(Trade::getEventId).thenComparing(Event::getValue) but no joy.
5. Note that both of these classes are third party so I cannot alter them.

Answer 1

Expression Comparator.comparing(Trade::getEventId().getValue()) does not work, because this is invalid use of method reference . With :: operator you can refer to a single method, like Trade::getEventId . This is an equivalent for following lambda expression:

trade -> trade.getEventId()

Although you may try using following Function<Trade, Integer> represented as a following lambda expression:

(Function<Trade, Integer>) trade -> trade.getEventId().getValue()

Then your comparator could be defined as:

Comparator.comparing((Function<Trade, Integer>) trade -> trade.getEventId().getValue())

PS: casting to Function<Trade, Integer> can be avoided probably.

Answer 2

It's as easy as :

trades.sort(Comparator.comparingInt(x -> x.getEventId().getValue()));

Also notice that your version with Comparator.comparing(Trade::getEventId).thenComparing(Event::getValue) would semantically mean that you are first sorting by eventId then by value which looks like not what you want in the first place.