在流中排序不适用

Question

我有这条Java8代码:

Set<Purchase> purchases = 
    user.getAcquisitions()
        .parallelStream()
        .map(a -> a.getPurchases())
    .sorted(Comparator.comparing(Purchase::getPurchaseDate).reversed());

但我有这个编译错误,我不知道为什么:

The method sorted(Comparator<? super Set<Purchase>>) in the type Stream<Set<Purchase>> is not applicable for the arguments 
 (Comparator<Purchase>)

Answer 1

之后.map(a -> a.getPurchases()),你似乎期待一个Stream<Purchase>,但你真正拥有的是一个Stream<Set<Purchase>>.

如果a Stream<Purchase>确实是你想要的,那么你应该使用

.flatMap(a -> a.getPurchases().stream())