Java8 lambda將List轉換為Maps的Map

Question

是否可以使用一個lambda表達式將List<Entry>轉換為Map<Employee, Map<LocalDate, Entry>> ？

public class Entry {
    Employee employee;
    LocalDate date;
}

到目前為止，我想出了類似的東西：

entries.stream().collect(Collectors.toMap(Collectors.groupingBy(Entry::getEmployee), Collectors.groupingBy(Entry::getDate), Function.identity()));

但這會產生編譯錯誤：

no suitable method found for 
toMap(java.util.stream.Collector<com.a.Entry,capture#1 of ?,java.util.Map<com.a.Employee,
java.util.List<com.a.Entry>>>‌​,java.util.stream.Co‌​llector<com.a.Entry,‌​capture#2 of ?, 
java.util.Map<java.time.LocalDate,java.util.List<com.a.Ent‌ry>>>,
java.util.func‌​tion.Function<java.l‌​ang.Object,java.lang‌​.Object>) 

謝謝

Answer 1

假設Entry有getter， Employee覆蓋hashCode()和equals() ：

Map<Employee, Map<LocalDate, Entry>> result = entries.stream()
        .collect(Collectors.groupingBy(Entry::getEmployee,
                Collectors.toMap(Entry::getDate, Function.identity())));

請注意，如果員工有重復日期，這將拋出異常。