Java8 lambda 将 List 转换为 Map of Maps

Question

是否可以转换List<Entry>至 Map<Employee, Map<LocalDate, Entry>>用一个 lambda 表达式？

public class Entry {
    Employee employee;
    LocalDate date;
}

到目前为止，我想出了这样的事情:

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

但这会产生编译错误:

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>) 

谢谢

Answer 1

假设 Entry 有 getter 并且 Employee 覆盖了 hashCode() 和 equals():

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

请注意，如果员工有重复的日期，这将引发异常。