Java8 lambda将List转换为Maps的Map

Question

是否有可能转换List<Entry>为Map<Employee, Map<LocalDate, Entry>>一个lambda表达式？

public class Entry {
    Employee employee;
    LocalDate date;
}

到目前为止,我想出了类似的东西:

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.Ent?ry>>>,
java.util.func??tion.Function<java.l??ang.Object,java.lang??.Object>) 

谢谢

Answer 1

假设Entry有getter和Employeeoverrides hashCode()并且equals():

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

请注意,如果员工有重复日期,这将抛出异常.