Java8 Lambda将List转换为Maps [英] Java8 lambda convert List to Map of Maps

问题描述

是否可以使用一个lambda表达式将 List< Entry> 转换为 Map< Employee，Map< LocalDate，Entry>> ?

Is it possible to transform List<Entry> to Map<Employee, Map<LocalDate, Entry>> with one lambda expression?

public class Entry {
    Employee employee;
    LocalDate date;
}

到目前为止，我已经想到了这样的东西:

So far i have came up with something like this:

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

但这会导致编译错误:

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

推荐答案

假设 Entry 具有吸气剂，并且 Employee 覆盖 hashCode()和 equals():

Assuming Entry has getters and Employee overrides hashCode() and 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.

Note that this will throw an exception if an employee has duplicate dates.

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