Java8 lambda convert List to Map of Maps

Question

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

public class Entry {
    Employee employee;
    LocalDate date;
}

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()));

But this gives a compilation error:

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.Co‌llector<com.a.Entry,‌capture#2 of ?, 
java.util.Map<java.time.LocalDate,java.util.List<com.a.Ent‌ry>>>,
java.util.func‌tion.Function<java.l‌ang.Object,java.lang‌.Object>)

Thanks

Are you sure that Employee is a suitable key for a map? BTW, define "does not work". — aventurin
– aventurin, Commented Jan 17, 2017 at 23:01
compilation error: 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.Collector<com.a.Entry,capture#2 of ?,java.util.Map<java.time.LocalDate,java.util.List<com.a.Entry>>>,java.util.function.Function<java.lang.Object,java.lang.Object>) — przem
– przem, Commented Jan 17, 2017 at 23:07

shmosel · Accepted Answer · 2017-01-17 23:06:20Z

13

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())));

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

answered Jan 17, 2017 at 23:06

shmosel

51.1k7 gold badges81 silver badges148 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

przem Over a year ago

wicket, works perfectly. Exception on duplicates is expected behavior. Thank you

przem Over a year ago

And what if i'd need Map<String, Map<LocaleDate,Entry>> where key for the map is Employee::getUsername ? Is it possible to manipulate the keys as well?

shmosel Over a year ago

@przem Sure, just replace Entry::getEmployee with entry -> entry.getEmployee().getUsername().

przem Over a year ago

noooooo i cant be that simple!!! :-D many many thanks! works like a dream

Collectives™ on Stack Overflow

Java8 lambda convert List to Map of Maps

1 Answer 1

4 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related