Java 8 streams, convert List of object to Map<String, Set<String>>

Question

I have already gone through few examples and those did not work for me.

Here is what I am trying to do:

I have a List<SomeClass> of the following class:

class SomeClass {
  String rid;
  String name;
  ...
}

The values in my List look like this:

SomeClass(1,"apple")
SomeClass(1,"banana")
SomeClass(1,"orange")
SomeClass(2,"papaya")
SomeClass(2,"peaches")
SomeClass(3,"melons")

I want to convert the above List into a Map<String, Set<String>>, where key is rid and value is Set of name field.

To solve this using Java Streams I am using groupingBy and I could come to below solution:

someClassList
            .stream()
            .map(SomeClass::getName)
            .collect(
                  Collectors.groupingBy(
                      SomeClass::getRid, Collectors.toSet()));

But this gives me compilation error. How do I solve this and what is the problem with my approach?

Eran · Accepted Answer · 2018-07-17 17:44:42Z

8

When you call .map(SomeClass::getName) on your Stream<SomeClass>, you get a Stream<String>. You can't execute collect(Collectors.groupingBy(SomeClass::getRid,...)) on a Stream<String> (you can only execute it on a Stream<SomeClass>). Therefore your map step is wrong.

You need to pass the Collector returned by Collectors.mapping() to Collectors.groupingBy() in order to map the SomeClass instances to Strings after they are grouped by getRid.

Map<String, Set<String>> map =
    someClassList.stream()
                 .collect(Collectors.groupingBy(SomeClass::getRid, 
                                                Collectors.mapping(SomeClass::getName,
                                                                   Collectors.toSet())));

edited Jul 17, 2018 at 17:44

answered Jul 17, 2018 at 6:00

Eran

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1 Comment

Bikas Katwal Over a year ago

Thanks, that was quick :)

Ousmane D. · Accepted Answer · 2018-07-17 07:45:57Z

3

Although not as readable as the groupingBy collector; you can use the toMap collector just as well:

myList.stream()
      .collect(toMap(SomeClass::getRid, e -> new HashSet<>(singleton(e.getName())), 
                 (l, r) -> {l.addAll(r); return l;}));

Ensure that you have the necessary imports for singleton and toMap or you can just use Collectors.toMap(...) and Collections.singleton(...) respectively.

answered Jul 17, 2018 at 7:45

Ousmane D.

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1 Comment

Holger Over a year ago

That’s what I use when I expect most of the groups to be rather small.

.collect(toMap(SomeClass::getRid, e -> singleton(e.getName()), (l, r) -> { if(l.size() == 1) { l = new HashSet<>(l); } l.addAll(r); return l;}));

if you want to use it in parallel, it might be worth to extend the merge function to check the sizes and add the smaller set to the bigger set.

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