Streaming a Map with a nested object in Java8

Question

I have an Map of the format

Map<Object1, List<Object2>> dependencies;

. Can someone please suggest what is the concise way to convert this to a Map<Object1, List < Object3 >> using Java Streams ? Failed attempt to do it with streams is listed below.

class Object2 {

List<Object3> subDependencies;

}

//What I was trying to do
Map<Object1, List<Object3>> results = dependencies.entrySet().stream().flatMap
   (entry -> entry.getValue().stream()
   .collect(Collectors.toMap(entry.getKey(), Object2::getsubDependecies));

Are you trying to combine all Object3 in subDependencies values as a single list? — ernest_k
– ernest_k, Commented Mar 3, 2021 at 19:27
@ernest_k yes. I am trying to combine all subdependencies of Object1 to one map where key is Object1 and Value is list of all the subdependencies. — KVM
– KVM, Commented Mar 3, 2021 at 19:30

ETO · Accepted Answer · 2021-03-03 22:50:45Z

1

Try this:

Map<Object1, List<Object3>> results = 
    dependencies.entrySet()
                .stream()
                .collect(toMap(Map.Entry::getKey, 
                               e -> e.getValue()
                                     .stream()
                                     .map(Object2::getSubDependecies)
                                     .flatMap(List::stream)
                                     .collect(toList())));

edited Mar 3, 2021 at 22:50

answered Mar 3, 2021 at 20:37

ETO

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3 Comments

Yoni Over a year ago

seems like you're missing 2 closing brackets at the end of your statement

ETO Over a year ago

@Yoni Right. Thanks.

KVM Over a year ago

Thank you @ETO .

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