How to merge a nested List into a simple List using Java 8 Stream API

Question

I have a List which looks as follows:

List<List<Class2>> list = new ArrayList<>();

List<Class2> l1 = new ArrayList<>();
l1.add(new Class2(new Class3()));
l1.add(new Class2(new Class3()));
l1.add(new Class2(new Class3()));
list.add(l1);

List<Class2> l2 = new ArrayList<>();
l2.add(new Class2(new Class3()));
l2.add(new Class2(new Class3()));
l2.add(new Class2(new Class3()));
list.add(l2);

How can I convert the List list into a List<Class3> using the Java 8 Stream API?

No attempts yet, except the brute-force method where I iterated over the list and extracted the Class3 object by myself — sfandler
– sfandler, Commented Sep 29, 2016 at 12:11

Eran · Accepted Answer · 2016-09-29 12:14:53Z

9

Since you are passing to the Class2 constructor an instance of Class3, I'm assuming Class2 has a Class3 member with a getter (lets call the getter getClass3()).

Based on this assumption, you can do the following to get a List<Class3> of all the Class3 members of all the Class2 members of all the lists contained in list :

List<Class3> listOf3 = 
    list.stream()
        .flatMap(List::stream) // convert a Stream<List<Class2>> to Stream<Class2>
        .map(Class2::getClass3) // convert a Stream<Class2> to Stream<Class3>
        .collect(Collectors.toList()); // collect to a List<Class3>

edited Sep 29, 2016 at 12:14

answered Sep 29, 2016 at 12:03

Eran

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