java stream nested objects

Question

I have an object as follows and a list as per the object Type:

public class A{
    private String x;
    private String y;
    private List<A> child = new ArrayList<>();
}

What I want to do is flatten the list and make a list of Object A together with its children;

Currently I am doing in three separate statements as per below:

List<A> ChildElements = map.values()
                .stream()
                .flatMap(entry -> entry.getChild().stream())
                .collect(Collectors.toList());
List<A> RootElements = map.values()
                .stream()
                .collect(Collectors.toList());
List<A> elements = Stream.concat(ChildElements.stream(),RootElements.stream()).collect(Collectors.toList());

I wanted to know if the above can be refactored in one statement.

Thank you.

@Turing85 just for my knowledge and see if it can be written in lesser number of lines of codes. — ashley
– ashley, Commented Aug 8, 2021 at 18:55
map.values().stream().flatMap(entry -> { final ArraryList<A> result = new ArrayList>(entry.getChild().stream())); result.add(entry); return result.stream(); }).collect(Collectors.toList()); — Turing85
– Turing85, Commented Aug 8, 2021 at 19:00

Nowhere Man · Accepted Answer · 2021-08-08 20:08:30Z

1

It is possible to concat streams immediately in the flatMap:

List<A> flattened = map.values()
        .stream()
        .flatMap(root -> Stream.concat(
            Stream.of(root),
            root.getChildren().stream()
        ))
        .collect(Collectors.toList());

However, initial implementation provides another order of the elements in the results:

initial: child11, child12,... child1N1, child21,... child2N2, ...childMNm, root1,... rootM
flattened: root1, child11,... child1N1,... rootM, childM1... childMNm

answered Aug 8, 2021 at 20:08

Nowhere Man

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Comments

Gaël J · Accepted Answer · 2021-08-08 19:00:53Z

1

One way would be like following:

map.values()
  .stream()
  .flatMap(entry -> {
    Stream<A> entryStream = Stream.of(entry);
    Stream<A> childStream = entry.getChild().stream();
    return Stream.concat(entryStream, childStream);
  })
  .collect(Collectors.toList());

answered Aug 8, 2021 at 19:00

Gaël J

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