Interview: Design an iterator for a collection of collections

Question

Design an iterator for a collection of collections in java. The iterator should hide the nesting, allowing you to iterate all of the elements belonging to all of the collect

Answer 1

This is an old question, but nowadays (2019) we have JDK8+ goodies. In particular, we have streams, which make this task straightforward:

public static  Iterator flatIterator(Collection> collections) {

    return collections.stream()
            .filter(Objects::nonNull)
            .flatMap(Collection::stream)
            .iterator();
}

I'm filtering null inner collections out, just in case...

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EDIT: If you also want to filter null elements out of the inner collections, just add an extra non-null filter aflter flatMap:

return collections.stream()
        .filter(Objects::nonNull)
        .flatMap(Collection::stream)
        .filter(Objects::nonNull)
        .iterator();