Using Java8 Streams to create a list of objects from another two lists

Question

I have the following Java6 and Java8 code:

List<ObjectType1> lst1 = // a list of ObjectType1 objects
List<ObjectType2> lst2 = // a list of ObjectType1 objects, same size of lst1

List<ObjectType3> lst3 = new ArrayLis<ObjectType3>(lst1.size());
for(int i=0; i < lst1.size(); i++){
  lst3.add(new ObjectType3(lst1.get(i).getAVal(), lst2.get(i).getAnotherVal()));
}

Is there any way in Java8 to handle the previous for in a more concise way using Lambda?

Answer 1

A Stream is tied to a given iterable/Collection so you can't really "iterate" two collections in parallel.

One workaround would be to create a stream of indexes but then it does not necessarily improve over the for loop. The stream version could look like:

List<ObjectType3> lst3 = IntStream.range(0, lst1.size())
         .mapToObj(i -> new ObjectType3(lst1.get(i).getAVal(), lst2.get(i).getAnotherVal()))
         .collect(toList());

Answer 2

You could create a method that transforms two collections into a new collection, like this:

public <T, U, R> Collection<R> singleCollectionOf(final Collection<T> collectionA, final Collection<U> collectionB, final Supplier<Collection<R>> supplier, final BiFunction<T, U, R> mapper) {
    if (Objects.requireNonNull(collectionA).size() != Objects.requireNonNull(collectionB).size()) {
        throw new IllegalArgumentException();
    }
    Objects.requireNonNull(supplier);
    Objects.requireNonNull(mapper);
    Iterator<T> iteratorA = collectionA.iterator();
    Iterator<U> iteratorB = collectionB.iterator();
    Collection<R> returnCollection = supplier.get();
    while (iteratorA.hasNext() && iteratorB.hasNext()) {
        returnCollection.add(mapper.apply(iteratorA.next(), iteratorB.next()));
    }
    return returnCollection;
}

The important part here is that it will map the obtained iteratorA.next() and iteratorB.next() into a new object.

It is called like this:

List<Integer> list1 = IntStream.range(0, 10).boxed().collect(Collectors.toList());
List<Integer> list2 = IntStream.range(0, 10).map(n -> n * n + 1).boxed().collect(Collectors.toList());
singleCollectionOf(list1, list2, ArrayList::new, Pair::new).stream().forEach(System.out::println);

In your example it would be:

List<ObjectType3> lst3 = singleCollectionOf(lst1, lst2, ArrayList::new, ObjectType3::new);

Where for example Pair::new is a shorthand for the lamdda (t, u) -> new Pair(t, u).