Vaadin components (e.g. Tree) with methods using Streams and Collections should be relaxed to ? extends T

Currently (8.4.2), Vaadin components that are generic (such as com.vaadin.ui.Tree) and that have methods that can produce data from a Stream and/or Collection objects (such as Tree::addItems) are constrained to use objects of the instance's exact generic type. Relaxing the requirement to something that extends the generic type would greatly improve user-land usability.

Current Solution:

public TreeData<T> addItems(T parent, Stream<T> items) { ... }
public TreeData<T> addItems(T parent, Collection<T> items) { ... }

Proposed Improved Solution:

public TreeData<T> addItems(T parent, Stream<? extends T> items) { ... }
public TreeData<T> addItems(T parent, Collection<? extends T> items) { ... }

If the proposed improvement is introduced, we could write code like this:

Tree<Car> tree = new Tree<>();
tree.addItems(root, Stream.of(new Volvo(), new Tesla(), new Ford());

provided that all the concrete car classes implements Car. This would simplify building trees with heterogeneous types.

PECS = (Producer Extends, Consumer Super)

Start by locating Tree::addItems and the related generic component methods that accept Stream or Collection values. Compare their current generic bounds with the proposed bounds and identify the affected APIs; done means compatible heterogeneous subtype collections or streams are accepted without breaking existing calls.