Return sublist instead of getChildren()

[Tom McLintock]

I have a question about the list returned by getChildren(). Is there an easy way to restrict the type of objects allowed in and out of that list, beyond just Nodes? Say I want to only allow Buttons into a list of Children and have that checked at compile time, and also have the signature of getChildren return ObservableList<Button> instead of ObservableList<Node>. How can I do that?

[Campbell Ritchie]

Welcome to the Ranch

Please go through the documentation for getChildren(). I think it simply returns the List in question. I don't think there are any facilities in that method to specify the type of element included in the List. It is a long time since I did any FX, but I think that ObservableList extends java.util.List, whose methods include those allowing you to choose a sublist by index, or to create a Stream which you can filter to produce a new List. That latter would not behave like part of the original List. I think a sublist might but I am not sure. It says in the documentation for subList() that,

. . . non-structural changes in the returned list are reflected in this list, and vice-versa. . . .

Is there a setChildren() method? What parameter does that take? If it is ObservableList<E extends Node> or ObservableList<? extends Node>, you might be able to set it to an ObservableList<Button>. Otherwise, don'know.

[Campbell Ritchie]

If the parameter's type is ObservableList<Node>, an ObservableList<Button> isn't a subtype of ObservableList<Node>, and I am afraid you are stuck with the type Node and can't change it. Can you create an anonymous class and override a method or create a new method with parameter type Button?
As I said, I haven't written any FX for a long time, but this looks more and more difficult every time I think about it.

[Junilu Lacar]

There are ways to do this but it's not going to be straightforward.

For example, you could extend Group and provide an overridden version of getChildren() that returns a list implementation that only accepts Button objects and throws an UnsupportedOperationException otherwise. But you can't have another getChildren() that returns an ObservableList<Button> instead of ObservableList<Node>. You could overload it though but the implementation will be kind of ugly (it's already ugly just thinking about it, it'll probably much uglier in actual code).