Question

I’ve read similar questions here but I’m still a little confused.

 MyCollection extends ArrayList<MyClass>

 MyClass implements Data

yet this gives me the “cannot convert from ArrayList to MyCollection”

 MyCollection mycollection = somehandler.getCollection();

where getCollection looks like this

 public ArrayList<Data> getCollection()

So my assumptions are obviously wrong. How can I make this work like I would like it to

Answer 1

ArrayList does not extend MyCollection. It’s the other way round.

There are several ways to fix this problem, depending on the functional requirement:

1. You need to provide a MyCollection constructor which can take another Collection.
2. If the actual type is MyCollection, then you need to cast ArrayList to MyCollection.
3. If the actual type is MyCollection, then you need to change return type to MyCollection.

That said, what makes MyCollection so different that it apparently doesn’t adhere the List‘s contract and you hence need to cast/convert it? This shouldn’t happen, I would rethink the design/approach.