Incompatible types with inheritance Java

Question

I'm trying to wrap my head around this inheritance problem I'm having. Here's my current situation:

So I have a planning that can be either a list of activities (class Activities) or a list of things to do (class Todos).
However, if I do this in Todos:

private List<Todo> todos;

public List<Activity> getPlanning(){
    return todos;
}

It says that the types are incompatible.

• Why are they incompatible?

Since Todo extends from Activity, aren't we certain that Todo provides at least the same functionality as Activity?

and perhaps more importantly:

• How do I adjust my design to make things work?

(I'm not an expert in UML so forgive me if there are some mistakes in my diagram)

Answer 1

Welcome to the nightmare and poor implementation of generics in Java. Try using public List<? extends Activity> getPlanning().

Answer 2

If Java allowed you to return your List<Todos> as a List<Activity, the caller could then add any Activity into it. Clearly, that must not be allowed to happen.

You could make the returned list read-only by changing the return type to List<? extends Activity>. This would disallow adding any objects to the list (except null or objects directly retrieved from the same list).