Implementations and Collections

Question

Why does this not work...

public ArrayList getEdges() {

return A;

//A is an Arraylist of type \'Action\'. Action implements Ed         


        

Answer 1

This is because ArrayList is not covariant on the type E. That is, you cannot substitute an instance of ArrayList for ArrayList just because Derived inherits from Base.

Consider this case: String inherits from Object; however, if this meant you could use an ArrayList as an ArrayList