Question

I have an interface HTTPSequence. I also have an abstract class AbstractHTTPFactory which in turn has an abstract method returning ArrayList<HTTPSequence>. In classes derived from AbstractHTTPFactory I want to override those methods to return ArrayList<[Class implementing HTTPSequence]>.

Is it possible ? Now compiler gives my an error suggesting that I change overriden methods signature to HTTPSequence.

// abstract class with abstract method returning ArrayList of objects implementing interface
abstract public class AbstractHTTPFactory {
    abstract ArrayList<HTTPSequence> make();
}
// Specific class that returns ArrayList of objects of the class implementing HTTPSequence
public class RecipesHTTPFactory extends AbstractHTTPFactory{
    public ArrayList<Recipe> make() {
    }
}
// interface
public interface HTTPSequence {
}
// one of the classes implementing the above interface
public class Recipe implements HTTPSequence {
}

And the message Eclipse gives me is:

Multiple markers at this line
– The return type is incompatible with AbstractHTTPFactory.make()
– implements ….ider.AbstractHTTPFactory.make

Answer 1

You could write your AbstractClass method to return ArrayList<? extends Interface>, and then you do not have to change the derived class method signatures