Question

I have an interface defining a method like this :

public List<IA> myMethod();

with IA another interface.

In my implementation of the method I declare :

public List<A> myMethod() { /* Do something */ }

with A a class implementing IA.

However, Java (or eclipse) doesn’t like it and forces me to have List<IA> in my implementation of myMethod. As A implements IA, I don’t see why this casts an error.

So here are the questions :

• Why do I have this error ?
• Which is the best way to avoid it (knowing I do not have much liberty on the code) ?

Answer 1

You have this error because it’s unsafe from a type-safety point of view. Consider:

// Interface declaration
List<Fruit> myMethod();

// Implemented by...
public List<Banana> myMethod() {
    return new ArrayList<Banana>();
}

// Called as:
List<Fruit> fruit = foo.myMethod();
fruit.add(new Apple());

Do you really want to be able to add an Apple to an ArrayList<Banana>?

You can fix this by changing the declaration to

List<? extends IA> myMethod();

That will prevent calls from adding to the list (or setting values on it).

If you can’t change the interface, you will have to return a List<IA> from your method, I’m afraid.

See the Java Generics FAQ for much more information.