Question

I’m trying to figure out why this code won’t compile.

I have interface A extended by interface B.
Class C which implements interface B.

When I call a method that takes in a single object of type A, I can pass in an object of type C and it’s fine.

When I call a method that takes in a java.util.List of type A, I cannot pass in a java.util.List of objects of type C. Eclipse generates the following error:
The method addAList(List) in the type Test1 is not applicable for the arguments (List)

Source code example is below.

import java.util.ArrayList;
import java.util.List;

public class Test1 {

    public void addASingle(A a) {
        return;
    }

    public void addAList(List<A> aList) {
        return;
    }

// **********************************

    public static void main(String[] args) {
        Test1 t = new Test1();

        C c1 = new C();
        List<C> cList = new ArrayList<C>();
        cList.add(c1);

        t.addASingle(c1); // allowed

        t.addAList(cList);  // The method addAList(List<Test1.A>)
    // in the type Test1 is not applicable for the arguments (List<Test1.C>)
    }

// **********************************

    public static interface A {
    }

    public static interface B extends A {
    }

    public static class C implements B {
    }
}

Answer 1

A List<Car> is not a List<Vehicle>. If it was, you could do the following:

List<Car> cars = new ArrayList<>();
List<Vehicle> vehicles = cars;
vehicles.add(new Bicycle());

and you would end up with a list of cars which contains a bicycle. It would ruin the type-safety of generic collections.

You probably should used a List<? extends A> instead of List<A>. List<? extends A> means: a List<some class which is A or which extends A>.