Question

I’d like to know how can I declare a method will return an instance of a specific class.

For example:

The abstract class A declares a method (method1) that must return a instance of “List”
the class B and C extends A. I want B to return an ArrayList and C to return a LinkedList.

How can I implement that? Thanks

Answer 1

If you want the caller to know that it’s getting a LinkedList or ArrayList then you can leverage generics.

It would look something like this:

public abstract class A<T extends List<String>> {
    public abstract T method()
}

public class B extends A<ArrayList<String>> {
    public ArrayList<String> method() {
        //...
    }
}

public class C extends A<LinkedList<String>> {
    public LinkedList<String> method() {
        //...
    }
}

Or, if you are going to be interacting with B and C directly (not via A) then you can leverage covariant return types and simply have the subclasses declare the return type as ArrayList and LinkedList respectively. It is legal in Java 5+ to narrow the return type of an overridden method.

If the concrete type doesn’t need to be known to the caller, then you just want to pass a strategy (i.e. a factory) for creating the list to A. This can be done by implementing a simple interface:

public interface ListFactory<T> {

    public List<T> newList();
}

and then passing it to the constructor of A. In fact, the factory can be used either way, however you only need classes B and C if you are in the first situation I described.