Question

I read this code in a book just now.

public class AdapterWrapper implements ListAdapter {
  ListAdapter delegate=null;

  // other code

}

The ListAdapter is a public interface and a reference of that interface has been made and assigned null. Is this valid? I’m really confused by this.

Answer 1

Yes, it’s valid. It just means that delegate is a reference to ListAdapter interface and it’s currently pointing at null.

You can later make it point to any class implementing the ListAdapter interface, such as SimpleCursorAdapter, WrapperListAdapter or any other implementation you want.

Using an interface type as reference is useful when you don’t know ahead of time what class is it going to be at runtime. So you just use a reference pointing to an interface.