Java: Interface vs Abstract Class (regarding fields)

Question

From what I have gathered, I want to force a class to use particular private fields (and methods) I need an abstract class because an interface only declares public/static/final fields and methods. Correct??

I just started my first big java project and want to make sure I'm not going to hurt myself later :)

Answer 1

It's fairly common to provide both, so that you end up with:

public interface Sendable {
    public void sendMe();
}

and

public abstract class AbstractSender implements Sendable {
    public abstract void send();

    public void sendMe() {
        send(this.toString());
    }
}

That way, anyone who is happy with the default implementation in the abstract class can quickly subclass it without rewriting a lot of code, but anyone who needs to do something more complex (or who needs to inherit from a different base class) can still implement the interface and be plug-and-play.