Java: interface vs abstract class (about fields)

From the content I collected, I want to force a class to use specific 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 wanted to make sure I wouldn't hurt myself later:)

Solution

It is common to provide both, so the final result is:

public interface Sendable {
    public void sendMe();
}

and

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

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

In this way, anyone who is satisfied with the default implementation in the abstract class can subclass it quickly without rewriting a lot of code, but anyone who needs to do more complex things (or inherit from different base classes) can still implement the interface and plug and play