public class SpecificDBDriver {

	public SpecificDB getDB(String dbname) {
		SpecificDB db = null;
		return db;
	}

}

public abstract class SpecificDB {

}

public class Test {

	public static void main(String[] args) {
		
		SpecificDBDriver dbDriver = new SpecificDBDriver();
		SpecificDB db = dbDriver.getDB("test");

	}

}

public class DBDriver extends SpecificDBDriver {

}

public abstract class DB extends SpecificDB{

}

public class Test {

	public static void main(String[] args) {

		DBDriver dbDriver_ = new DBDriver();
		DB db_ = dbDriver.getDB("test");

	}

}

Type mismatch: cannot convert from SpecificDB to DB