A new feature coming in JDK 8 allows you to add to an existing interface while preserving binary compatibility.
The syntax is like
public interface S
I know this is a old post, but as i'm working with this stuff...
You will have an error from the compiler, telling you that:
 class TimeTravelingStudent inherits unrelated defaults for present() from types Attendance and Timeline reference to present is ambiguous, both method present() in Timeline and method present() in Attendance match.