Java Design Issue: Enforce method call sequence

Question

There is a question which was recently asked to me in an interview.

Problem: There is a class meant to profile the execution time of the code. The class

Answer 1

I suggest something like:

interface WatchFactory {
    Watch startTimer();
}

interface Watch {
    long stopTimer();
}

It will be used like this

 Watch watch = watchFactory.startTimer();

 // Do something you want to measure

 long timeSpentInMillis = watch.stopTimer();

You can't invoke anything in wrong order. And if you invoke stopTimer twice you get meaningful result both time (maybe it is better rename it to measure and return actual time each time it invoked)