Hello, welcome to JavaRanch. Can I get you to take a look at our
display name policy and
edit your profile to give us a display name that looks a little more like a real name, rather than an abbreviation or random line noise?
Thanks.
If your objective is just to get the list sorted, using Collections.sort() is probably best. This already uses a mergesort with slight modifications. You can use Collections.sort(List,Comparator) to specify a Comparator object which implements a particular sorting criterion. (Like sorting by last name then first name.)
If this is an assignment where the instructor wants to see you write a mergesort yourself, then, well, you really should do most of it yourself. Do you have a text or something describing how a mergesort works? Have you tried writing any code yet? What specific issues or questions do you have?
In either case, you also need to know what exactly is in the ArrayList. Are the objects Strings? Or some other class? If they're Strings, are they stored like "Jim Yingst" or "Yingst, Jim"? This will make a difference in terms of how you implement the sorting criterion. The latter format is really simple to deal with - you can just sort using the natural order of the Strings, since this will sort by last name, then first name.