Modify your UserProfile class to meet the following requirements:

1. It must include a field called wall to store an ArrayList of wall posting objects. It must also provide a getter called getWall() that returns the ArrayList of wall posting objects. The list should initially be empty for a newly created UserProfile.
2. It must provide a method called postToWall() that takes two parameters: the author of the posting (a user profile, not a string) and a string message. This method should add a new wall posting with the given contents.

Note that wall postings should be stored in reverse-chronological order, meaning that newly created posts should be stored at the beginning of the list, not at the end. Also, this method should ensure that the total number of posts does not exceed 20. Once the wall gets that full, delete the oldest post to make room for a new one. Study the API documentation for ArrayList to find suitable methods to accomplish this arrangement. You may assume that the clock is consistent, that is, postings created more recently will also have a more recent time stamp.