Putting objects of your own classes in order. Comparable and compareTo give a class a natural order, which Collections.sort and a stream's sorted() use; a Comparator, written as a lambda or built with Comparator.comparing and reversed, sorts by any other rule; thenComparing sorts by several criteria, in lists and in streams. Last, where an order belongs, in the class or where it's used. MOOC section 10.2.