When using HashSet or LinkedHashSet, the objects you add to them
must override hashCode(). If they don�t override hashCode(), the default Object.hashCode() method will allow multiple objects that you might consider "meaningfully equal" to be added to your "no duplicates allowed" set.
if (c == e) System.out.println("c == e"); // true (so 'e' being unboxed)
Behind the scenes, the format() method uses the java.util.Formatter class to do the heavy formatting work. You can use the Formatter class directly if you choose, but for the exam all you have to know is the basic syntax of the arguments you pass to the format() method. The documentation for these formatting arguments can be found in the Formatter API.