Create a class named MapSet and use it to implement your generic Set interface using a HashMap.

a$.$ Your constructor should create an empty HashMap. Use the Set's type paratemer as the key type in the map. The value type does not matter $-$ use any type you'd like.

b$.$ When an element is added to the set, add it to the map as a key. Again, the value does not matter, so use null.

c$.$ Use the other methods on the HashMap to implement the remaining methods in your set interface.

d$.$ Write a unit test to verify that your Set works as expected, including iterating over the elements using a for loop. Hint: just return the iterator for the key set.