CIS 212 Assignment 6 Solution

The goal of this assignment is provide exposure to building a new generic data structure. Specifically, this project will involve building a new Set structure that maintains the “add” counts of its unique elements so that they can be iterated in sorted order with respect to how many times they’ve been added to the set. The Set implementation will be optimized for efficient add/remove/contains over efficient iteration, based on the assumption that we’ll generally be modifying the structure more often than accessing its elements in sorted order.


1.  [70] Create a new class OccurrenceSet<T that implements Set<T. Your OccurrenceSet implementation should create and maintain a HashMap<T, Integer; doing so will allow you to easily track the integer “add” count for each element in the set. All methods should function as specified in the Set documentation. Additionally:


•       (10) The add and addAll methods will need to keep track of how many times an element has been added to the set. We are optimizing for efficient add, so add should be O(1) and addAll should be O(n) for n added elements.

•       (10) The remove, removeAll, and retainAll methods should remove the necessary


elements from the set completely (i.e., not just decrement their counts). We are optimizing for efficient remove, so remove should be O(1) and removeAll should be O(n) for n removed elements.


•       (10) The contains and containsAll methods should behave as documented and operate in O(1) and O(n) time (i.e., for n query elements), respectively.

•       (10) The size method should return the number of unique elements currently in the set (i.e., not considering “add” counts). This method should be O(1). The clear and isEmpty methods should behave as documented.

•       (20) The iterator and toArray methods should return an Iterator or array, respectively,


with elements sorted by their “add” counts in ascending order. We are optimizing for efficient add/remove/contains over iteration, but these methods should still be O(n lg n). The Iterator remove method does not need to remove the element from the set (see extra credit). Hint: Creating a new List and using the Collections sort method is reasonable here, though note that you’ll need to implement a Comparator object so that the elements can be sorted by their associated counts (we’ll discuss this in class).


•       (10) Add a toString method that returns a string representation of the elements in the


list in sorted order (i.e., ascending with respect to their “add” counts). This method should be O(n lg n).


2.  [10] Create a Main class that creates a few OccurrenceSets of various types to test the functionality of your new data structure. For example, a test like:

OccurrenceSet<Integer intSet = new OccurrenceSet<Integer();



















OccurrenceSet<String stringSet = new OccurrenceSet<String();
















Should have the output:


[1, 5, 3]


[here, hello, world]


You’ve now got a data structure which will allow you to easily do things like sort words with respect to word counts without ever needing to write this code again (in Java, at least)! Yay! J


3.  [20] Write code that is clear and efficient. Specifically, your code should be indented with respect to code blocks, avoid unnecessarily repetitive code, avoid code that is commented out or otherwise unused, use descriptive and consistent class/method/variable names, etc.


4.  [+10] (Extra credit) Implement your own Iterator class to be returned by the iterator method above. Implement the remove method of the Iterator such that the current element is removed from the set.

Zip the Assignment6 folder in your Eclipse workspace directory, name the .zip file <Your Full (e.g.,, and upload the .zip file to Canvas (see Assignments section for submission link).
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