30 points, due Friday, Oct 15th by 10PM. Like all assignments, there is a 48-hour, no questions asked extensions policy. If you have a tech issue, a health issue, or some other issue that impedes making the deadline, use this policy. You or your partner need to notify me that you’re using the policy via email, CC’ing the partner. Extensions beyond the 48-hour policy will only be considered in extenuating circumstances, and circumstances that arise between the original deadline and the extension are generally not extenuating - the deadline is still Friday, not 48 hours later.


To gain a better understanding of linked lists by implementing one that inserts values into sorted positions and maintains forward and backward pointers.

Assignment Type and Collaborative Learning Expectations

This is a homework assignment that you’ll be handing in on Moodle.

You must work on this homework with your assigned partner (if you have one) via pair programming. That means that you cannot write any code without your partner and you must both be fully engaged and discussing the code at all times while working. See the collaboration policy for details.

Setup and Requirements

Mount COURSES, download the HW4 starter code and move it to your StuWork/username folder. You should follow the same process of compressing your files and uploading them to Moodle to submit your homework.

Create a file Collaborations.txt and put in any help that you get on this assignment including sources that you reference and help from lab assistants or the prefect. Make sure to refer to the Collaboration page on what collaborations are allowed for homework assignments.

The code that you write for this assignment will build on top of the Country class that you worked with previously. I’ve provided my own version of it and the boilerplate code for reading in from a file and creating a Country.


For this assignment, you’ll make a new class, CountrySorterList that consists of a linked list that adds items in sorted order according to a previously set indicator and is able to provide the user with the minimum and maximum countries for the given indicator.


If you were thinking after the Country Stats Displayer assignment that there has got to be a better way of sorting, you are right! In Java, there is an interface called the Comparator that focuses on allowing two objects of the same type to be compared easily. This is slightly different than the Comparable interface discussed and used in the book because it doesn’t assume that the class has an inherent order, and instead orders the objects of the class based on the user specified indicator. First you will create a comparator that will then be stored in your linked list later.

Create a new class CountryComparator that implements the Comparator interface for Country:

public class CountryComparator implements Comparator<Country> {

This class will consist of just two methods: 1) a constructor, and 2) the compare method that is required for a Comparator.


Your comparator needs to store the indicator that it compares by. Create that instance variable and a constructor that takes in a value for indicator and sets the instance variable.

compare method

Next create a method compare that takes two Country objects as parameters. The Comparator’s compare method works similarly to the Comparable interface in that for two objects a and b, it returns:

  • -1 if a comes before b,
  • 0 if a and b could be in either order, and
  • 1 if b comes before a.

Your compare method should work the same based on the indicator chosen by the constructor.

You should test your CountryComparator by making a new one in the main(String[] args) method of CountryComparator.java with some test Country objects.


Now that you have an object for comparing two countries, let’s make a sorted linked list for your countries. Note that you are not allowed to use any built-in List classes or methods! The point of this homework is to understand linked lists by implementing your own.

Using CountryComparator

Your CountrySorterList should use an object of type CountryComparator to determine the order of the countries in the list.

add(Country newEntry)

You should create an add method that takes a Country object and puts it into the correct spot in the sorted list based on the comparator. The reading gives you a good start on this method and you can and should copy over the relevant code and credit it in your Collaborations.txt document. You will need to adapt the code to work with your CountryComparator though.

Double linking

You should adapt the starter code and reading code to have a doubly-linked list, which is a list where the Nodes have a link to the node before them, often called previous and you maintain a lastNode pointer as well as a firstNode pointer.

Max and min

You should enable the user to get the following information:

  • The top X countries for any of the indicators, where X can be any number that the user specifies (i.e. the top 10 countries for Population Growth)
  • The bottom X countries for any of the indicators

You should retrieve this information efficiently, meaning that you should not have to loop all the way to the end of the list to get either the maximum or minimum countries. Instead, you should use the double linked nature of the list effectively.

User interaction

As usual, it is up to you how you get information from the user and return information to them and you should consider how to do those tasks in a way that makes it easiest for the user.


As always, you should include a detailed README for this homework. It should include a brief overview of your project, a short example of how to run your program and see its interesting behavior, and a more detailed section that has demonstration input for each of the rubric items. Remember, you want to make it as easy as possible for the grader to see that your homework does everything its supposed to!


In addition to demonstrating each rubric item, include a separate discussion about why you are using the CountryComparator class instead of having Country implement the interface Comparable as discussed in Java Interlude 5 of the textbook.


This is how you will be graded on this homework:

Item Points
User can get list of top X countries for each indicator 6
User can get list of bottom X countries for each indicator 6
Information from end of list retrieved efficiently 6
CountryComparator implemented and used correctly 3
README clear and complete 6
Java-docs style documentation 3


Remember to update your Collaborations.txt file with any sources that you consulted.

Then make a .zip of your files and upload it to Moodle. If you are working with a partner, only one of you should submit on Moodle, the grader will handle giving you both the same score.