25 points, due Monday, January 11th by 10PM Central time. 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 need to notify me that you’re using the policy. 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 Monday, not 48 hours later.


Demonstrate your understanding of private and class variables, ability to create objects of your own making, and use other Java functionality.


You should complete this assignment on Repl.it. Open the HW1 project.

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


For this project you have the opportunity to be creative! Instead of telling you a specific thing to make, since the goal is just to get some practice making a Java class and using it, you can come up with your own example. Below I have listed the things that your code must incorporate, but you are encouraged to go beyond these requirements and have some fun making a small project that is of interest to you. The cool thing about object-oriented programming is that you can simulate nearly anything in the real world, so while I give some example ideas if you are stuck, you really can make a class for anything that interests you.


  • Create a new class in its own file, which I’ll refer to here at YourClass
  • YourClass has and uses at least one private variable
  • YourClass tracks how many instances of YourClass have been created (using a class variable which uses the static keyword!)
  • Main.java’s main creates at least 5 instances of YourClass
  • Main takes at least one command line argument and uses it meaningfully
  • Either Main or YourClass prints out something useful
  • Follows style guide
  • JavaDocs-style method comments for all public methods


  • Library catalogue with at least five books where command line argument is the title of a book and outputs information about that book
  • Sport stats displayer with at least five players where command line argument is the name of a player and outputs some stat about the player (sorry I don’t follow professional sports much!)
  • Song recommender with at least five songs where command line argument is type of song someone wants to hear and output is the song of that category
  • Tweak the above ideas to be about cooking recipes, video games, workouts, or anything else that interests you

Code Notes and Tips

  • Command line arguments are what is captured in args in your main(String[] args) function, NOT just user input after the program is running!
  • Remember that you have to construct variables before you use them. If you run into null pointer exceptions or exceptions about not using a variable before it has been initialized, one possible cause is that you have declared a variable (e.g., List<Integer> list;) but you haven’t assigned it a value (e.g. list = new ArrayList<>()).
  • Your code should not produce any warnings. Warnings are extra messages from the compiler like: Note: RandomList.java uses unchecked or unsafe operations. Note: Recompile with -Xlint:unchecked for details.

If you see a message like this, you should follow the instructions and add -Xlint:unchecked after your compiler command.

  • If you want to turn a command line argument into something other than a String, you need to use a method to do so. You will probably want either parseInt(String in) or parseDouble(String in):
      int myInt = Integer.parseInt("1");
      double myDouble = Double.parseDouble("1.0");


Item Points
Compiles without warnings and runs 4
New class created correctly 3
Private variable used correctly 3
Class variable used correctly 4
5 instances of class created correctly 4
Command line argument used correctly 2
Useful output 1
Sufficient JavaDocs-style documentation 2
Follows style guide 2


Download the zip of your code from Repl.it (by clicking on the three vertical dots across from “Files” and selecting “Download zip”) and upload that zip to Moodle.