Project 1A: Data Structures

This project is in two parts. Part 1A will be due 09/14, and Part 1B will be due 09/18.

Rather than simulating the glorious gravitational physics of our universe, we’ll instead be building data structures. Not the most glamorous thing, but this will provide you with plenty of opportunities to practice everything we’ve been doing in lecture before our midterm rolls around.


In project 1A, we will build implementations of a “Double Ended Queue” using both lists and arrays. In project 1B, we will learn how to write our own tests for those data structures, and will use the Double Ended Queue to solve some small real world probelms.

In this part of the project you will create two Java files: and, with public methods listed below.

Unlike Project 0, we will provide relatively little scaffolding. In other words, we’ll say what you should do, but not how.

For this project, you must work alone! Please carefully read the Policy on Collaboration and Cheating to see what this means exactly.

We strongly encourage you to switch to IntelliJ for this project. While it’s not absolutely required, you will have a better time. The ability to visually debug your code can be incredibly useful, it’s also nice to have a development environment that catches syntax errors as you are typing, and it avoids the need to type javac and java (or pressing arrow keys) a bajillion times. If you need a refresher on how to import a project, you can follow the Intellij setup guide

Additionally, we will be enforcing style. You must follow the style guide or you will lose points on the autograder.

Getting the Skeleton Files

As with Project 0, you should start by downloading the skeleton files.

To do this, head to the folder containing your copy of your repository. For example, if your login is ‘s101’, then head to the ‘fa20-s101’ folder (or any subdirectory).

To make sure you have the latest copy of the skeleton files, use git pull skeleton master. If you’re using a newer version of git, you might need to run git pull skeleton master -allow-unrelated-histories.

You should now see a proj1a directory appear with files that match the skeleton repository.

If you get some sort of error, STOP and either figure it out by carefully reading the git guide or seek help at OH or Ed. You’ll potentially save yourself a lot of trouble vs. guess-and-check with git commands. If you find yourself trying to use commands recommended by Google like force push, don’t. Don’t use force push, even if a post you found on Stack Overflow says to do it!

The only provided file in the skeleton is This file provides examples of how you might write tests to verify the correctness of your code. We strongly encourage you try out the given tests, as well as to write at least one of your own.

You may find writing tests quite annoying. However, in part B of this project, we will use a library called JUnit, which will make writing tests much easier and organized. We will talk more about testing in a later section.

The Deque API

The double ended queue is very similar to the SLList and AList classes that we’ve discussed in class. Here is a definition from

Deque (usually pronounced like “deck”) is an irregular acronym of double-ended queue. Double-ended queues are sequence containers with dynamic sizes that can be expanded or contracted on both ends (either its front or its back).

Specifically, any deque implementation must have exactly the following operations:

Your class should accept any generic type (not just integers). For information on creating and using generic data structures, see lecture 5. Make sure to pay close attention to the rules of thumb on the last slide about generics.

In this project, you will provide two implementations for the Deque interface: one powered by a Linked List, and one by a resizing array.

Project Tasks

1. Linked List Deque

Note: We covered everything needed in lecture to do this part in Lectures 4 and 5 (9/2 and 9/4).

Create a file called in your proj1a directory.

As your first deque implementation, you’ll build the LinkedListDeque class, which will be linked list based.

Your operations are subject to the following rules:

Implement all the methods listed above in “The Deque API” section.

In addition, you also need to implement:

You may add any private helper classes or methods in if you deem it necessary.

While this may sound simple, there are many design issues to consider, and you may find the implementation more challenging than you’d expect. Make sure to consult the lecture on doubly linked lists, particularly the slides on sentinel nodes: two sentinel topology, and circular sentinel topology. I prefer the circular approach. You are not allowed to use Java’s built in LinkedList data structure (or any data structure from java.util.*) in your implementation.

2. Array Deque

Note: We’ll have covered everything needed in lecture to do this part by lecture 6 (9/7).

Create a file called in your proj1a directory.

As your second deque implementation, you’ll build the ArrayDeque class. This deque must use arrays as the core data structure.

For this implementation, your operations are subject to the following rules:

Implement all the methods listed above in “The Deque API” section.

In addition, you also need to implement:

You may add any private helper classes or methods in if you deem it necessary.

You will need to somehow keep track of what array indices hold the Deque’s front and back elements. We strongly recommend that you treat your array as circular for this exercise. In other words, if your front item is at position zero, and you addFirst, the new front should loop back around to the end of the array (so the new front item in the deque will be the last item in the underlying array). This will result in far fewer headaches than non-circular approaches. See the project 1 demo slides for more details.

Correctly resizing your array is very tricky, and will require some deep thought. Try drawing out various approaches by hand. It may take you quite some time to come up with the right approach, and we encourage you to debate the big ideas with your fellow students or TAs. Make sure that your actual implementation is by you alone.

Frequently Asked Questions

Q: How should I print the items in my deque when I don’t know their type?

A: It’s fine to use the default String that will be printed (this string comes from an Object’s implementation of toString(), which we’ll talk more about later this semester). For example, if you called the generic type in your class Jumanji, to print Jumanji j, you can call System.out.print(j).

Q: I can’t get Java to create an array of generic objects!

A: Use the strange syntax we saw in lecture, i.e. T[] a = (T[]) new Object[1000];. Here, T is a generic type, it’s a placeholder for other Object types like “String” or “Integer”.

Q: I tried that but I’m getting a compiler warning?

A: Sorry, this is something the designers of Java messed up when they introduced generics into Java. There’s no nice way around it. Enjoy your compiler warning. We’ll talk more about this in a few weeks.

Q: How do I make my arrows point to particular fields of a data structure?

In your diagram from lecture it looked like the arrows were able to point to the middle of an array or at specific fields of a node.

A: Any time I drew an arrow in class that pointed at an object, the pointer was to the ENTIRE object, not a particular field of an object. In fact it is impossible for a reference to point to the fields of an object in Java.


Testing is an important part of code writing in industry and academia. It is an essential skill that can prevent monetary loss and hazardous bugs in industry, or in your case, losing points. Learning how to write good, comprehensive unit tests, and developing a good habit of always testing code before shipping are some core objectives of CS 61B.

We will focus more on testing in the next part of this project, project 1B. For now, we have provided you a very simple sanity check, To use the sample testing file, you must uncomment the lines in the sample tests. Only uncomment a test once you have implemented all of the methods used by that test (otherwise it won’t compile). Execute the main method to run the tests. When testing your project, remember you can use the visualizer from inside IntelliJ!

You will not submit It is for your benefit to write more comprehensive tests for both LinkedListDeque and ArrayDeque before submitting. Note, passing the given tests in does not necessarily mean that you will pass all of the autograder tests or receive full credit.