Project 1: Data Structures

The due date for this project is 02/16, but to keep you on track we have an extra credit checkpoint on 02/05 that will be a smaller version of the autograder for the project. More details later in the spec.

You will also push your snaps-sp21-s*** repo at the end of the assignment to get your Gradescope score into Beacon. More on that later.

Recall you can only use 2 slip days on this project because of the dependency between Project 1 and Lab 4.


In Project 1, we will build implementations of a “Double Ended Queue” using both lists and arrays in a package that other classes can use. The project is roughly split into two halves: the data structure portion and the application portion.

In the data structure part of the project, you will create two Java files: and, with public methods listed below. You will be verifying the correctness of these data structures yourself using the randomized and timing test skills you gained from Lab 3.

In the application part of this project, you’ll create a Java file as well as use your package to ultimately implement a sound synthesizer capable of playing music from Guitar Hero. You must test your MaxArrayDeque, but we’ll provide the tests for sound synthesizer.

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.

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 ‘sp21-s101’ folder (or any subdirectory).

To make sure you have the latest copy of the skeleton files, use the command:

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 proj1 directory appear with two folders:

├── deque
│   └──
└── gh2

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 files in the skeleton are the deque/ file as well as some skeleton for the second part of this project located in the gh2 folder (guitar hero 2). The deque/ 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 your own, as these tests are not comprehensive.

The tests in this file are also the exact tests that will be used in the checkpoint to assess your progress on your LinkedListDeque implementation, but we will also have additional tests for ArrayDeque that we do not give you. More details on the appear checkpoint later in the spec.

Before we get into the details of the Deque API and the implementation requirements, let’s briefly talk about packages and why we are using them in this project.


Part of this project is using packages to separate logic and functionality. At the end of the project, you’ll have two packages: the deque package that provides an implementation of the Deque data structure, and the gh2 package that implements a synthesizer used to play guitar hero. You should already see folders with these names in the starter code, and your job is to implement them. Let’s look at the specifics for what a package really is.

A package is a collection of Java classes that all work together towards some common goal. We’ve already seen packages in CS 61B without knowing it. For example, org.junit is a package that contains various classes useful for testing, including our familiar Assert class, which contains useful static methods like assertEquals. In other words, when we saw org.junit.Assert.assertEquals, the org.junit was the package name, Assert was the class name, and assertEquals was the method name. We call org.junit.Assert.assertEquals the “canonical name” of the method, and we call assertEquals the “simple name” of the method.

When creating a package, we specify that code is part of a package by specifying the package name at the top of the file using the package keyword. For example, if we wanted to declare that a file is part of the deque package, we’d add the following line to the top of the file.

package deque;

If a programmer wanted to use a class or method from our deque package, they would have to either use the full canonical name, e.g. deque.ArrayDeque, or alternately use import deque.ArrayDeque, at which point they could just use the simple name ArrayDeque. So import statements just allow you to use the simple name of a class/method.

Typically, package names are the internet address of the entity writing the code, but backwards. For example, the JUnit library is hosted at, so the package is called org.junit.

Why are packages useful? It all boils down to that word “canonical”. As long as no two programmers use the same package name for their package, we can freely use the same class name in several different contexts. For example, there might exist a class called com.hrblock.TaxCalculator, which is different from com.turbotax.TaxCalculator. Given the requirement to either use the full canonical name or to use an import, this means we’ll never accidentally use one class when we meant to use the other.

Conceptually, you can think of packages as being similar to different folders on your computer. When you are building a large system, it is a good idea to organize it into different packages.

From this point forwards, most of our code in CS 61B will be part of a package.

With that out of the way, let’s talk about the methods that a Deque should have.

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:

In addition, we also want our two Deques to implement these two special methods:

ADDED 2/12: You should not have your Deque interface implement Iterable but rather just the two implementations LinkedListDeque and ArrayDeque. If you do the former, our autograder will give you API errors.

You’ll learn what an Iterator is in lecture 11 (2/12), so don’t worry for about it right now. This project is meant to be done little by little as you learn more things from lecture and discussion, and it’s a fantastic opportunity to practice all the things you learn in this course.

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 (1/27 and 1/25) with the exception of Iterators, which you’ll learn about in lecture 11 (2/12).

Create a file called in your proj1/deque directory. Make sure you declare that it is in the deque package using the special package keyword.

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. If you do, please add helpful javadoc comments for your and your TAs sake.

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 and the autograder will instantly give you a 0 if we detect that you’ve imported any such data structure.

2. Array Deque

Note: We’ll have covered everything needed in lecture to do this part by lecture 7 (2/03) with the exception of Iterators, which you’ll learn about in lecture 11 (2/12).

Create a file called in your proj1/deque directory. Again, use the package keyword to tell this file that it is part of the deque package.

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.

Extra Credit: Checkpoint

To keep you on track for this project, we have a checkpoint due on 2/05 for 16 extra credit points. The autograder for the checkpoint will test basic functionality of your LinkedListDeque and ArrayDeque classes: specifically, the given tests in will be used to test your LinkedListDeque implementation, and we will use our own tests to test your ArrayDeque implementation. By “basic functionality”, we mean all the add methods, all the remove methods, the size method, the isEmpty method, and the get method. This means we will not test your equals(Object o) nor your iterator() methods.

NOTE: the isEmpty has been on the grader the whole time but we added this to the spec today (2/5). However, if you made the size method, this method should be a piece of cake (you can make it a one-liner).

Importantly, we will insert no more than 8 items in your ArrayDeque, meaning you shouldn’t have to worry about resizing for this checkpoint.

You may choose not to do the checkpoint, but we highly suggest you do so that you stay on track and earn some extra credit while you’re at it.

The rest of the assignment will not be considered in the checkpoint.


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.

In the start code, 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 on the full autograder

Because the part of the goals for this project is for you to build something and assess the correctness by yourself, we do not want you to become to reliant on the full autograder to verify correctness. Thus, you will receive one autograder token that recharges every 8 hours. You cannot “stack” these tokens, so if you do not submit anything to the autograder for 3 days, you still only have one token.

On Saturday 2/13, the recharge rate will permanently reduce to every 20 minutes.

So how do you verify correctness of your data structure? You use your skills that you got from Lab 3! You are encouraged to copy and paste those tests for SList and AList and adapt them for these data structures. The tests will look very similar and only require basic changes.

While it does seem very daunting and scary to do an entire project with very minimal access to the autograder, you should feel very confident in your implementation if your randomized tests are really big. With just a few lines of code, you could test your data structures with sizes in the 100,000’s and all sorts of random method calls in random order. In other words, you are testing a whole lot of cases on your data structures and are likely testing every possible edge case! This is the beauty of randomized testing: it allows us to leave the creativity of thinking of edge cases to randomness.

The tests you create will not be graded, but there is an additional extra credit portion of this project in which you will write your own autograder. Details near the bottom of the spec.

Your code will not compile on the full autograder until you implement the Deque interface and all of the required methods. So if you’ve done everything on the spec up until this point, you should be using the checkpoint grader and not the full grader.

Extra Credit: Autograder

For 32 points of extra credit, you can create your own autograder for deques! Full details are on this spec (to avoid cluttering this one). It is due the same day as Project 1 (2/16). You can use slip days on extra credit assignments, but understand that staff will prioritize the required portions when helping students on Ed and in Office Hours. We don’t suggest working on this extre credit assignment until you’ve finished the entire project, so keep this in mind but continue with the assignment for now.


After you’ve fully implemented your ArrayDeque and tested its correctness, you will now build the MaxArrayDeque. A MaxArrayDeque has all of the methods that an ArrayDeque has, but it also has 2 additional methods and a new constructor:

The MaxArrayDeque can either tell you the max element in itself by using the Comparator<T> given to it in the constructor, or an arbitrary Comparator<T> that is different from the one given in the constructor.

We do not care about the equals(Object o) method of this class, so feel free to define it however you think is most appropriate. We will not test this method.

If you find yourself starting off by copying your entire ArrayDeque implementation in a MaxArrayDeque file, then you’re doing it wrong. This is an exercise in clean code, and redundancy is one our worst enemies when battling complexity! For a hint, re-read the second sentence of this section above.

There are no runtime requirements on these additional methods, we only care about the correctness of your answer. Sometimes, there might be multiple elements in the MaxArrayDeque that are all equal and hence all the max: in in this case, you can return any of them and they will be considered correct.

You should write tests for this part as well! They do not need to be nearly as robust as your randomized and timing tests you created for the two Deque implementations above since the functionality you’re adding is fairly simple. You’ll likely be creating multiple Comparator<T> classes to test your code: this is the point! To get practice using Comparator objects to do something useful (find the maximum element) and to get practice writing your own Comparator classes. You will not be turning in these tests, but we still highly suggest making them for your sake.

You will not use the MaxArrayDeque you made for the next part. It is it’s own isolated exercise.

Deque Interface

In the last part of this project, we’re actually going to use the data structure you made to solve a real world problem.

Recall that we defined the Deque API, or behavior, by the following methods:

public void addFirst(T item)
public void addLast(T item)
public boolean isEmpty()
public int size()
public void printDeque()
public T removeFirst()
public T removeLast()
public T get(int index) 

Since your program will rely on this behavior, it shouldn’t matter to it what Deque implementation it is provided, ArrayDeque or LinkedListDeque, and should work for both. To achieve this, we will use the power of interfaces.

This first task is going to be a little tedious, but it won’t take long.

Create an interface in a new file named that contains all of the methods above. In IntelliJ, use “New → Java Class”. IntelliJ will assume you want a class, so make sure to replace the class keyword with interface. Don’t forget to declare that the Deque interface is part of the deque package!

Modify your LinkedListDeque and/or ArrayDeque so that they implement the Deque interface by adding implements Deque<T> to the line declaring the existence of the class. If IntelliJ yells at you with an error message like:

The method ... of type LinkedListDeque has the same erasure as ... of type Deque but does not override it.

It means you forgot the generic T in the implements line (i.e. you wrote implements Deque instead of implements Deque<T>).

If you used something other than T for your generic type parameter, use that instead. Add @Override tags to each method that overrides a Deque method.

Now, in the Deque interface, give isEmpty() a default implementation, which returns true if the size() is 0. Since your LinkedListDeque and ArrayDeque implement the Deque interface, given the default isEmpty() implementation, you can remove that method from the LinkedListDeque and ArrayDeque that you implemented earlier.

Now, after you’ve implemented the Deque interface and removed the isEmpty() method from your LinkedListDeque and ArrayDeque implementations, your code will compile on the full autograder.

Guitar Hero

In this part of the project, we will create another package for generating synthesized musical instruments using the deque package we just made. We’ll get the opportunity to use our data structure for implementing an algorithm that allows us to simulate the plucking of a guitar string.

The GH2 Package

The gh2 package has just one primary component that you will edit:

We’ve provided you with skeleton code for GuitarString which is where you will use your deque package that you made in the first part of this project.


We want to finish the GuitarString file, which should use the deque package to replicate the sound of a plucked string. We’ll be using the Karplus-Strong algorithm, which is quite easy to implement with a Deque.

The Karplus-Algorithm is simply the following three steps:

  1. Replace every item in a Deque with random noise (double values between -0.5 and 0.5).
  2. Remove the front double in the Deque and average it with the next double in the Deque (hint: use removeFirst) and get()) multiplied by an energy decay factor of 0.996 (we’ll call this entire quantity newDouble). Then, add newDouble to the back of the Deque.
  3. Play the double (newDouble) that you dequeued in step 2. Go back to step 2 (and repeat forever).

Or visually, if the Deque is as shown on the top, we’d remove the 0.2, combine it with the 0.4 to form 0.2988, add the 0.2988, and play the 0.2.


You can play a double value with the method. For example will tell the diaphragm of your speaker to extend itself to 1/3rd of its total reach, will tell it to stretch its little heart backwards almost as far as it can reach. Movement of the speaker diaphragm displaces air, and if you displace air in nice patterns, these disruptions will be interpreted by your consciousness as pleasing thanks to billions of years of evolution. See this page for more. If you simply do and never play anything again, the diaphragm shown in the image would just be sitting still 9/10ths of the way forwards.

Complete so that it implements steps 1 and 2 of the Karplus-Strong algorithm. Note that you will have to fill you Deque buffer with zeros in the GuitarString constructor. Step 3 will be done by the client of the GuitarString class.

Make sure to open your project with Maven, as usual, otherwise IntelliJ won’t be able to find StdAudio.

For example, the provided TestGuitarString class provides a sample test testPluckTheAString that attempts to play an A-note on a guitar string. If you run the test should hear an A-note when you run this test. If you don’t, you should try the testTic method and debug from there. Consider adding a print or toString method to that will help you see what’s going on between tics.

Note: we’ve said Deque here, but not specified which Deque implementation to use. That is because we only need those operations addLast, removeFirst, and get and we know that classes that implement Deque have them. So you are free to choose either the LinkedListDeque for the actual implementation, or the ArrayDeque. For an optional (but highly suggested) exercise, think about the tradeoffs with using one vs the other and discuss with your friends what you think the better choice is, or if they’re both equally fine choices.


You should now also be able to use the GuitarHeroLite class. Running it will provide a graphical interface, allowing the user (you!) to interactively play sounds using the gh2 package’s GuitarString class.

The following part of the assignment is not graded.

Consider creating a program GuitarHero that is similar to GuitarHeroLite, but supports a total of 37 notes on the chromatic scale from 110Hz to 880Hz. Use the following 37 keys to represent the keyboard, from lowest note to highest note:

String keyboard = "q2we4r5ty7u8i9op-[=zxdcfvgbnjmk,.;/' ";

This keyboard arrangement imitates a piano keyboard: The “white keys” are on the qwerty and zxcv rows and the “black keys” on the 12345 and asdf rows of the keyboard.

The ith character of the string keyboard corresponds to a frequency of $440 \cdot 2^{(i - 24) / 12}$, so that the character ‘q’ is 110Hz, ‘i’ is 220Hz, ‘v’ is 440Hz, and ‘ ‘ is 880Hz. Don’t even think of including 37 individual GuitarString variables or a 37-way if statement! Instead, create an array of 37 GuitarString objects and use keyboard.indexOf(key) to figure out which key was typed. Make sure your program does not crash if a key is pressed that does not correspond to one of your 37 notes.

Just For Fun: TTFAF

Once you’re relatively comfortable that GuitarString should be working, try running TTFAF. Make sure your sound is on!

You can read the GuitarPlayer and TTFAF classes to figure out how they work. TTFAF in particular includes (as commented-out code) an example of how to use it another way.

Even More Fun

This part of the assignment is not graded and just for fun.

Why It Works

The two primary components that make the Karplus-Strong algorithm work are the ring buffer feedback mechanism and the averaging operation.

Submission and Grading

To submit the project, add and commit your files, then push to your remote repository. Then on Gradescope go to the assignment and submit there.

After your final submission on Gradescope, you must push your snaps repo.

Your Gradescope score will not be transferred to Beacon until you’ve pushed your snaps repo and submitted to the Snaps Gradescope assignment. To push your snaps repo, run these commands:

git push

After you’ve pushed your snaps repository, there is a Gradescope assignment that you will submit your snaps-sp21-s*** repository to (similar to Lab 1A). This is only for the full grader (not the checkpoint nor the extra credit assignment).

You can do this up to a week after the deadline as well in case you forget. If you forget to push after a week, then you’ll have to use slip days.

NOTE: we slightly edited the above snaps thing as of 2/07 from simply pushing your snaps repo to pushing and submitting on Gradescope so it’s more obvious for you, the student, that your submission went through.

The entire project is worth 640 points

And there are a total of 48 extra credit points available:

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.

Guitar Hero

I’m getting a “class file contains wrong class” error.

Make sure all of your Java files have the right package declaration at the top. Also make sure that anything that is part of the gh2 package is in a folder called “gh2”.

I’m getting a message that I did not override an abstract method, but I am!

Chances are you have a typo. You should always use the @Override tag when overriding methods so that the compiler will find any such typos.

When I try to run the provided tests I get “No runnable methods”.

Make sure you’ve uncommented the tests, including the @Test annotation.

When I try to compile my code, it says type K#1 is not compatible with type K#2, or something similar.

If you’re defining an inner class, make sure it does not redeclare a new generic type parameter, e.g. the first <Z> given in private class MapWizard<Z> implements Iterator<Z>{ should not be there!

I’m getting a strange autograder error!

While GuitarString is a guitar string simulator, it should not involve playing any sounds. The playing should be done by the GuitarString client.

Credits: RingBuffer figures from wikipedia. This assignment adapted from Kevin Wayne’s Guitar Heroine assignment.