HW 1: Packages, Interfaces, Generics, Exceptions, Iteration

Getting the Skeleton Files

As usual git pull skeleton master to get the starting files.

Note that for this homework, unlike the earlier projects, we’ll be providing lots of starter code for you to fill in.


In this homework, we will create a package for generating synthesized musical instruments. You will learn how to write and use packages, as well as get some hands-on practice with interfaces. We’ll also get an opportunity to implement a simple data structure as well as an algorithm that’s easy to implement using that data structure. Finally, we’ll add support for iteration and exceptions to our data structure.


A package is a collection of Java classes that all work together towards some common goal. We’ve already seen packages in 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 es.datastructur.synthesizer package, we’d add the following line to the top of the file.

package es.datastructur.synthesizer;

If a programmer wanted to use a class or method from our es.datastructur.synthesizer package, they would have to either use the full canonical name, e.g. es.datastructur.GuitarString, or alternately use import es.datastructur.GuitarString, at which point they could just use the simple name GuitarString.

Typically, package names are the internet address of the entity writing the code, but backwards. For example, the JUnit library is hosted at junit.org, 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 61B will be part of a package.

The Synthesizer Package

The es.datastructur.synthesizer package has three primary components:

We’ve provided you with skeleton code for ArrayRingBuffer and GuitarString, but you’ll need to implement the BoundedQueue interface from scratch. In this HW, we’ll work our way down the hierarchy from most abstract to most concrete.

Task 1: BoundedQueue

Review: What is an Interface? Why would you want one?

As discussed in class, an interface is a formal contract between a class and the outside world. If your class claims to implement an interface, then all methods defined by that interface must appear in your class (or somewhere in your superclass) before the class will successfully compile. This is a way of enforcing promised behavior. All methods that you declare in an interface are automatically public and abstract (even if you omit the public keyword).

Your Task

We will start by defining a BoundedQueue interface. The BoundedQueue is similar to our Deque from Project 1, but with a more limited API. Specifically, items can only be enqueued at the back of the queue, and can only be dequeued from the front of the queue. Unlike our Deque, the BoundedQueue has a fixed capacity, and nothing is allowed to enqueue if the queue is full.

Create a file BoundedQueue.java in the es.datastructur.synthesizer folder. You can do this easily in IntelliJ by right-clicking on the es.datastructur.synthesizer folder in the project structure sidebar and click New → Java Class. Be sure to set “Kind” to “Interface”.

Your BoundedQueue interface should contain the following methods:

int capacity();     // return size of the buffer
int fillCount();    // return number of items currently in the buffer
void enqueue(T x);  // add item x to the end
T dequeue();        // delete and return item from the front
T peek();           // return (but do not delete) item from the front

You should also create default methods isEmpty() and isFull() that return the appropriate answer if the BoundedQueue is empty or full.

default boolean isEmpty()       // is the buffer empty (fillCount equals zero)?
default boolean isFull()        // is the buffer full (fillCount is same as capacity)?

For example, given an empty BoundedQueue<Double> of capacity 4, the state of the queue after each operation is shown below:

isEmpty()       //                       (returns true)
enqueue(9.3)    // 9.3
enqueue(15.1)   // 9.3  15.1
enqueue(31.2)   // 9.3  15.1  31.2
isFull()        // 9.3  15.1  31.2       (returns false)
enqueue(-3.1)   // 9.3  15.1  31.2  -3.1
isFull()        // 9.3  15.1  31.2  -3.1 (returns true)
dequeue()       // 15.1 31.2  -3.1       (returns 9.3)
peek()          // 15.1 31.2  -3.1       (returns 15.1)

Of course, your BoundedQueue.java file won’t actually do anything (since it’s an interface), but it will define the contract that any BoundedQueue implementing class must follow.

Make sure to declare this interface as part of the es.datastructur.synthesizer package. As noted above, the syntax for declaring yourself to be part of a package is package <packagename>;. For example, if you are part of the animal package, the top of your file should have a package animal; line. Your package name should say es.datastructur.synthesizer, nothing else. (If you created the file using IntelliJ, this should have already been done for you.)

Before moving on, ensure that there are no compilation errors in BoundedQueue.

If you’re stuck, see the List61B interface for an example of an interface declaration with generics.

Task 2: ArrayRingBuffer

The ArrayRingBuffer class will do all the real work by implementing BoundedQueue. That means we can happily inherit isEmpty(), and isFull() without having to override these, but we’ll need to override all of the abstract methods. In this part, you’ll fill out ArrayRingBuffer.java, which will use an array to represent the queue and the elements in it.

A naive array based implementation of a BoundedQueue would store the newest item at position 0, the second newest item in position 1, and so forth. This is an inefficient approach, as we see in the example below, where the comments show entries 0, 1, 2, and 3 of the array respectively. We assume that the array is initially all nulls.

BoundedQueue x = new NaiveArrayBoundedQueue(4);
x.enqueue(33.1) // 33.1 null null  null
x.enqueue(44.8) // 33.1 44.8 null  null
x.enqueue(62.3) // 33.1 44.8 62.3  null
x.enqueue(-3.4) // 33.1 44.8 62.3  -3.4
x.dequeue()     // 44.8 62.3 -3.4  null (returns 33.1)

Note that in this setup, the call to dequeue is very slow as it requires moving every single item to the left, to make sure the new first element is at index 0. For larger arrays this would result in unacceptable performance.

The ArrayRingBuffer will improve this runtime substantially by using the ‘ring buffer’ data structure, similar to the circular array from Project 1A. A ring buffer first starts empty and of some predefined length. For example, this is a 7-element buffer:

empty buffer

Assume that a 1 is written into the middle of the buffer (the exact starting location does not matter in a ring buffer):

one item

Then, assume that two more elements are added, 2 and 3, which get appended after the 1. Here, it is important that the 2 and 3 are placed in the exact order and places shown:

three items

If two elements are then removed from the buffer, the oldest two values inside the buffer are removed. The two elements removed in this case are 1 and 2, leaving the buffer with just 3:

one item again

If we then enqueue 4, 5, 6, 7, 8, and 9, the ring buffer is now as shown below:


Note that the 6 was enqueued at the leftmost entry of the array (i.e. the buffer wraps around, like a ring). At this point, the ring buffer is full, and if another enqueue() is performed, then an Exception will occur. We will add the exception throwing logic at the end of this assignment.

We recommend you maintain one integer instance variable first that stores the index of the least recently inserted item; maintain a second integer instance variable last that stores the index one beyond the most recently inserted item. To insert an item, put it at index last and increment last. To remove an item, take it from index first and increment first. When either index equals capacity, make it wrap-around by changing the index to 0. Our skeleton file provides starter code along these lines. You’re welcome to do something else if you’d like, since these variables are private and thus our tester will not be able to see them anyway.

In the last section of this homework, we’ll implement our ArrayRingBuffer to throw a run-time exception if the client attempts to enqueue() into a full buffer or call dequeue() or peek() on an empty buffer.

The skeleton code contains TODOs that will guide you through writing this class. Once you’ve fleshed out the TODOs, make sure ArrayRingBuffer compiles before moving on. Optionally, you can add tests to the TestArrayRingBuffer class (either before or after your write ArrayRingBuffer). TestArrayRingBuffer.java will not be graded.

For homework and labs (but not projects), you’re welcome to share test code. Feel free to share your tests for this homework on Ed.

Task 3: GuitarString

Finally, we want to flesh out GuitarString, which uses an ArrayRingBuffer to replicate the sound of a plucked string. We’ll be using the Karplus-Strong algorithm, which is quite easy to implement with a BoundedQueue.

The Karplus-Algorithm is simply the following three steps:

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

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


You can play a double value with the StdAudio.play method. For example StdAudio.play(0.333) will tell the diaphragm of your speaker to extend itself to 1/3rd of its total reach, StdAudio.play(-0.9) 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 StdAudio.play(0.9) and never play anything again, the diaphragm shown in the image would just be sitting still 9/10ths of the way forwards.

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

Make sure to import javalib into your project, 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 uncomment the code in this class at this point, you 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 GuitarString.java that will help you see what’s going on between tics.


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 es.datastructur.synthesizer 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.

Why It Works

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

Task 4: Iteration, Exceptions, and Equals

Now that you’ve hopefully had lots of fun, let’s do a somewhat bland exercise in giving your data structures more industrial strength. In the final task, we’ll add the ability to iterate through a BoundedQueue, change its behavior so that it throws exceptions when given invalid inputs, and finally, create an equals method to test the equality of two ArrayRingBuffers.


First, modify your BoundedQueue<T> interface so that it extends Iterable<T> and add the required abstract method to the interface. You’ll need to import java.util.Iterator.


Now add the required iterator() method to ArrayRingBuffer. You’ll need to define a private class that implements the Iterator interface. See lecture 11 for an example.


Now modify ArrayRingBuffer so that it throws a RuntimeException with the String “Ring Buffer overflow” when a user attempts to enqueue into a full ArrayRingBuffer, and “Ring Buffer underflow” when a user attempts to call dequeue or peek on an empty ArrayRingBuffer.


Now, modify ArrayRingBuffer so that it overrides the equals(Object o). This method should return true only if the other object is an ArrayRingBuffer with the exact same values. For example, if we have two ArrayRingBuffers and called dequeue() on both at the same time, we would ge the same elements if they are equal. This method should be nondestructive.

Note: Once you implement equals and you run the style checker, you will get a message that says you must define a hashCode() function. Ignore this for now; we’ll talk about this method later in the semester.


You should submit the usual way, by pushing to GitHub and then submitting on Gradescope.

Frequently Asked Questions

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 es.datastructur.synthesizer package is in a folder called “synthesizer”, which is in a folder called “datastructur”, which is in a folder called “es”.

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.

I’m getting … in ArrayRIngBuffer and … in BoundedQueue have the same erasure, yet neither overrides the other.

Make sure your classes are defined as ArrayRingBuffer<T> implements BoundedQueue<T> (or whatever type parameter you used instead of T).

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

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

I’m failing the nested iteration test. What does this mean?

Consider what happens when you run the following:

int[] someInts = new int[]{1, 2, 3};
for (int x : someInts) {
    for (int y: someInts) {
        System.out.println("x: " + x +  ", y:" + y);

And think about how your code is not doing what is listed above.

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.

Additional Thoughts

For simplicity’s sake, there are a few things we did that would be bad programming practice in a real package. One important one is that we make all 3 classes public. In a real world scenario, it’d probably be better design to make only the GuitarString class public (since people using synthesizers don’t really care about ArrayRingBuffers or BoundedQueues), and restrict the other two so that they could only be used from within the package itself. To do this, we’d simply omit public from the ArrayRingBuffer and BoundedQueue definitions. However, we’ll be leaving all of our classes public for this HW for testing convenience.