Homework 1: JUnit testing, basic syntax, and linked lists

A. Introduction to JUnit

In hw1, you saw an example of unit testing, the testing of individual components (methods) of a program. To do this, you write extra code that is not used in the actual operation of your program, but is instead intended for use during development to find and localize bugs as they happen.

Why is testing important?

Let's say we have a giant project to keep track of users' accounts and the overall worth of their stocks. The project comprises 10 classes, each with a wide range of methods and constructors. In testing our project, we find out that one of the user's overall worth for 2020 is off by 19 cents.

Where do we start looking for the problem? Do we look for errors in how we store each user's balance? Do we look for errors in how we calculate interest over time? Determining where the bug is coming from may take longer than writing the program itself!

Testing each individual piece helps us avoid this problem. When we finish our interest calculation method, we write a small test which helps us feel confident that our method is working correctly, and fix whatever bugs we are able to find. When we know that our foundation is solid (that all of the individual methods we wrote should be working correctly), we can move forward to actually using our code without worrying.


To support unit testing our programs, we'll rely on a widely used testing package called JUnit. Your instructor has been quoted saying that "it is one of the most poorly documented bunches of Java code I've seen," so we'll jump right into using it, rather than going to any official documentation.

As with any assignment, start this homework by running the following commands in your local repo directory.

git fetch shared
git merge shared/hw1 -m "start HW 1"
git push

You'll receive a hw1 folder with three subdirectories: Arithmetic, CompoundInterest, and MultiArr. Arithmetic contains a fully implemented sample program and JUnit tests, and the other two folders are programs that you'll need to implement for this homework.

Open the hw1 folder in IntelliJ using Import Project or File > New > Project from Existing Sources..., and not with the Open command. Remember to import the libraries in cs61b-software/lib by File > Project Structure > Libraries and clicking on the green plus button.

Ad-hoc Testing in Java

Let's start by examining the already completed contents of the Arithmetic folder. In it, you'll see a very simple arithmetic package in a file named Arithmetic.java, along with a couple of test clients named ArithmeticTest.java and ArithmeticJunitTest.java. In case you're unfamiliar with the term, a program X is said to be a client of the program Y if X uses any data or methods from program Y. In this case, the purpose of our two clients will be to test the class Arithmetic.

The ArithmeticTest test client is an ad-hoc test written entirely from scratch. Don't try to understand the details or even the flow of the test, just briefly look at the overall structure, noting the length and the nature of the methods implemented.

You'll observe that the source code is 56 lines long, and has to manually implement common tasks like approximate floating point comparison, tallying of tests passed, and provision of useful test output for the human user. There are various ways to run the tests. Try running the file ArithmeticTest.java in IntelliJ. You should see the output:

product OK.
sum FAILS.

On Unix or MacOS, another alternative is to use the command line to compile the program with make (or compile ArithmeticTest.java with javac on Windows) and then run it with java ArithmeticTest (or do both with make adhoc-check). This should give the same output.

JUnit Testing

The JUnit package does a lot of the kludgy work for us, avoiding implementation of common testing tasks such as those we saw in ArithmeticTest. Basic JUnit tests tend to leverage a few key components:

  1. A set of methods with names like assertTrue and assertEquals that perform some simple tests and cause an error if it fails.
  2. A number of "annotations," such as @Test, which marks a method as being a unit test.
  3. Various main testing routines that examine specific classes at execution time and call all of their annotated test methods, i.e. those methods with @Test proceeding their definition.

As an example, look at the Arithmetic/ArithmeticJUnitTest.java JUnit- based arithmetic test client:

In this homework, you'll write your own such JUnit tests, which can be compiled and executed with the command make check.

There are a number of advantages to using JUnit-based testing over the ad-hoc test above: the JUnit test is only 29 lines long, is easier to read, and avoids implementation of common tasks like approximate floating point comparisons, and so forth. Furthermore, when run, it also provides us with a more useful output for deubgging purposes:

Time: 0.018
There were 1 failures:
1) testSum(ArithmeticJUnitTest)
expected:<11.0> but was:<30.0>
at ArithmeticJUnitTest.testSum:14 (ArithmeticJUnitTest.java)
Ran 2 tests. 1 failed.

JUnit tests are easy to write once you learn the basics and give you useful output, straight out of the box. We hope you'll grow to love them.

B. Arithmetic

Open up Arithmetic/ArithmeticJUnitTest.java. Try looking through the file. Try running the tests. Do they pass or fail? Make sure the test fail and then open Arithmetic/Arithmetic.java. Look through the code until you find the mistake. Try running your tests again. They should pass now.

C. Compound Interest

"Compound interest is the most powerful force in the universe." - Albert Einstein (maybe)

Investment income grows faster than inflation, and thus the choices you make about investment at an early age can make a huge difference in how much money you'll have when you retire. In this homework problem, we'll build some code to explore this idea, and we'll also get some practice with the idea of test-driven-development using JUnit.

Go into the CompoundInterest folder, and you should see CompoundInterest.java, CompoundInterestTest.java, and Makefile. Each of these files is a skeleton. Your goal in this problem is to fill in all the methods in both .java files to match the comments.

As you work, try to use the test-driven development methodology where you do things in the following order: Write the test. Run the test (you should fail). Write the code. Run the test (you should pass). Refactor if desired and if so, re-run test.

To run the tests in CompoundInterestTest.java, select the file and click on Run > Run 'CompoundInterestTest' in IntelliJ. You'll see that the unit tests report that all tests have passed. This is bad, because it means that our starter test is garbage, as it believes our incomplete CompoundInterest.java is flawless.

By the way, you can also run the starter test from the command line like this (if you have make installed):

$ make check

which (as you can see from Makefile) runs the command

java CompoundInterestTest

after first making sure that CompoundInterestTest.class is up to date.

The rest of this part of the homework describes a suggested path to completion. You do not have to follow it, but it is recommended. If you set off on your own from this point on to Part 1, please give the test-first approach a fair shake. We strongly believe it will save you grief in the future.


Start this homework by opening CompoundInterestTest.java and CompoundInterest.java in intelliJ. In CompoundInterestTest.java, you'll see a bunch of tests you're supposed to implement. Using Arithmetic/ArithmeticJUnitTest.java as a guide, edit the testNumYears method so that it acts as a good test of whether or not numYears obeys the specifications given in the documentation comments in CompoundInterest.java. Two assertEquals statements are probably good enough. We're throwing you right in at the deep end with a bunch of sharks and megalodons here, so don't hesitate to ask for help (in lab, office hours, piazza, HKN, etc.).

Useful fact: numYears returns an int, so you don't need to specify a tolerance when you write your assertEquals statements (since we don't have to worry about rounding error when comparing integers).

After you've created better tests, run them, and your numYears method should now fail the test. Ironically, this shows that the test is working! In fact, experienced programmers get suspicious when they write a bunch of tests that don't fail out of the box.


Now edit numYears in CompoundInterest.java so that it passes the test. It should be a straightforward method to write.

While it might be a little silly to write a unit test for something as trivial as numYears, once you get used to JUnit testing, the time taken to write a test becomes so small that you may as well write at least a basic test for every method. This will save you sweat and tears down the line.


Repeat the exercise from before, but now with the testFutureValue and futureValue methods. Write the test first, and verify that it compiles and fails before moving on to writing futureValue. Feel free to use the example in the documentation comments as one of your JUnit tests.

Make sure your test includes negative appreciation rates.


Now we'll write a method that computes the future value of an appreciating (or depreciating) asset taking inflation into account. Having a million dollars today is very different from what it will be in 60 years.

To correct for inflation, one simply considers how much an asset would be worth if it hypothetically depreciated at the inflation rate for the appropriate time frame. For example, if we want to know how much 1,000,000 dollars in cash will be worth in 40 years and we assume the inflation rate will be 3 percent over the next 40 years, we'd see it would be worth $\$1,000,000 \times (0.97)^{40}$ or $295,712.29 in 2020 dollars. Not bad, but not quite so impressive.

Again, start by writing the tests, then run the tests to see they successfully compile and fail, and then finally write code for futureValueReal that passes the tests.

printDollarFV and CompoundInterest.main

Using what we've written so far, we can answer our first interesting question: How much money is future-you losing every time present-you spends a dollar? They say a penny saved is a penny earned, but this is only true if you're a bad investor. In fact, each penny is worth many pennies.

Try running CompoundInterest's main function, and you'll see that it tells you something that is clearly not true (assuming that we don't go through an apocalyptic event that eradicates the value of all money). Update the printDollarFV function so that it gives you a correct result.

totalSavings and totalSavingsReal

Another more interesting question: How much money will you have if you set aside some fixed amount each year? To lay the groundwork, repeat the same exercise as above for totalSavings and totalSavingsReal.

printSavingsFV and CompoundInterest.main

As the final step in this assignment, edit printSavingsFV so that it gives you useful information about how much money you'll have if you save perYear dollars every year until targetYear.

D. Multidimensional Arrays

Test-driven development particularly shines when you have a task whose outcome is conceptually easy to understand but hard to implement. Let's try out the TDD methodology in the context of recursive data structures.

This assignment will be diving into the world of multidimenstional arrays! We know that an array is a list of objects (integers, strings, Objects, etc). But what happens when we put an array inside an array? What about when those internal arrays are themselves filled with arrays? It's turtles all the way down!

Indexing into these types of data structures can get confusing as the number of dimensions increases, but if you can draw a mental map of the dimensions the indexing follows. Row indices always comes first, followed by column, and then by any additional following dimensions. For example, this is what a two dimensional array looks like.

To cement the idea of indexing into multiple dimensions, try implementing printRowAndCol (which will not be graded).

Then, start by opening MultiArrTest.java and implementing testMaxValue. Run the test to ensure it fails. After writing testMaxValue, implement maxValue to return the maximum value found in a multidimensional array. Then, move on to testAllRowSums followed by allRowSums. Further details of functionality and examples can be found in the skeleton code.

E. Starting Signpost

For this part of the assignment you will be introduced to Project 0: Signpost. Signpost is a board game that is played on an $N x M$ grid of squares. For this assignment, we are only concerned with a small component of the overall project. We provided the Place class, which represents one $(X,Y)$ coordinate on the board. This class has a method called successorCells, which for every square on the board, computes all the other squares reachable by a queen move (unlimited movement in a single direction) from that square. In code, directions are numbered 1-8, such that direction d is d * 45 degrees clockwise from straight up (i.e., toward higher y coordinates). Thus, direction 1 is "northeast", 2 is "east", ..., and 8 is "north".

Game board with coordinates

For example, on the above board all reachable squares from (0,0) in direction 8 (north) are (0,1), (0,2), and (0, 3). All reachable squares from (1, 1) in direction 1 (northeast) are (2, 2) and (3, 3).

The provided method is almost fully implemented, but includes two small bugs.

Open this assignment in IntelliJ. Read over the description for this method, run the unit test (which will fail since the code provided is buggy), and observe its output. Once you've read the debugging guide and feel comfortable with how the debugger works, try running through successorCells by setting a breakpoint. This is code you will use for Project 0, so you are already starting on the project! One of your tasks in Project 0 will be to implement the method successorCells, which is included here. Once you fixed the two bugs here, feel free to use your solution from this homework on the actual project.

Start by reading through the debugging guide thoroughly!

F. Submission

To get full credit for this assingment, turn in a functioning Arithmetic.java, CompoundInterest.java, and MultiArr.java (each with their approriate testing files!) and the corrected signpost code!

To submit, follow the same instructions as for lab1. Here is a summary of those:

When you're done with the assignment and have properly committed your changes (git commit -m ...), submit them:

git tag hw1-0       # or hw1-1, hw1-2, for resubmissions
git push
git push --tags

These are required in order to get credit for the homework. In order to recieve full credit your submission must pass 10/15 of the autograder tests on Compound Interest, 3/5 on Multidimensional Arrays, and 1/5 of unit test tests. You must pass all of your own unit tests.