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

A. Introduction to JUnit

In lab 1, you saw an example of unit testing, the testing of individual components of a program (methods). To do this, you write extra code that is not actually used in the actual operation of your program, but is instead intended for use during development to find and localize bugs as they happen.

We'll rely on a widely used testing package called JUnit. Your instructor has been quoted as saying "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 lab1, start this homework by using the following commands in your local repo directory.

git fetch shared
git merge shared/hw1
git push

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

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, the 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. Try running the code by compiling with make (on Unix or MacOS) and then java ArithmeticTest. (On Windows, you can compile with javac instead). You should get the following:

product OK.
sum FAILS.

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 test 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 specified classes at execution time and call all of the methods in them that are annotated to be tests, 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.

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. 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 simply 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 starter test, we can use:

$ make check

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

java CompoundInterestTest

after first making sure that CompoundInterestTest.class is up to date. Those of you on Windows can simply use this command.

This will compile the starter code and run the JUnit test, and 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.

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 Part 1, please give the test-first approach a fair shake. We believe it will save you grief in the future.


Start this homework by opening CompoundInterestTest.java and CompoundInterest.java in your text editor of choice. 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 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, 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 the make check command, 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 bunches 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.


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 if we take into account inflation. Having a million dollars today is very different than 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 will be worth in 40 years and we assume the inflation rate will be 3 percent over the next 40 years, we'd see they'd be worth $1,000,000 * (0.97)^(40) or $295,712.28 in 2014 dollars. Not bad, but not quite so impressive.

Again, start by writing the tests, then use make check to ensure that the tests correct 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 everytime 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.

In case you're curious about the parametric assumptions made in main, see the end of this assignment for more.

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. If you want, you can check your answer using this web tool.

C. IntLists

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.

In the IntList folder, you'll see an implementation of the IntList class as discussed in class during lecture 4. Your task for this problem is to add four new methods for manipulating IntLists: dcatenate, subtail, sublist, and dsublist. The desired behavior for these functions is given in the starter code for IntList.

Start by implementing testDcatenate in IntListTest.java. Run make check to ensure your test fails (since you haven't implemented dcatenate yet).

After writing testDcatenate, move on to the dcatenate method in IntListTest. You're welcome to use either iterative or recursive code in your solution, but we recommend a recursive approach.

Once you've completed testDcatenate and dcatenate, repeat this process for the next three methods of IntList.

When you're done with all four tests and IntList methods, and have properly committed them (git commit -a ...), submit them as for lab1:

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

This is required in order to get credit for the homework.

D. Regarding Investment Assumptions

We were seemingly quite optimistic in our assumptions, expecting a 10% yearly return and a 3% inflation rate. This rate of return was assuming that you've invested your money in what is known as an index fund. Index funds have become particularly popular in recent years, and for good reason. Over the past half century, an investment portfolia that simply consists of every single company in the S&P 500 would have earned an effective interest rate of 10% per year (the actual returns are highly volatile, but averaged over a very long period of time it comes to about 10%).

This seemingly naive (and easy to automate!) strategy of simply investing in a large number of large companies actually does quite well. In contrast, there are also managed funds, which are steered by analysts who try to beat the market, i.e. try to do better than the naive strategy of investing in the entire S&P (or similar). Intriguingly, despite their highly capable employees, such managed funds have traditionally been unable to beat managed funds in performance (see this, for example). And analysts don't work for free. If you invest in managed funds, the company running it will take a cut that ends up making a huge difference in your overall investment payoff.

Caveat emptor: While the S&P 500 has historically done quite well, the stock markets of other nations have not been quite so lucky. It is entirely possible that we are experiencing a phase shift in the American (and perhaps global) economy; those sweet 10% returns might evaporate for good, and who knows what sort of financial dystopia we might find outselves in.