Lab 10: More Debugging

A. Intro

Today, we will go over how to use some of the more powerful debugger functionalities. Your job will be to debug a variety of buggy classes, primarily using a debugger.

B. Debugging

A Review of Debugging Tools

If you're still not familiar with debuggers, please take the time to review their documentation. You have a variety of options to choose from, but the two most popular debuggers are listed below.

In IntelliJ, in addition to traditional breakpoints you can also have conditional breakpoints. Understanding how to use them will be useful for this lab.

Exercise: A Debugging Mystery

An important skill to learn is how to exhaustively debug. When done properly, debugging should allow you to rapidly narrow down where a bug might be located, even when you are debugging code you don't fully understand.

Your company, Flik Enterprises, has released a fine software library called that is able to determine whether two Integers are the same or not.

You receive an email from someone named "Horrible Steve" who describes a problem they're having with your library:

"Dear Flik Enterprises,

Your library is very bad. See the attached code. It should print out 500 but actually it's printing out 128.


Using you choice of debugger, figure out whether the bug is in Horrible Steve's code or in Flik Enterprise's library.

Once you find the bug, fix it. You might find conditional breakpoints useful for this exercise.

Exercise: SortedListâ„¢ Helper

After the embarrassing incident in the previous part, Flik Enterprises rebranded itself as Flik Enterprises Corp, and sought to clear its name by releasing a new, improved library. This library allows for easy insertion and maintenance of sorted lists -- that is, lists whose elements are sorted in increasing order.

The Flik Enterprises Corp issued a press release assuring the public that this library was 100% bug free: after all, it ships with JUnit tests, and the tests pass! However, an anonymous whistleblower involved in the development of SortedListâ„¢ Helper tells you that they fear the provided tests are incomplete, and the library isn't so bug-free after all...

1. Add statements to the testInsertIntoSortedList test in SortedListTester to more thoroughly the behavior of SortedListHelper#insertIntoSortedList. Feel free to use SortedListHelper#isListSorted in your test. Hint: edit the @Test annotation above your test into @Test(timeout=1000) to impose a time limit of 1000 milliseconds on your test.

2. Is insertIntoSortedList correct? If not, find and fix the bug. Hint: if the code is taking an unusually long time to complete, run the code in Debug mode, and click the blue 'pause' button to arbitrarily suspend the execution of your program, allowing you to more easily figure out what's taking so long.

Exercise: Plumbum|Beta

After the demise of Flik Enterprises Corp in the previous part, you decide to leave the company and start your own software company, specializing in mathematical software.

Your first product is Plumbum|Beta, a computer algebra system (CAS) written in Java.


One of your software engineers submits an initial version of the SparseIntVector class (along with a SparseIntVectorTest) for review. To your dismay, you find that the provided tests don't pass. Note: a sparse vector is a vector (i.e. 1D ordered list of numbers) that contains very few non-zero elements.

Ignoring the fact that your engineer submitted code which clearly doesn't work, please fill in the static variables in PlumbumBetaAnswers with your answers to the following questions as you find and fix the bugs in the code. You should only modify existing lines or replace the entire contents of a line marked // replace this comment with something?.

  1. Start by running the provided test, which fails with an exception. A. What is the name of the exception thrown? B. Which line (in the format for an exception thrown on line 239 of is the exception thrown by? C. Is the line in the stack trace corresponding to the method which threw the exception on the top or bottom (of the stack trace)? Answer "top" or "bottom".
  2. Now fix the first bug. In one to two sentences, describe the cause of the bug.
  3. Run the tests again. Unfortunately, there's another bug. Find and fix this bug. A. Which line (in the same format as above) contained the bug? B. Describe the bug in a single phrase. C. Which line (in the same format as above) called the method that contained the bug? Hint: look at the stack trace thrown by the second exception.

Now, you want to add swizzling functionality to Plumbum|Beta.

Swizzling is an operation that reorders the elements of a vector (or list), given a list of indices into the list.

For example, the list [2, 4, 6, 8, 10] may be swizzled with the list of indices [0, 1, 2, 0, 0, 4], producing the output array [2, 4, 6, 2, 2, 10].

Each element i in the index arrays corresponds to an element in the output array, copying the i-th element of the input array.

However, all of your engineers were busy, so you decided to hire an outside software firm to write the Swizzler class for you. Unfortunately, the Swizzler class you commissioned does not work.

Find the bug, and fix the code so that the tests pass.


Finally, you want to create a ListUtilities class for internal use. This class will provide several helpful utility methods for working on lists which will be used throughout the Plumbum|Beta codebase.

You start by creating ListUtilities.filter, a method that takes an input list and a predicate (a function that takes a single argument and returns a boolean).

The intended functionality is for filter to remove all of the elements from the input list that don't satisfy the predicate (that is, predicate(element) returns false).

Despite this simple functionality (and short and simple implementation!), the filter method is buggy.

As usual, find the bug and fix the code!

C. Submission

Please submit all of the Java files in the lab.