Lab 4: Linked Lists

The purpose of this week’s lab is to give you some hands-on experience writing code that manipulates a linked structure. Your task is to complete the Laboratory Assignment from Section 9.10 of your text (Bailey), entitled Lists with Dummy Nodes (pp. 215—217). You will start with code that already works: a class called LinkedList that extends the structure5 class DoublyLinkedList. The goal is to produce a new class that has the same interface as the old class, but with a simpler implementation. To achieve this, you will add two “dummy” nodes to your LinkedList.

The Laboratory Assignment in Section 9.10 walks you through the refactoring process. You will have the best experience if you follow the instructions in that description carefully!

PRE-LAB: Step 0

This is a partner lab. You may work with one other person of your choosing, or if you are looking for an extra challenge you may work entirely by yourself. Although you are permitted to jointly develop code with your partner, each of you must independently submit your code. No copying of code is permitted. You must independently retype any code you develop with your partner.

Indicate your partnering arrangement (including those flying solo) by filling out the following form.

You would help finding a partner, please indicate so on the form and we will do our best to find you one.

PRE-LAB: Step 1

Before lab, please do the following:

  • Read Chapter 9 up to and including 9.5 Implementation: Doubly Linked Lists, and bring your questions to lab.
  • Study the code in the file before coming to lab, and think carefully about how you might modify the various methods as described in the assignment. A copy of the file will also be provided in your starter repository.

Lab Assignment

  1. Complete Laboratory Assignment 9.10, which begins on p. 215 of Bailey. The starter file will be included in your team’s private GitLab repository in addition to a file,, that includes tests. The tests are not exhaustive, so please add additional tests as you consider the various edge cases.
  2. In the comment block for each method in your LinkedList class, provide the running time (in Big-O notation) for each method, along with a brief justification.

Big-O and Pre/Post-conditions

As with your previous lab, be sure to provide Javadoc annotations at the top of every method. We talk more about Javadocs below if you need a refresher.

In addition to @pre and @post annotations, you should also provide a @big-o annotation that states the asymptotic runtime of the method.

For example:

   * A method that randomly prints out the same number over
   * and over.
   * @pre 0 <= low < 10.
   * @post Prints 10 - low numbers.
   * @big-o O(1).
  public static void iSwearItIsRandom(int low) {
    Assert.pre(low >= 0 && low < 10, "low must be between 0 inclusive and 10 exclusive.");
    Random random = new Random(441287210);
    for (int i = low; i < 10; i++) {
      System.out.print(random.nextInt(10) + " ");


As usual, we will be checking for good code style. We will be checking that you follow the same style guidelines we’ve introduced in the past. This section describes some additional guidelines that we’d like you to follow.

Requirement: checkstyle

This week, we are expanding checkstyle to encourage you to write modular and reusable code.

Rule: Don’t Repeat Yourself (DRY)

As a programmer, you should never type the same code over and over again. That would be a waste of your time. The ability to copy/paste useful code makes this tempting, but it is a bad practice! When you copy code, you copy errors. If you ever fix those errors, you have to remember to fix them in every location. This “fixing” rarely happens. Trust us. We’ve been there. We’re not really any smarter than you, we’ve just made all the mistakes already.

So what should you do instead? Write a helper method! Whenever you find yourself writing the same code more than once (it could be a common loop, searching a data structure, computing an equation) you should refactor that code into a method. Then you can call that method many times, but only ever have to fix bugs in one place.

Similarly, if you find yourselves writing a function that is very long, that is problematic. The long function can probably be broken into smaller parts that can then be composed to solve a larger problem. As a bonus, you can now reuse those parts to solve other problems.

Breaking long functions into byte-size chunks also has the benefit of making your code easier to read and debug. By isolating functionality inside independent units, you can convince yourself that each of those units is correct in isolation. Then, to reason about the entire program, you only need to convince yourself that the combination of those correct units is correct.

What checkstyle looks for

Since checkstyle does not directly check for copy/pasted code, instead we encourage concise methods.

The checkstyle tool will report an [ERROR] if your program has a method that is more than 40 lines long (excluding whitespace and single-line comments). This new rule is in addition to previous style requirements.

In total, checkstyle will enforce the following guidelines:

  • All class variables that are not final must be declared private or protected (i.e., no public member variables unless they are constants).
  • All public methods must include a “Javadoc” comment (starts with /** and ends with */; it should include descriptions of the function, the arguments, and any pre/post conditions).
  • No method should be longer than 40 lines (exluding whitespace and single-line comments).

To run checkstyle, type the following command at the terminal:

$ ./checkstyle

The ./ is peculiar to Unix: it tells the terminal to look for the checkstyle program in the current directory. This command runs checkstyle on every Java program in your directory. To run checkstyle on a specific Java file, type:

$ ./checkstyle

Requirement: javac

In addition to checkstyle, we will continue to enforce proper use of Java generics.

  • Compiling your code with javac must not produce the following message:
    Note: uses unchecked or unsafe operations.
    Note: Recompile with -Xlint:unchecked for details.

This message tells you how to get more information. Recompile your program as follows (replace with the appropriate file name):

$ javac -Xlint:unchecked 

The output will give you details about the issue.

To fix this problem, make sure that you specify type parameters for every generic class, both when declaring the variable’s type and when instantiating an object. For example, to create a Vector that stores Integer objects, one would type:

    Vector<Integer> intVec = new Vector<Integer>();

Lab Deliverables

By the start of lab, you should see a new private repository called lab04-dummylists in your GitLab account.

For this lab, please submit the following:


The file contains starter code, and you should write all of your functions inside that file. The file contains a convenient main method pre-populated with a variety of helpful tests that should help you get started.

As with all labs, you will be graded on design, documentation, style, and correctness. Be sure to document your program appropriately: include pre/post conditions and assertions where appropriate. We will also be looking at how well you organize your code. Whenever you see yourself duplicating functionality, consider moving that code to a helper method. There are several opportunities in this lab to simplify your code by using helper methods.

Submitting Your Lab

As you complete portions of this lab, you should commit your changes and push them. Commit early and often. When the deadline arrives, we will retrieve the latest version of your code. If you are confident that you are done, please use the phrase "Lab Submission" as the commit message for your final commit. If you later decide that you have more edits to make, it is OK. We will look at the latest commit before the deadline.

We will know that the files are yours because they are in your git repository. Do not include identifying information in the code that you submit. We grade your lab programs anonymously to avoid bias.

  • CSCI 136, Fall 2022

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