Lab 2: IntelliJ and IntLists

A. Before You Start

B. Introduction

In this lab, we will finish our setup (Whew!) and introduce to you a powerful IDE called IntelliJ. Then, we will learn about IntLists and destructive vs. non-destructive.

As usual, you can get the start files for this lab from git. Make sure you have committed and pushed any current work in your repo directory, and then use

git fetch shared
git merge shared/lab2 -m "Start lab2"
git push

C. Software Setup

Recall from Lab 1 that you weren't able to run your testing files on your personal computer. That's because we did not provide you the necessary software to do so. Upon completion of this section, you should no longer have any issues with missing packages.

Regardless of if you're using a lab computer of a personal computer, do the following:

git clone cs61b-software

These two commands place a cs61b-software folder in your home directory. This folder should be kept separate from your repo folder.

Mac/Linux Setup

You should have a hidden file called .bash_profile in your home directory. Open this file (with either a text editor or Vim) and add the line:

source $HOME/cs61b-software/adm/login

Important: Add this line to a file, DON'T run it on the command line.

Then, on your command line, execute:

source ~/.bash_profile

Important: Run this line on the command line, DON'T add it to a file.

This should set your PATH and CLASSPATH variables properly.

Windows Setup

See part 5B of the README inside the cs61b-software folder that you just obtained. Follow the instructions to set up your CLASSPATH.

D. IntelliJ Setup

IntelliJ is an IDE (Interactive Development Environment). It's like a text editor (i.e. Sublime Text) but it's chock full of extra features. In order to run your files in this special environment where we can work our IDE magic, we need to import our files into a project. Fortunately, this is a fairly painless process. You should be using IntelliJ for every homework, lab, and project that comes your way this semester!

Installing IntelliJ

Again, if you're only planning on using the lab computers, skip the setup. If not:

  1. You'll need to install the Community Edition of IntelliJ from the Jetbrains website.
  2. After selecting the appropriate version for your OS (Mac OSX, Windows, or Linux), click download.
  3. Run the install file and follow the prompts to install IntelliJ onto your computer. You can stick to all the default settings.

Installing the CS 61B and Python Community Plugins

  1. Start up IntelliJ. If you're on the lab computers, you can start up IntelliJ by using the command /share/instsww/bin/ in a terminal window.
  2. In the Welcome window, click the Configure -> Plugin button on the bottom right of the window.
  3. Click the button labeled Browse Repositories... on the bottom. Browse Repositories
  4. In the window that appears, enter "CS 61B" in the search bar at the top. The CS 61B plugin entry should appear.
  5. Click the green Install button. Browse Repositories
  6. Wait for the plugin to download and install.
  7. Now, repeating steps 3-6, install the "Python Community Edition" plugin.
  8. Click the grey Restart IntelliJ IDEA button to finalize the installation.

For more information on using the plugin, read the plugin guide. You can skip this reading for now.

Creating Projects

The following instructions apply for both initial setup and for future assignments. After running git fetch/merge to retrieve a new assignment, you will notice that you have a new assignment directory (next week, you'll get lab3/). Therefore, you can simply run through the following steps again. Don't be intimidated by the long section; this is because we've carefully screenshot every step. In future iterations, setting up will likely involve pressing next for all steps and, if IntelliJ asks you to overwrite various housekeeping files (such as .iml files) because they already exist, respond "Yes" or "Overwrite" to those popup windows. This is so IntelliJ can automatically mark the new directories for your assignment for you as opposed to you manually marking source folders and/or modules.

  1. Start up IntelliJ. If you're on the lab computers, you can start up IntelliJ by using the command /share/instsww/bin/ in a terminal window.
  2. Upon opening IntelliJ, click on the "Import Project" option.

    IntelliJ Start Menu

  3. Find and choose the directory of your current assignment (for today's lab, it should be ~/repo/lab2), then press the OK button.

    Select Folder

  4. Make sure "Create project from existing sources" is selected and press next. You shouldn't have to change anything at this step.

    Import Project

  5. Leave these fields untouched and press next.

    Project Name

  6. Do nothing here and press next. For context, IntelliJ is automagically detecting what your Java files are and self-configuring itself to edit and run them.

    Import Sources

  7. You may not actually see this next window pop up. If it does, click next. If it doesn't, that's fine.

    Import Lib

  8. You may not actually see this next window pop up. If it does, click next. If it doesn't, that's fine.

    Module Config

  9. You may not actually see this next window pop up. If it does, typically it should look like the screenshot below. If it does, then your project is setup and you may click next and skip steps 10 to 12. If you are missing either the Java SDK or Python, go to their respective steps below to add them. For the first time you use IntelliJ, you must do steps 10 to 12.

    Select SDK

  10. Click the plus in the upper left corner and click jdk from the drop-down menu.

    Add JDK

  11. Find where your jdk is saved, select it, and press the OK button. On a Mac, it is at "/Library/Java/JavaVirtualMachines/jdk1.8.082.jdk/Contents/Home". If you're on Windows, your path may look like "C:\Program Files\Java\jdk1.8.092". If you're on a lab or Ubuntu computer, it should be at "/usr/lib/jvm/Java-8-oracle/". The actual number after jdk1.8.0 may differ depending on which update of Java 8 you downloaded. It does not matter as long as you are using Java 8.

    Select JDK

  12. Next, we'll set up your Python SDK. Click the "plus" again, and this time click Python SDK -> Add local. Find where your python3 executable is saved, select it, and click "OK". On a Mac or on Ubuntu, you can figure out the path to python3 by typing which python3 into your terminal. On Windows, you can find python3 by inspecting your environment variable settings.

    Select Python

  13. Once your screen looks similar to the screenshot in step 9, press "Next", then "Finish". Once you've pressed finish, you should see something really similar to the following image. You may need to click the little triangle next to lab2 in the top left to get the source files ( and for lab2 to show up.

    Main Screen

  14. Finally, you'll want to import libraries. You'll have to do this for every project you set up. Click File -> Project Structure at the top left of IntelliJ. A window should pop up, and you should click "Libraries" on the left panel of this window. When you've done so, it'll look as shown below:

    Project Structure

  15. Click the green plus in the top left of this window and pick Java from among the given choices. This will pop up a window. Navigate to your cs61b-software's lib folder, and select all of the jar files. It should look something like this (the jar files in the image below may not match exactly with what you have in your folder; that's okay). Press OK several times to exit.

    Library Files

You should be all done now! Again, this may seem like a lot, but for most of these steps, all you did was click "Next"! And some of these steps like setting up your JDK and Python will only be done once.

E. Using IntelliJ

Running Java Files

Let's get more familiar with the IntelliJ interface.

Click on the file from the left sidebar. Next, go to the top of the screen and press Run -> Run... (there are two 'Run' options here, the first one is greyed out).

A small box should appear. You should click on IntList, and the java file will run.

A box should appear on the bottom of the screen; this box will output the result of your program. Right now, it looks like we have an exception (error):


The blue link indicates the file and line number at which the exception occurred. We can click on it to go directly to that line in our code.

It looks like that particular print statement prints Hello and then whatever args[0] is. What is args[0]? These are arguments that you usually provide on the command line. To mimic this functionality in IntelliJ, we click on Run -> Edit Configurations. A new popup should appear. On the blank that sides Program Arguments, put your name. After you have done this, click OK. Let's run our file again. Go to Run -> Run 'IntList'. Now the first run option is no longer greyed out. Clicking on it will run will run whichever file you last ran.

You should get an output: Hello, <Your Name>.

Now, let's take a look at the main method again. We have created an IntList for you. We will talk about what an IntList is, but for now, let's print it out. On the line after IntList L1 = list(1, 2, 3, 4, 5);, type sout then press Enter. Did you see what happened? IntelliJ converted sout to System.out.println();, which is how we print things in Java. Cool right? Throughout this course, you will learn even more IntelliJ functionality that makes coding in Java much easier.

Inside the parentheses of your new System.out.println();, add L1. Now run IntList again. Verify that your output is now Hello, <Your Name> on the first line and [1, 2, 3, 4, 5] on the second line.


Let's try running the debugger on IntelliJ. The debugger is single-handedly the most important tool you will use in this course, and it is not close. If you ever have a bug and you ask your TA about the bug, most of the time, the words that come out of said TA's mouth will be 'Have you tried using the IntelliJ debugger?' Therefore your goal should always be to answer 'Yes!' every time.

First let's set a breakpoint. You can set breakpoints by clicking to the left of each line. A breakpoint is set when a red circle appears left of the line.

Let's set a breakpoint on the line IntList L1 = list(1, 2, 3, 4, 5); from before. Then, go to Run -> Debug 'IntList'. The debugger now opens on the bottom of your screen. The program execution has been paused at the breakpoint we've just set! From here, we can step through each line of code that helped us create our IntList, line by line.

Near the top of the debugger console, you will see some blue and red arrows. Hover your cursor over them to see what they say. Then, press the arrow that says Step Into. This allows us to enter the method in that line (in this case, the list method). Verify that after you've pressed Step Into, that the program is now inside the list method.

What if you wanted to skip entering the list method and simply continue the debugger after the list was created? Then you would have pressed Step Over.

Let's step out of the list method and return back to the main method. Press the Step Out button and you should return back to the main method.

Say we are done debugging, and we would like to finish the execution of our program. Or, perhaps you've set two breakpoints, and you wish to jump to the next breakpoint. On the left side of the debugger console, there is a green triangle. Hover over it; you will see the text Resume. Press it, and our debugger will continue executing until it hits another breakpoint or the end of your program.

Finally, let's end our debugger. Again, on the left side of the debugger console, there is a red square. Hover over it; you will see the text Stop. Press it, and our debugger will exit. It is important that you press this square every time after you have finished with your debugger, or else the debugger will run forever!

Creating JUnit Tests

Unlike the previous sections, you do not have to explicitly do this section; we will save the generation of JUnit test files to next lab. However, you should read the section below.

We have provided you a second file, called We have also written some test methods for you. Normally, to generate a JUnit test file, navigate to the file you'd like to test, then go to Navigate -> Test. A small window will pop up; click Create New Test.... A new window should open. For testing library, select JUnit4 from the dropdown menu. You will be able to check boxes to indicate which methods you want to test for. Although you can always write in more tests, the check box option is nice since IntelliJ will automatically generate the method header for these tests for you. You can run these tests the same way you'd run a regular Java file (as described above).

Again, we will leave the creation of JUnit tests to the next lab. For now, we have provided you the testing file.

Using the Plugin

Style Checking (Optional)

In this class, you will eventually be required to make sure your code conforms to the official style guide. The plugin includes a helpful style checker, which will check your code and inform you of any style errors and their locations.

To run the style checker, simply right click any file or directories you want to check, and select Check Style in the menu that appears: Check Style Menu

Click it, and the style checker will run. A tool window will appear with the results of the style check, and a list of any errors. Click the links to jump directly to the problematic line of code: Check Style Results Tool Window

Java Visualizer (Optional)

The plugin contains a built-in version of the Java Visualizer, a tool similar to the Python Visualizer you may have used to CS 61B. This tool is intended to help you debug and understand your code, and is integrated into IntelliJ's Java debugger.

To use the built-in visualizer, debug your code, setting breakpoints as necessary. When your code stops, you can click the Java Visualizer icon: Java Visualizer Button

The Java Visualizer will appear, displaying the stack of the currently paused program: Java Visualizer In Action

As you continue to step through and pause your code, the visualizer display will update accordingly to show you what's going on in your program.

Finishing Up

There are many more convenient shortcuts that you can find in IntelliJ. Poke around the IDE a little more, and don't be afraid to Google the nuances!

F. IntLists

Introduction to IntLists

As discussed in Wednesday's lecture, an IntList is our CS61B implementation for a linked list of integers, which you will have already seen if you took 61A, 61AS, or other introductory programming courses.

As we saw in lecture, an IntList has a head and tail property. We named it IntList because a set of these objects can represent a list (sequence) of Java int values. The head of the first IntList object is the first element of the list. The tail (another IntList) represents the list of remaining elements (that is, it is a pointer to the IntList object whose head is the second item of the list.)

As a result of this correspondence between sets of IntList objects and lists, we often conflate pointers to IntList objects with the lists they head and use the term "IntList" to refer both to individual IntList objects and to the entire set of objects reachable by following the tail pointers. Usually, the distinction is clear from context.

See in the IntList directory for a refresher. We've added a method called list, which makes it easier to create IntLists. For example, to create an IntList containing the numbers 0, 1, 2, and 3, we could use the method as follows:

IntList myList = IntList.list(0, 1, 2, 3);

which will create the IntList 0 -> 1 -> 2 -> 3 -> null

The IntList.list method makes it much easier to create IntLists compared to the naive approach:

IntList myList = new IntList(0, null);
myList.tail = new IntList(1, null);
myList.tail.tail = new IntList(2, null);
myList.tail.tail.tail = new IntList(3, null);

There is also a convenient toString method. As we'll see later in the course, this is a useful method, because among other things, the standard print... methods in System.out use it whenever they are asked to print a value of type IntList, so that


will print

[0, 1, 2, 3]

Likewise, the "+" (append) operation on values of type String will use the toString method to figure out what string to append when adding an arbitrary object to a String.

Destructive vs. Nondestructive

Let's consider a method dSquareList that will destructively square every item in a list.

IntList origL = Intlist.list(1, 2, 3)
// origL is now (1, 4, 9)

Where dSquareList is defined as follows:

public static void dSquareList(IntList L) {
    while (L != null) {
        L.head = L.head * L.head;
        L = L.tail;

This is a classic example of a destructive method. It iterates through the list and squares each item, causing the values linked by L to change. In other words, after calling this method once on L, every element in L will be squared.

Examining the code above, we see that the origL variable contains a reference to the created IntList. The value of this variable never changes. By contrast, the L variable in dSquareList moves around inside the method when the line L = L.tail is executed.

As we iterate through the method, origL always points to the beginning of the IntList, but L moves to the right until it reaches the null value at the end.

The reason we say that dSquareList is destructive is that we change the values of the original IntList objects, destroying the original data. As we go along, we replace each head value with its square.

By the end of the method, L is null, and origL is still pointing at the beginning of the IntList, but every value in the IntList that origL points to is now squared.

If these ideas don't yet make total sense, ask a TA or lab assistant to draw a diagram of the code execution. Pointers and IntLists might seem confusing at first, but it's important that you understand these concepts!

Now, look at at squareListIterative and squareListRecursive. These methods are both non-destructive. That is, the underlying IntList passed into the methods does not get modified.

Look at the recursive version--try to reason why this is non-destructive. If you don't understand this yet, you should make sure you do before proceeding.

Now look at squareListIterative. The iterative version of a non-destructive method is often quite a bit messier than the recursive version, since it takes some careful pointer action to create a new IntList, build it up, and return it. Try to understand what this code is doing, but don't stress if it doesn't all make sense right away.

Finally, look at the test method testdSquareList in This test checks whether or not dSquareList is destructive by making sure that the original list is mutated.

You're done reading code for now. Phew! Create a test for squareListRecursive that checks that it is not destructive. You will also want to test whether or not squareListRecursive is correct. Once you have checked both requirements and your own test passes, you're done!

Submit this lab per the submission guidelines from Lab 1 (pushing and tagging). You should have:

G. Recap

  1. IntelliJ is an amazing tool. Use it! Just remember to set up your project the same way every time you start a new project.
  2. The debugger in IntelliJ is the single most powerful tool you are given. When in doubt, use it. When not in doubt, use it anyways.
  3. Testing and debugging are the two most important skills you will learn in this course. If you choose to learn nothing in this course but two things, learn these two! If you plan on continuing your computer science career, you will need these skills. In the real world, the process of writing tests and debugging probably consumes most of time in a project. Never think that because you have "finished coding", you are anywhere near being finished!
  4. Do not rely on "print" statements (although they can be useful in small cases) to test your program! Many students simply rely on "print" statements, which mess up one's program, produce incomprehensible volumes of output, and somehow never manage to get removed.
  5. Oh yeah, IntLists are cool too! You will work with them more in the next lab.