6.005 — Software Construction
Fall 2015

Getting Started

Follow this guide to set up the Eclipse IDE and the Git version control system on your computer and learn how to use them in 6.005.

Learning Java

6.005 requires you to get up to speed quickly with the basics of writing and running Java. If you are not familiar with Java:

Tool Setup

6.005 requires you to set up a development environment on your computer. If you are not familiar with the Eclipse IDE and the Git version control system, or you have trouble following the instructions below:

  • visit lab hours on Wednesday or Thursday evening where TAs & LAs can help you out

Office and lab hours

Wednesday, Sep 9

Thursday, Sep 10

Friday, Sep 11

7pm to 10pm in 32-044

7pm to 10pm in 32-044

1pm in class: You must have Java, Eclipse & Git set up & working on your laptop

3pm to 5pm in the 32-G7 lounge

See the course calendar for a schedule of office and lab hours. Regular office hours will begin on Monday, September 14. Lab hours will change depending on deadlines, so always check the calendar to see when lab hours are scheduled.

Step 1: Install software

Install the software you’ll need on your laptop. You need to install three things for 6.005:

  • JDK 8 (for Windows, Linux, or OS X): From this page, download Java SE Development Kit 8 (you don’t need demos, samples, NetBeans, or Java EE).

    The latest version of Java is required.

  • Eclipse 4.5 (a.k.a. Eclipse Mars): Choose Eclipse IDE for Java Developers (not Java EE, just Java). For Windows users, see determining whether you’re running a 32- or 64-bit OS. Unpack the downloaded .zip or .tar.gz file and run Eclipse.

    The latest version of Eclipse is required.

  • Git: We will be using the command-line interface to Git.

    If you already have Git installed, you do not need to install the latest version. Windows users should look for the Git shell. OS X & Linux users should try running git in a terminal.

    The Git site should prompt you to download the appropriate version. Follow the instructions in the README file in the downloaded .zip or .dmg file.

    Windows: choose “use Git from the windows command prompt”, “checkout windows-style, commit unix-style line endings”, and select to add a shortcut to Desktop during installation (this creates a Git Bash shortcut which you will use for all Git commands).

    OS X: if you receive an “unidentified developer” warning, right-click the .pkg file, select Open, then click Open.

Step 2: Configure Eclipse

The Eclipse integrated development environment (IDE) is a powerful, flexible, complicated, and occasionally frustrating set of tools for writing, modifying, and debugging programs. It is especially useful for working in Java.

When you run Eclipse, you will be prompted for a “workspace” directory, which is where Eclipse will store its configuration and metadata. The default location is a directory called workspace in your home directory. You should not run more than one copy of Eclipse at the same time with the same workspace, or the metadata will become corrupted.

The first time you run Eclipse, it will show you a welcome screen. Click the “Workbench” button and you’re ready to begin.

You may store your code in the workspace directory, but this is not recommended. On the left side of your Eclipse window is the Package Explorer, which shows you all the projects in your workspace. The Package Explorer looks like a file browser, but it is not. Don’t be fooled — it rearranges files and folders, includes things that are not files or folders, and can include projects stored anywhere on disk that have been added (but not copied into) to the workspace.

  1. Open Eclipse preferences.

    Windows & Linux: go to Window → Preferences.

    OS X: go to Eclipse → Preferences.

  2. Make sure Eclipse is configured to use Java 8.

    1. In preferences, go to Java → Installed JREs. Ensure that “Java SE 8” (or “jre1.8.0_60”) is the only one checked. If it’s not listed, click Search.

    2. Go to Java → Compiler and set “Compiler compliance level” to 1.8. Click OK and Yes on any prompts.

  3. Make sure assertions are always on. Assertions are a great tool for keeping your code safe from bugs, but Java has them off by default.

    In preferences, go to Java → Installed JREs. Click “Java SE 8”, click “Edit…”, and in the “Default VM arguments” box enter: -ea (which stands for enable assertions).

  4. Tab policy. Configure your editor to use spaces instead of tabs, so your code looks the same in all editors regardless of how that editor displays tab characters.

    In preferences, go to Java → Code Style → Formatter. Click the “Edit…” button next to the active profile. In the new window, change the Tab policy to “Spaces only.” Keep the Indentation size and Tab size at 4. To save your changes, enter a new “Profile name” at the top of the window and click OK.

Read: the 6.005 Eclipse FAQ, which also has some tips and tricks to help you make the most of this powerful but complex tool.

Step 3: Open the command line

One thing that makes learning Git hard for many students is that it’s a command-line program. If you’re not familiar with the command-line, this can be confusing.

A command-line is just an interface to your computer, totally analogous to the Finder or Windows Explorer, except that it’s text-based. As the name implies, you interact with it through “commands” — each line of input begins with a command and might have zero or more arguments, separated by spaces. The command-line keeps track of what directory (folder) you’re in, which is important to many of the commands you might be running.

Common commands

On UNIX-like operating systems (OS X or Linux) open the Terminal application to use these commands.

On Windows you should install Git for Windows from the Git site. Then run the program Git Bash to open a terminal where you can run all these commands, in addition to Git commands.

Here are some of the most common commands:

  • cd (stands for “change directory”)

    Changes the current directory. In you’re in a directory that has a subdirectory called hello, then cd hello moves into that subdirectory.

    Use cd .. to move to the parent directory of your current directory.

  • pwd (“print working directory”)

    Prints out the current directory, if you’re not sure where you are.

    On a well-configured system, your current directory is displayed as part of the prompt that the system shows when it’s ready to receive a command. If that’s not the case on your system, post on Piazza to get help configuring your prompt.

  • ls (“list”)

    Lists the files in the current directory.

    Use ls -l for extra information (a “long” listing) about the files. Use ls -a (stands for “all”) to show hidden files, which are files and subdirectories whose names begin with a period.

  • mkdir (“make directory”)

    Creates a new directory in the current directory. To create a directory called goodbye, use mkdir goodbye.

  • up and down arrow

    Use up arrow to put the command you just ran back on the command line. You can now edit that command to fix a typo, or just press enter to run it again.

    Use the up and down arrow keys to navigate through your history of commands, so you never have to re-type a long command line.

Step 4: Configure Git

Before using Git, let’s do some required setup, and make it behave a little nicer.

  1. Who are you?

    Every Git commit includes the author’s name and e-mail. Make sure Git knows your name and email by going to the terminal (Git Bash on Windows) and running:

    git config --global user.name "Your Name"
    git config --global user.email username@mit.edu
  2. Editing commit messages.

    Every Git commit has a descriptive message, called the commit message. Configure the default text editor you will use to write them.

    Option 1(a): use nano on OS X and Linux

    nano is a simple text editor. It does not come with the Windows version of Git, so Windows users should choose a different option.

    To see if you have nano, try running:


    in the terminal. The result should be a simple editor with instructions at the bottom of the screen; quit with ctrl-X. If that worked:

    git config --global core.editor nano

    will configure Git to use the nano editor. The commands to use the text editor (like copy, paste, quit, etc.) will be shown on the bottom of the screen. The ^ symbol represents the ctrl key. For example, you can press ctrl-O to save (nano calls it “write out”) and then ctrl-X to quit.

    Option 1(b): use Notepad on Windows

    You can change the default editor to Notepad with:

    git config --global core.editor notepad

    If you prefer to edit your commit messages in the terminal, choose the next option instead.

    Option 2: use Vim on Linux, OS X, and Windows

    On OS X and Windows, your default editor will be Vim. On Linux, the default editor depends on your distribution.

    Vim is a popular text editor, but it’s tricky to use.

    Before making your first commit, try running:


    in the terminal.

    You start in a mode called “normal mode”. You can’t immediately type anything into the file!

    In order to start typing, press i (stands for “insert”). This will bring you to “insert mode”, so named because in this mode you can type text into the file.

    When you are done typing, press esc. This will bring you back to “normal mode”.

    Once you’re back in normal mode, you can type commands that start with :.

    To save your work, type :w (stands for “write”) and press return.

    To exit (quit) Vim, type :q and press return.

    To save and quit in one command, combine them: type :wq and press return.

  3. Add some color.

    Out of the box, it can be hard to see and understand all the output that git prints out at you. One way to make it a little easier is to add some color. Run the following commands to make your git output colorful:

    git config --global color.branch auto
    git config --global color.diff auto
    git config --global color.interactive auto
    git config --global color.status auto
    git config --global color.grep auto
  4. LOL.

    As we’ll see in the next step of this guide, git log is a command for looking at the history of your repository.

    To create a special version of git log that summarizes the history of your repo, let’s create a git lol alias using the command (all on one line):

    git config --global alias.lol "log --graph --oneline --decorate --color --all"

    Now, in any repository you can use:

    git lol

    to see an ASCII-art graph of the commit history.

Step 5: Learn the Git workflow

This section includes links to a Git tutor called GitStream. GitStream allows you to practice Git on your machine: for each exercise, you clone a GitStream repository, then follow the instructions on the web page. GitStream will give you feedback in both the terminal and on the web as you complete each exercise.

As you read: complete the GitStream exercises to practice using Git.

You can find general instructions and a list of exercises on the GitStream page.

GitStream is a new project! If you encounter a problem or a bug, please post on Piazza.

  • It may not work with multiple exercise pages open at the same time. If an exercise doesn’t work, please close other exercise windows and try again.

What is Git?

Git is a Version Control System (VCS). If you have used other version control software before, like SVN or CVS, many of the concepts and procedures of git will be familiar to you. The Pro Git book (which you can read online) describes what Git is used for:

What is version control, and why should you care? Version control is a system that records changes to a file or set of files over time so that you can recall specific versions later. […] It allows you to revert files back to a previous state, revert the entire project back to a previous state, compare changes over time, see who last modified something that might be causing a problem, who introduced an issue and when, and more. Using a VCS also generally means that if you screw things up or lose files, you can easily recover.

Some of the most important Git concepts:

  • repository: A folder containing all the files associated with a project (e.g. a 6.005 problem set or team project), as well as the entire history of commits to those files.

  • commit (or “revision”): A snapshot of the files in a repository at a given point in time.

  • add (or “stage”): Before changes to a file can be committed to a repository, the files in question must be added or staged (before each commit). This lets you commit changes to only certain files of your choosing at a time, but can also be a bit of a pain if you accidentally forget to add all the files you wanted to commit before committing.

  • clone: Since git is a “distributed” version control system, there is no concept of a centralized git “server” that holds the latest official version of your code. Instead, developers “clone” remote repositories that contain files they want access to, and then commit to their local clones. Only when they push their local commits to the original remote repository are other developers able to see their changes.

  • push: The act of sending your local commits to a remote repository. Again, until you add, commit, and push your changes, no one else can see them.

  • pull: The act of retrieving commits made to a remote repository and writing them into your local repository. This is how you are able to see commits made by others after the time at which you made an initial clone.


You start working with Git repos in 6.005 by cloning a remote repository into a local repository on your computer.

To do this, open the terminal (or the Git Bash for Windows) and use the cd command to change to the directory where you would like to store your code. Then run:

git clone URI-of-remote-repo


git clone URI-of-remote-repo project-name

Replace URI-of-remote-repo with the location of the remote repository, and replace project-name with the appropriate project name, like ps0. The result will be a new directory project-name with the contents of the repository. This is your local repository.

GitStreamPractice git clone

Cloning problem sets: for each problem set in 6.005, you will have a Git repository on Athena.

Initially this remote repository only contains some template code.

To start working on the problem set, you will clone that repository onto your machine.

As you complete each part of the problem set, you will commit your changes to the local repository and then push them to the remote repository.

When the time comes for grading your assignment, we will clone the remote repository and look at the last commit you made and pushed there before the deadline.

Getting the history of the repository

After you have cloned the repository, you should navigate into the repository on your command prompt using cd. This lets you run git commands on the repository.

For example, you can see the last commit on the repository using git show. This will show you the commit message as well as all the modifications.

You can see the list of all the commits you made (along with their commit messages) with git log. If you do git log -p, it will show you the full commit history, including the changes each commit made.

Long output: if git show or git log generate more output than fits on one page, you will see a colon (:) symbol at the bottom of the screen. You will not be able to type another command! Use the arrow keys to scroll up and down, and quit the output viewer by pressing q.

Commit IDs: every Git commit has a unique ID, the long hexadecimal numbers you see in git log or git show. These numbers are in fact a cryptographic hash of the contents of your commit. One neat feature is that this ID is unique not just within your repository, but actually within the universe of Git commits. In other words, if your commit ID is something like ab1312313febc241..., that commit is (extremely likely) to be the only commit in the world with that name.

You can reference a commit by its ID (or frequently just by the first several characters). This is most useful with something like git show, where you can look at a particular commit, rather than just the most recent one.

You will also see commits identified by ID in tools like gitweb and Didit.

Creating a commit

The basic building block of data in Git is called a “commit”. A commit represents some change to one or more files (or the creation of one or more files).

When you first create a file or change a file, that data is unknown. To add it, run:

git add file.txt (where file.txt is the file you want to add)

You’ll either need to run that command from the same directory as the file, or include directory names in the file path.

This “stages” the file. Once you’ve staged all your changes, run:

git commit

This will pop up an editor that will give you a chance to write a commit message. When you save and close the editor, the commit will be created.

Getting the status of your repository

Git has some nice commands for seeing the status of your repository.

The most basic of these is git status. You can run this at any point to see which files Git sees have been modified and are still unstaged and which files have been modified and staged (so that if you git commit those changes will be included in the commit). Note that the same file might have both staged and unstaged changes, if you changed the file more after running git add.

When you have unstaged changes, you can see what the changes were (relative to the last commit) by running git diff. Note that this will not include changes that were staged (but not committed). You can see those if you run git diff --staged.


After you’ve made some commits, you might want to push them to a remote repository. Again, in 6.005, you really only have one remote repository to push to, called origin. To push to it, you run the command:

git push origin master

The origin in the command specifies that you’re pushing to the origin remote. The master refers to the master branch. Branches are an advanced feature of Git that we’re not going to be using in 6.005, but since Git has them, you do have to specify a branch. For now, just include this part when you push.

Once you run this, you will be prompted for your password and hopefully everything will push. You’ll get a line like this:

a67cc45..b4db9b0  master -> master


Sometimes, when you try to push, things will go wrong. You might get an output like this:

! [rejected]      master -> master (non-fast-forward)

What’s going on here is that Git won’t let you push to a repository unless all your commits come after all the ones already in your remote repository. If you get an error message like that, it means that there is a commit in your remote repository that you don’t have in your local one (probably because a teammate pushed before you did). If you find yourself in this situation, you have to pull first and then push.


To perform a pull, you should run git pull. When you run this, Git actually does two things:

  1. It downloads the changes and stores them in its internal state. At this point, your repository doesn’t appear any different—it just knows what the state of the remote repository is and what the state of your repository is.
  2. It incorporates the changes from the remote repository into the new repository via a process called merging (see next section).


If you made some changes to your repository and you’re trying to incorporate the changes from another repository, you need to merge them together somehow. In terms of commits, what actually needs to happen is that you have to create a special merge commit which encompasses both changes. How this process actually happens depends on the changes.

If you’re lucky, then the changes you made and the changes that you downloaded from the remote repository don’t conflict. For example, maybe you changed one file and your partner changed another. In this case, it’s safe to just include both changes. Similarly, maybe you changed different functions of the same file. In these cases, Git can do the merge automatically. When you run git pull, it will pop up an editor as if you were making a commit—in fact, this is the commit message of the merge commit that Git automatically generated. Once you save and close this editor, the merge commit will be made and you will have incorporated the changes. At this point, you can try to git push again and hopefully it will work this time.

Merge conflicts

Sometimes, you’re not so lucky. If the changes you made and the changes you pulled edit the same part of the same file, Git won’t know how to resolve it. This is called a merge conflict. In this case, you will get an output that says CONFLICT in big letters. If you run git status, it will show the conflicting files with the label Both modified. You now have to edit these files and resolve them by hand.

First, open them up in your text editor (probably Eclipse for 6.005). The parts that are conflicted will be really obviously marked with obnoxious <<<<<<<<<<<<<<<<<<, ==================, and >>>>>>>>>>>>>>>>>> lines. Everything between the <<<< and the ==== lines are the changes you made. Everything between the ==== and the >>>> lines are the changes you pulled in. It’s your job to figure out how to combine these. The answer will of course depend on the situation. Maybe one change logically supercedes the other, or maybe they can be merged somehow. You should edit the file to your satisfaction and remove the <<<</====/>>>> markers when you’re done.

Once you have resolved all the conflicts (note that there can be several conflicting files, and also several conflicts per file), git add all the affected files and then git commit. You will have an opportunity to write the merge commit message (where you should describe how you did the merge). Now you should be able to push.

Avoid merges and merge conflicts:

Pull before you start working

Before you start working, always git pull. That way, you’ll be working from the latest version of your code, and you’ll be less likely to have to perform a merge later.

Reverting to previous versions

If you’d like to practice using the version history to undo a change:

GitStream → (optional exercise) Practice git log and git revert

Technical detail: what is the remote repository?

Unlike other similar systems, Git doesn’t have built-in a notion of a “central repository.” Instead, any repository can push to any other repository by specifying it as a “remote.” A “remote” is just a pair of a name (which can be anything) and a URI, which is a string indicating how it can find the other repository. The URI might look something like this:


Breaking that down:

  • ssh:// — this specifies the protocol git should use to transfer the data. SSH is a protocol that lets you send data securely, which is useful to us because we have to type in a password. But in principle this is totally analogous to, for example, the http:// which you see in web browsers (HTTP is a protocol commonly used for data on the Web).
  • USERNAME@athena.dialup.mit.edu — this actually has two parts. The USERNAME is the username you use to log in to the server. The athena.dialup.mit.edu is the address of the server itself. athena.dialup.mit.edu is the name of an Athena server IS&T runs. It accepts Kerberos logins, so your username can just be your Kerberos name.
  • /mit/6.005/git/SEMESTER/psets/PSET/USERNAME.git — this is the path on the server where the repository is stored. (Note that you should use the actual path specified in the pset that you’re about to work on, so that SEMESTER and PSET are replaced with the right things.) One noteworthy thing here is the USERNAME.git part at the end. In 6.005, this will always be your username, or the usernames of your group members in a group project. However, there’s nothing in Git to say that the username you log in with (the thing before the @ sign) and repository name at the end of the path have to match. In 6.005, we just set it up that way.

Now, even though Git doesn’t have the idea of a central repository, it’s very useful for 6.005. Thus, in 6.005, all of your repositories are actually created by cloning a remote repository which we create (and which acts as the “central” repository). You’ve done this with the git clone URI directory command. This actually does a couple of things:

  1. Create an empty directory called directory (i.e. the last argument to git clone).
  2. Initialize it as an empty Git repository.
  3. Add a remote with the URI you specified and the name origin.
  4. Download the data from the remote.

So for those of you who were wondering, that’s what the origin means. It’s just the default name of the remote repository that you cloned your repository from.

Have fun in 6.005!