6.031 — Software Construction
Spring 2019

Project: Crossword Extravaganza

Team contract due
Wednesday, April 24, 10pm
Warmup due
Wednesday, May 1, 11am
Specs due
Wednesday, May 8, 11am
Project deadline
Wednesday, May 15, 11am
Reflection deadline
Wednesday, May 15, 11am

We tend to think of solving crossword puzzles as a solitary activity, but not anymore! For this project, you will be creating a venue for competitive crossword puzzle solving. Specifically, you will be constructing a system that will allow users to challenge each other to competitive crossword puzzle matches.

The project will exercise many of the concepts you learned in class. You will implement a client/server architecture and develop your own wire protocol; you will use ParserLib to read in crossword puzzles, and will develop your own abstract datatypes to represent different aspects of the system. Most importantly, you will also practice working in a small group, using the software engineering techniques we have learned this semester.

Design Freedom

On this project, you have complete design freedom to choose the packages, interfaces, classes, and method signatures you use in your code. Choose them wisely. You will be expected to use specs, tests, abstraction functions, rep invariants, safety arguments, checkRep and other assertions.

The specification in this handout constrains what your solution must do, but you will have many design questions that are not answered in this handout. You are free to come up with your own answers to these questions – just be reasonable, consistent, safe from bugs, easy to understand, and ready for change. You can always ask your TA mentor or on Piazza for advice, but there is unlikely to be a single hard-and-fast answer.

On this project, Didit will run only your own tests. As always, correctness is your responsibility, and you must rely on your own careful specification, testing, and implementation to achieve it.

Your team’s repository

To get started, (note: only available after the project announcement in class)

  1. One member of your team should ask Didit to create a remote projects/crossword repository, which will create a single repository shared by all members of your team.

  2. Then all team members will be able to go to Didit, follow the link to your team repo page, and find the ssh:// URL at top. Clone and import into Eclipse.


During the project, class meetings will be partly or completely replaced by team work time. Your team is expected to meet and work together during these times.

Checking in

Your team will be assigned a TA mentor who will help you with your design and help you stay on track as you implement it. You are required to check in with your TA during every team work time class. At this check-in, each team member should be prepared to:

  • say what they have accomplished since the last check-in
  • say what they plan to accomplish by the next check-in
  • say what, if anything, is blocking their progress
  • show that their own working copy of the project is committed, pushed, and up to date with the remote repo

Use the check-in to review how each person’s plans fit together and decide how to resolve blocking problems.

Working together

Other than reflections at the end of the project, all parts of the project should be committed to the repository you share. Each commit to the repository should have a useful commit message that describes what you changed.

Use the code review skills you’ve practiced on Caesar to review one another’s code during the project. Caesar won’t have your project code loaded into it, however, so you can use in-person discussion or email for these reviews.

You are also strongly encouraged to try pair programming, where two people collaborate on a single computer. Pair programming is a skill that requires practice. Be patient: expect that pairing will mean you write code more slowly, because it’s like code review in real time, but the results are more correct, more clear, and more changeable. You can find plenty of advice on the Internet for how to structure your pairing.

Note that “pair programming” normally means just one keyboard, with one person driving (typing) and the other person in the backseat (code reviewing). So you are editing only one working copy, and committing the changes just once. When multiple people contribute to a commit, mention them in the commit message. Your TA will be reviewing the Git log to see individual contributions.

What we do in class with Constellation is a closer kind of collaboration than normal pair programming. Using Constellation during the project is possible but not encouraged, because it makes identical changes on two computers that require careful coordination to avoid merge conflicts. But if you really want to use Constellation, then before starting the collaboration, make sure both sides are in a clean state, with no uncommitted changes and fully in sync with the remote repo. After collaborating, both sides should commit the changes to their local repos, then push and pull until the commits are merged and both sides are again in a clean state. Note that you must be extremely disciplined to use Constellation and Git together successfully. Never walk away from a Constellation collaboration without cleaning up and getting merged.

In general, working together will be easier if you:

  1. pull every time you sit down to start working
  2. commit and push frequently
  3. don’t break the build! (don’t commit code that doesn’t compile)


  • Team contract. Before you begin, write and agree to a team contract.

  • Understand the problem. Read the project specification carefully.

  • Warmup. There is a warmup exercise that your group needs to do, explained below. It will help you understand the puzzle file format and serve as a good milestone for overall progress on the project.

  • Design. You will need to create abstract datatypes for various parts of your system, both client and server, and a grammar for parsing puzzle file input.

    Your software design is perhaps the most important part of the project: a good design will make it simpler to implement and debug your system. Remember to write clear specifications for classes and methods; write data type definitions for your expression data types; define abstraction functions and rep invariants, and write toString and checkRep; and document safety from rep exposure.

  • Iterate. Design the parts of the system required for the warmup. Test and implement them. Then revisit your design, improve the specifications, and test and implement more. And so on.

  • Test. You should write JUnit tests for the individual components of your system. Your test cases should be developed in a principled way, partitioning the spaces of inputs and outputs, and your testing strategy should be documented as we’ve been doing all along. Parts of your program may require manual testing, which you should document clearly as in problem set 4.

  • Implement. Write implementation code so that your tests pass. Iterate on the design and tests as needed to make the system work.

  • Reflection. Individually, you will write a brief commentary saying what you learned from this project experience, answering the reflection questions. Your reflection may not exceed 300 words, and should be submitted to the reflection form.

Dividing the work

Every team member must not only make a substantial contribution to the project, but every team member must make a concrete contribution to every major component of the project.

For example, you may not assign one team member who is solely responsible for the parser.

Contributions include writing specifications, writing testing strategy, writing tests, prototyping, writing internal docs, writing implementation code, fixing bugs, and giving code review feedback. Contributions you work on as a pair or a whole team are great, as long as everyone is involved. Tasks like running meetings, taking notes, and tracking bugs are important, but don’t count as contributions under this requirement.

You must also divide the work such that every team member makes several different kinds of contributions.

For example, you may not assign one team member who is solely responsible for writing tests.

Here is one way to break down the work for a 3-person group that satisfies the constraints above. Fill in the cells of this grid so that it is a Latin square, i.e. no person is mentioned more than once in each row or column:

Server-side logic Game logic Client-side logic




The server-side logic includes all the code necessary to accept clients, manage sessions, and transmit updates to clients. The game logic includes all the rules of the game itself, including the logic to check puzzles for consistency, and the client-side logic includes all the GUI logic, as well as the logic necessary to support live GUI updates.

Note that you may need to rebalance your work breakdown among group members as the design and implementation challenges become more clear, so be prepared to revisit this Latin square and change the rows or columns if necessary.

Automated testing

Didit will compile your project and run your tests every time you push.

The automated testing page describes how Didit finds your tests, what tests can and cannot do, and how to exclude tests from execution.

You are not required to run any of your tests on Didit, but your project must compile and have a green build on Didit in order to be graded. If it doesn’t build on Didit, your TA won’t be able to build it either.


The project specification describes how the Crossword service must work.

Running the command java -cp path crossword.Server puzzle_folder should start the server, where path is the path to the location of all your class files (you can find more details of running programs from the command line here). After it is started, the server should read all the *.puzzle files in the puzzle_folder given in the command line; it should then wait for client connections on port 4949.

The client should also be started through the command line java -cp path crossword.Client machine_address. The machine_address argument should correspond to the address of the server to connect. For example, to connect to a server running on your own machine, you can run java -cp path crossword.Client localhost.

Provided code

We have provided you with starter code for the client interface to give you a starting point on how to set up a user interface and draw crossword puzzles in it.

It’s also fine to use any code provided in this semester’s class. For example:

We also provide some example crossword files that you can use to test your system. These files are found in the puzzles directory in your Git repo.


The warmup will involve a client and a server component. The server must read a single puzzle file from the folder given in the command line and perform the consistency checks outlined in the project specification. If the puzzle file is correct, it should wait for connections from a client, and as soon as the client connects, it must communicate the puzzle to the client (but not the answers, only the length, position, and orientation of the words, as well as their associated hint as explained in the security section).

The client must connect to the designated server and display the puzzle that the server transmitted.

If you are having trouble with the warmup, seek help early!

This is an initial iteration, but you should still include a first draft of specs, tests, datatype definitions, rep invariants, abstraction functions, and equals()/hashCode()/toString() defined where appropriate.

Deliverables and grading

Wed Apr 24, 10pm
Your team contract must be committed to your group repository in a PDF file called team-contract.pdf in the top level of your repo.
Wed May 1, 11am
Warmup milestone meeting: you will have a meeting with your group’s TA mentor, at which you must demonstrate the deliverables described under warmup. You will demo the code and discuss your work at the meeting. Have a laptop with the program ready to run. Make sure to commit your warmup code to your group’s repository before this meeting.
Wed May 8, 11am
Specs milestone meeting: by this meeting, you should have committed (1) a ParserLib grammar for puzzles; (2) specs for all your classes and methods, as well as datatype definitions, AFs and RIs, and a concurrency design and thread safety arguments, in a design that addresses all the requirements of the Crossword Extravaganza specification; and (3) some progress on testing and implementation.
Wed May 15, 11am
Project deadline. Your specifications, tests, and implementation should be complete and committed to your group’s repository.
Wed May 15, 11am
Reflection deadline. Individually, you should write a brief reflection and submit it using the reflection form. Your reflection should be at most 300 words of plain text.

Grades will be determined according to the following breakdown:

  • Team contract: 5%
  • Design: 25%
  • Implementation: 40%
  • Testing: 25%
  • Reflection: 5%

The warmup and specifications deadlines are graded as binary checkoffs. Missing each of these intermediate deadlines will cost 5 points on the overall project grade. The associated milestone meetings also contribute to your design and implementation grades.

Check-ins with your TA mentor are also graded as binary checkoffs, either passed or missed. Missing a check-in costs 1 point on the overall project grade. You should check in with your mentor during class on:

  • Wed Apr 24
  • Fri Apr 26
  • Mon Apr 29 (after a short class)
  • Wed May 1 (warmup milestone meeting)
  • Wed May 8 (specs milestone meeting)
  • Fri May 10
  • Mon May 13

Additional pages referenced above

Crossword Extravaganza specification

Automated testing

Team contract