CSCI 121: Computer Science II

Debugger Lab

Overview

The goal of this lab is for you to learn about and practice using the debugger built into DrJava. In addition, you will see an example of a program where multiple classes work together to solve a larger problem.

Workflow

You will use the same git workflow for this lab as you did for last week's lab:

1. Fork the assignment repository
2. Clone your forked repository to the machine you are working on
3. Complete the lab
4. Add any files you created to the repository
5. Commit
6. Push
7. Review your repository on GitHub to make sure everything looks good
8. Create a pull request

If you forget the details of these steps, review last week's lab.

In last week's lab you created a text file and a Java file for your submission. This week's lab requires you to answer a series of questions and fix some Java code. The changes you make to the Java code will be detected and added to the repository by git automatically when you commit. Your answers to the questions in the lab should be placed in a Markdown file.

Editing Markdown Files

Markdown is a markup language. Markup languages allow you to add special formatting syntax to text files so that they can be converted to nice looking web pages or documents. This lab document was written in Markdown and saved as README.md in the lab repository. GitHub looks for a file called README.md in every repository. If it exists, GitHub displays it nicely on the home page for the repository.

In the lab repository there is a file called ExampleAnswers.md. Click on this file in GitHub and notice how it is formatted. Now click on the "Raw" button above the formatted text. This will show you what the file looks like before GitHub applies the formatting.

Open ExampleAnswers.md on your computer by opening up Finder and navigating to your local clone of the lab repository. By default OS X will open the file using TextEdit. When you edit a Markdown file in a text editor you will see the raw text as opposed to the formatted text.

TextEdit is fine for editing simple text files, but more sophisticated text editors will detect that this file is a Markdown file and add some syntax highlighting to make it easier to read and edit. There is a nice text editor called TextWrangler installed on the lab machines (this is a free program you may want to install on your machine as well if you are using your own Macbook for the lab). Open ExampleAnswers.md in TextWrangler and note that TextWrangler highlights the formatting syntax for you.

Put your answers to the lab questions in a file called Answers.md. If you use TextWrangler you should open a new text file and immediately save it as Answers.md in the assignment repository directory. TextWrangler will note the .md extension on the filename and apply Markdown highlighting automatically.

Pig

In CS I there was a lab in which you implemented the dice game pig. For those of you who haven't been playing pig regularly since then, here are the rules of the game to refresh your memory:

The object of pig is to be the first player to reach 100 points. On your turn, you repeatedly roll a six-sided die until you choose to stop or you roll a one, whichever comes first. If you choose to stop, then you score the sum of all your die rolls. However, if you roll a one before stopping, you score 0 for your turn.

Pig Implementation

In the git repository for the lab you will find a directory called src. This stands for "source" and is a common naming convention for directories that contain source code. In the src directory you will find four Java files. Open all four files in DrJava.

The following questions are designed to help you become familiar with the code you just opened.

Question 1: Why is cutoff not a parameter to the method playTurn in the PigGame class?

Question 2: What would the following code print?

ScoreSheet s = new ScoreSheet();
System.out.println(s.getTurnAverage());

Question 3: In the PigGame class, numBusts is incremented in the playGame method. Describe how this statement could be moved to another method in the class without affecting the results.

Fixing the Pig

Compile and run the program in DrJava. Main.java must be the currently selected file in DrJava when you press Run, because it is the class that contains the main method for our program.

When you run the program you should see > run Main in the Interactions pane, and it should say "Running main Method of Current Document" on the bottom bar of DrJava. However, the program does not seem do anything.

The program is stuck in an infinite loop. If you open up Activity Monitor (start spotlight with cmd-space and start typing Activity Monitor) and look at the CPU tab, you will see that java is using a significant portion of your CPU. This is because the program is running, but it is not producing output or terminating. Press "Reset" in DrJava to halt execution of the program.

Clearly this infinite loop behavior is not what we want this program to do. Your goal is to find the error(s) causing this behavior and to fix the code so that it executes correctly. You may not add any additional print statements to the code. Instead, you should use the debugger to trace the execution of the program. Before you begin, be sure to answer the following question:

Question 4: Based on your current understanding of the code, where do you think the problem(s) might be located? Are there portions of the code where you are fairly certain the problem(s) could not possibly be?

Using the Debugger in DrJava

For this portion of the lab, please do not tell other students what you discover. The point of the exercise is for each of you to determine what is wrong by examining lines of code until you find the one(s) that are incorrect.

To trace code in the debugger you need to toggle "Debug Mode" in the "Debugger" menu. You will need to set up breakpoints and watches to step through the code and see the values of variables. Start by setting a breakpoint at the first line of execution on line 12 of Main.java. Place your cursor on the line and select "Toggle Breakpoint on Current Line". You may also use the keyboard shortcut cmd-b. The line should now be highlighted in red.

Now when you run the program it will pause when it hits the breakpoint. You can step through the program line by line by pressing the "Step Into" button. DrJava will step forward one line of execution each time you press the button. If a method is called, it will take you into the method. After the method returns, you will be returned to the calling method. Keep pressing "Step Into" and notice what happens. Read on when you're sure you understand how this works.

We would like to see the values of different variables while we are stepping through the execution. To do this we need to set up watches. When you have stepped to a part of the code where you would like to see the value of a variable, enter the name of the variable in the "Name" field of an empty row in the "Watches" pane. Now every time a variable with that name is in scope it will show you the variable's value and type.

Find the issue in the code by stepping through and finding the variable(s) whose values are not what they should be, and the statement(s) that are causing the problem. Change the code so that it works correctly.

Question 5: Describe the problem(s) with the program and the way(s) you made the program execute correctly.

Question 6: Using the correct program, what are the average number of turns for cutoff values 10, 15, 20, and 25?