CS1027b Computer Science Fundamentals II

Lab 6

Learning Outcomes

Upon completion of this lab, you should be able to do the following:

Debug a large program
Use try-catch blocks to deal with exceptions

General Lab Instructions

Read through the lab instructions before coming to the lab.
Read the lecture notes on inheritance.
Download the file filesLab6.zip and uncompress it. This file contains a solution for Assignment 1 that does not increase the length of the snake when the snake eats an apple. The solution contained in this compressed file also has some errors that you need to fix, as explained below.
Download file AnswersLab6.txt and open it with a text editor.

IMPORTANT. Make sure you attend the lab session to which you registered, show the result(s) of each exercise to your TA, and submit the .java files that you wrote and AnswersLab6.txt through OWL by 11:55pm on the same day as your lab session to get your marks for the lab.

Exercise 1: Debugging a Large Program

Create a new project for this lab. Add all the java files from filesLab6.zip to your project. Place all the image files and file board1.txt in the root directory of your project (this is the same thing you did for assignment 1). Run the program PlayGame give as input board1.txt. Move the snake around using the arrow keys in your keyboard; you will see that as you move the snake many snake heads are created because the program has errors in it. You have to debug the program and fix the errors.

Many of you will feel overwhelmed by the task of debugging a program like this that consists of hundreds of lines of code, many of which that were not written by you. How on earth can we figure out why this program does not work as expected so we can fix it?

When debugging a program we do not look at the entire code and try to figure out what is wrong. Instead we look at each class and method of the program and try to find out which contain errors. To do this we use a test harness in each class or a separate test program that invokes each method one at a time and checks whether the method produces the expected output. We should design the testing program so that it tries to test methods independently from each other; this will simplify the task of finding the erroneous code.

When designing a test program for a method we should think what input will be passed to the method and what output the method must produce. Then, after running the testing program we check whether the output is the one that we expect; if it is not, then we examine the code of that method closely looking for the bug.

The debugger is a very useful tool in the process of finding bugs and correcting them. You have used the debugger in past labs, but this time you will use it to find bugs in a much larger program than the ones you considered in other labs. Keep in mind that a debugger is a tool that you will use throughout your career, as it greatly simplifies the task of finding and fixing errors in programs.

To debug a program you will follow these steps:

Use the testing program to find the method m that contains errors.
Use the debugger to find the statement of method m that causes the error. If the statement is an invocation to another method m1, then you might need to restart the debugger and find the statement inside method m1 that causes the error. Notice that this step might require stopping and restarting the debugger several times.
Study the statement that causes the error to figure out how it must be modified so that the program works correctly.

We wrote a testing program for you called TestLab6.java.

Run the program TestLab6. Tests 6 and 7 will fail. Set a breakpoint at line 113 (testPassed = true;) of TestLab6; this is the line where test 6 starts. Run the debugger; the program should stop execution when it reaches line 113.
Repeatedly press F6 or click on the "Step over" button; every time that you press F6 look at the value of variable testPassed. The value of this variable is initialized to true; stop when the value of the variable changes to false.
Which if statement causes the value of variable testPassed to be set to false? Write the line number of this if statement in AnswersLab6.txt.
Terminate the execution of the progam by clicking on the red square beside the Console panel. Clear the breakpoint of line 113 and add a new breakpoint at the line of the if statement that caused the value of testPassed to be set to false.
Restart the debugger. Method newHeadPostion is invoked at the line where the debugger paused the execution of the program, which suggests that the problem with the program is the implementation of method newHeadPosition. The specification of this method given in Assignment 1 states that the method must compute the position where the head of the snake should move without actually moving the snake. Since in line 114 (theSnake = new Snake(5, 8);) of TestLab6 the inital position of the snake is (5,8), then after invoking method newHeadPosition for the first time in line 119 (if (!theSnake.newHeadPosition("up").equals(up))), the snake must still be in postion (5,8).
Click on the ">" sybmol beside variable theSnake to expand it. Then click on the ">" symbol beside variable snakeBody (if you rember, this is the array that contains the positions of the game board occupied by the body of the snake). Then click on the ">' symbol beside entry [0] of this array. In which row and column is the snake currently positioned? Is this the right position for the head of the snake? Write your answers in AnswersLab6.
Since the content of array snakeBody was incorrectly changed, and lines 113-118 do not access this array, then the problem must be with method newHeadPostion invoked in line 119. Terminate the execution of the program. Clear the breakpoint and add a new breakpoint at line 119.
Testart the debugger. Use F5 or click on "Step into" to get inside method newHeadPosition. In the variables window, click on the ">" symbol beside this, then select snakeBody[0] as explained above. Note that the position of the head is still (5,8).
Repeatedly press F6 until you reach line 42 of class Snake.java. Check that the snake's head is still in position(5,8). Press F6 once more; since direction is "up", the statement head.setRow(row-1); will be executed. Press F6 one more time. Now the position of the snake's head is (4,8), which is incorrect. Why did the statement head.setRow(row-1); changed the data stored in snakeBody[0]? Write your answer in AnswersLab6.
Now that you have discovered the error, modify method newHeadPostion so it does return the correct answer, but it does not modify the data stored in snakeBody[0].

Run program TestLab6 again. Test 6 will still fail. Line 61 of class Snake.java contains this statement: if (snakeBody[i] == pos); is this the correct way to compare two non-primitive variables (snakeBody[i] and pos are of type Position)? How should they be compared? Write your answer in AnswersLab6.

Fix statement if (snakeBody[i] == pos); and then run the program PlayGame with input board1.txt. You will see that the snake does not move when you press the arrow keys and the program will repeatedly print the word "null" in the Console window. This message "null" is printed because the program is trying to access a null object. Use what you learned above to fix this error. Which method tried to access a null object, causing the error in the program? Write your answer in AnswersLab6. Run the program PlayGame and make sure that the snake moves around with the arrow keys, as expected.
Please try to figure out what the error is and how to fix it on your own. If you are really stuck, the read these hints.

Exercise 2: Experimenting with Exceptions

Download the Exception Examples 1, 6, 7, and 8 from the Sample Code section of the course's webpage.

Run ExceptionExample1. Why does the program crash? Write your answer in AnswersLab6.
Fix the program by adding a try-catch statement. Print an appropriate message when the exception is caught.
Run ExceptionExample6 and ExceptionExample7. They both produce the same exception, but notice that the try-catch statement is in a different method. This is an example of propagating an exception.
- Which statement causes the exception to be thrown in ExceptionExample6 and ExceptionExample7? Write your answer in AnswersLab6.
- In ExceptionExample7 does the execution of method change continue after the statement that generates the exception? Write your answer in AnswersLab6.
Run ExceptionExample8. Notice that both main and change have a try-catch statement. In which method is the exception caught? Write your answer in AnswersLab6.