CS1027b Computer Science Fundamentals II

Learning Outcomes

• Run the debugger in Eclipse.
• Execute a java progam step by step using the debugger.
• Add a breakpoint to your program.
• Find errors in a simple Java program using the debugger.
• Understand the scope of variables.

General lab instructions to help labs run smoothly

• Read through the lab instructions before coming to the lab.
• Do the pre-lab preparation.

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 through OWL by 11:55pm on the same day as your lab session to get your marks for the lab.

Exercise 1

Eclipse comes with a debugger which allows you to execute your code one statement at a time, see the values of variables, and stop the execution of the program at any point. The debugger is a very powerful and useful tool to help you find errors in the code. In this exercise you will use the debugger to trace through the program DebuggingExercise.java.

• Open DebuggingExercise.java and run it. You will see that it compiles but it crashes when you run it and the java virtual machine prints an error message. Read the error message and try to understand it. It will give you some information that you can use to fix the code.
• Check that you have a Debug button in the top right corner of your Eclipse window (a button with a little green bug in it), beside the Java button. If you don't, select Window, Perspective, Open Perspective, Debug.
• Click on the Debug button to change to the Debug view. You can, at any time, switch back to the Java view by clicking on the Java button in the top right corner (the button with the blue letter J located to the left of the debug button).
• In the window displaying your code, you will now add a breakpoint to the line for (int j=1; j<=6; j++). You do this by right-clicking in this line at the very left, and then clicking Toggle Breakpoint. You can remove a breakpoint at any time in the exact same way.
• When executing your program, the debugger will stop every time this line is reached. Run your program in the debug mode by selecting on Run, Debug (alternatively, press the key F11).
  You will see that the line for (int j=1; j<=6; j++) is highlighted in green. The execution of the program has stopped at this point. In the Variables window (upper right corner) you will see the variables of the program: args, testArray, and i and their values. Since testArray is a variable referencing a 2-dimensional array, or an array in which every entry is an array of integers, when you click on the ">" symbol to its left, the debugger shows 5 new variables ([0], [1], [2], [3], and [4]), each one of them references an array of length 6; these are the rows of the 2-dimensional array. If you click on the ">" symbol to the left of [0], six new variables will be shown; these are the columns of the first row of the two dimensional matrix.
  
• Now you can use F5 to execute your program one instruction at a time (this is called single-stepping). Press F5 once. Note how the variables i and j, and the values stored in the 2-dimensional array are changing in the Variables window. Why did testArray[0][0] not change?
• Keep pressing F5 and stop when the value of testArray[0][5] changes value from 0 to 5 and the statement for (int j=1; j<=6; j++) is highlighted in green. Push key F5 one more time. What is the value that j has? What value does i have? Is it correct that the program tries to store the value (i+1)*j in testArray[i][j]? Write your answers in AnswersLab3.txt
• After answering the previous questions, you should know what is wrong with the program. Fix the error. Run the program and make it sure that it does not crash; it should print 30 values.
• Modify the code so all entries of testArray are properly initialized. The first row of testArray should store the values 1, 2, 3, 4, 5 6; the second row should store the values 2, 4, 6, 8, 10, 12, the third row 3, 6, 9, 12, 15, 18, the fourth row 4, 8, 12, 16,20,24, and the last row 5, 10, 15, 20, 25, 30.

Exercise 2

• For this and the following exercises you CANNOT add any print or println statements to the code that you are testing.
• Open DebuggingExercise2.java and run it. It will also crash.
• Add a breakpoint to the line i = process(2); and keep pressing F5 until you reach the statement return result; (do not execute this statement yet). Using the debugger find out what the values of the variables i, step, result and load are. Write these values in AnswersLab3.txt.
  Since load is a static variable, to see its value in the Variables window you need to click on the little inverted triangle in the upper right corner of the Variables window; this will bring two options: "Layout" and "Java". Click on Java and then select "Show static variables" (see figure at the end of this document).
• Terminate the program by selecting Run, Terminate.

Exercise 3

• Run again DebuggingExercise2.java and stop execution at statement i = process(2). This time press the key F6 instead of the key F5. You will see that the debugger now does not go inside method process, but it goes directly to statement total = (i * 100) / load. Key F5 tells the debugger to "step into" a method, while key F6 makes the debugger "step over" a method. What is the value of load? Why does the program crash if you keep pressing F6? (Do not press F5 or the debugger will try to step into the code of the Java's virtual machine). Write your answers in AnswersLab3.txt.
• Fix the program so that it does not crash and it prints the message "The value of total is infinity". Change the statement i = process(2) to i = process(1) and run the program again; this time it must print "The value of total is 5".
  Hint. Check if load is 0 before computing total = (i * 100) / load.

Exercise 4

• Open DebuggingExercise4.java and run it. It will not compile.
• Which line causes the compilation error? Why the compiler shows this error even though i has been declared in the statement
      for (int i = 1; i < 2; ++i)
  Write your answers in AnswersLab3.txt.
  Hint. What is the scope of i?
• Fix the code by commenting out the line with the compilation error. There will be one warning; ignore it. Run the program; it should not produce any output.
• Study the code for this class and try to determine without running the program what the values for the variables var1 and obj1 are immediately after method2(obj1) is executed. Write your answers in AnswersLab3.txt. Now try to determine the values of the variables immediately after method1(var1) is executed. Write your answers in AnswersLab3.txt.
• Now use the debugger to determine the actual values of the above variables and write the values in AnswersLab3.txt. To see the string stored in obj1, in the Variables window click on the ">" symbol to the left of obj1; you will see now the value of name.
  If you made a mistake when predicting the values of the variables, study the code and try to understand why the variables have the values determined by the debugger.
• Add a breakpoint at the line if (obj1 == obj2). Press F11 to run the program in debugging mode and stop at this statement.
• Press F5. Why is the result of the comparison false even though both obj1 and obj2 contain the same information, namely "joe"? Write your answer in AnswersLab3.txt.
• Look at the value of obj1 in the Variables window of the debugger. Since obj1 is a non-primitive variable its value is the address of an object of the class MyObject. Note that the debugger does not show the actual address of the object referenced by obj1; instead it displays the address in a user-friendly format: MyObject (id = 19) —you might get a different value for id. This means that the address stored in obj1 is the address of an object of class MyObject and the debugger assigns it a unique internal identifier (in the above example the identifier is 19). If you were to print the value of obj1 with System.out.println(obj1) you would get a similar output, not the actual address of the object but a string of the form MyObject@5ccd43c2; the java virtual machine will assign a different identifier to the object (5ccd43c2) than the debugger.