Lab 6: Debugging Java programs using BlueJ

For the TA

The first part of the lab shows you to find and correct syntax errors in a Java program.

This part of the lab is highly synchronized:

In part 1, the TA goes through the lab exercise with the students where students do the same things under the direction of the TA
This part teaches students how to find syntax errors
While doing the lab, the TA should ask the students:
(So this lab is a "learn to debug" Java program exercise under guidance of the TA.)

Part 2: find a logic error in a program.
Let students find the error and correct it. Help them understand the logical error.
Part 3: write the upperCase() method that returns a string where all lower case characters are converted to uppercase.

Warning for students and TA:

When stop at a break point, you must TERMINATE a running program before compiling the program again.
If you compile without TERMINATING, BlueJ may hang

How to recover for this problem:

Find the process id of BlueJ

Kill it:

kill BlueJ-process-id (Use: kill -9 BlueJ-process-id if it refuses to die....)

Restart BlueJ....

Reading materials
- Relevant material:
You should read the material before coming to the lab.
If you have not done so, try to following the instructions in this webpage (they are very detailed and self-explanatory).
You should read the webpages above after the lab to re-enforce what you have learned.

Lab goals

What you will learn in this lab:

Use BlueJ to find and correct syntax errors in a Java program
Set break points in a Java program
Use the single step technique in BlueJ to find and correct logical errors in a Java program

Preparation: Create your ~/cs170/lab6/, and copy files

Copy-and-paste these terminal commands:

mkdir ~/cs170/lab6 cp ~cs170002/share/lab6/* ~/cs170/lab6 cd ~/cs170/lab6

Open ~/cs170/lab6/ in BlueJ
- Enter this command in a terminal window:
- Open ~/cs170/lab6/ in BlueJ
  (If you need detailed instruction on how to open a directory as BlueJ project, please see lab3.)
- You should see this when you are in lab6:

Part 1: find and correct all the syntax errors in Mystery.java

Open the program Mystery.java --- double-click on the Mystery.java file) and the following window will pop open:

TODO:

Find and correct all syntax errors in the program:

To find syntax errors, click compile:
BlueJ will high light the first syntax error:
The error is: not a statement.
Correct the error:

TA:

Ask the students what is the meaning of the error messages.
And how to fix it
Let the students fix the problem !
Only move on to the next problem after everyone has fixed the current problem.

If you have corrected all errors, you can run the program as follows:

In the main BlueJ window, right-click on the Mystery.java program and select main():
Enter a string as the argument to main(), e.g., "25":
Make sure you type "25" between the braces { ... } (or else you will get an error message).
You should see this output for 25:

TA:

Show the algorithm in one window.
Run the program in another window using the input: 43 and then 44
Ask students if they can figure out what the algorithm is doing...

Part 2: find and correct a logical error in IsPrime.java

TODO:
The IsPrime.java has a logical error.
You must find and correct the logical error

Students do this exercise under direction of the TA

TA should discuss briefly how to determine when a number is prime:

x = 5 First: Assume x is prime 5 % 2 remainder = 1 x is still prime 5 % 3 remainder = 2 x is still prime 5 % 4 remainder = 1 x is still prime DONE ===> x (5) is prime
x = 8 First: Assume x is prime 8 % 2 remainder = 0 x is NOT prime

Lab exercise script:

Open the program IsPrime.java (double-click on the IsPrime.java file):
Compile the IsPrime.java program with BlueJ (right click on IsPrime.java in BlueJ main window and select compile):
Run the program (right click on IsPrime.java in BlueJ main window and select main()):
Just click OK in the BlueJ: Method Call window (no parameter necessary)
You should see this output:
Conclusion:

The algorithm to determine if a number x is prime is given in this method:

public static boolean isPrime(int x) { // input: x = number // output: true if x is prime, false otherwise boolean prime = true; /* ------------------------------------------------ Check if x is divisible by any of the numbers between 2 and x-1 ------------------------------------------------ */ for ( int i = 2; i < x; i++ ) { if ( x%i == 0 ) { prime = false; // x is divisible by i, x is not prime } else { prime = true; // x is not divisible by i, x is prime } } return (prime); }

Set a break point at these 2 statement:

How to set break points:

Click on the left margin at the line containing the statement:

Note:

You can only set a break point when the program has been compiled
You can tell if a program has been compiled by the color of the left margin
If the color of the left margin is gray, you need to compile it:

Execute the IsPrime() using as parameter = 8:
The execution will stop at the first break point (see the arrow over the stop sign and the statement is high lighted):
The BlueJ: Debugger window contains the current values of the variable used in the IsPrime() method:
Press Step:
Notice the program has detected that 8 is not prime (because it is devisible by 2).
Keep pressing Step to find out what the algorithm is doing
Keep an eye on the BlueJ: Debugger window to find out the current value of the variables.

Note for TA:

Student should learn to read the information in the Debugger window
So: ask students to predict the execution of the if-else statement for several iterations

TODO:

Find out HOW come the IsPrime() method returns true for the input 8 ...
In other words:

(TA: let each student find out the problem for himself/herself. But assist the students and show them what to do if they ask.)

Part 3: write the toUpperCase() method

Open the UpperCase.java program in BlueJ.

You will see the following code:

/* ---------------------------------------------------------------- Complete the method "toUpperCase()" ---------------------------------------------------------------- */ public class UpperCase { public static String toUpperCase( String s ) { String output; output = ""; // For each character in input string s, do: // { // if character is a lowercase: // append the uppercase character to output // otherise // append the character to output // } // Don't forget the return the output !!! } /* ------------------------------------------------------ Method main is our main entry point. ------------------------------------------------------ */ public static void main(String args[]) { String in, out; in = "Hello4"; out = toUpperCase(in); System.out.println("Input: " + in + "\tUppercased Output: " + out); in = "7*h$npo"; out = toUpperCase(in); System.out.println("Input: " + in + "\tUppercased Output: " + out); } }

The method toUpperCase contains an input variable s which holds the input string.

Write statements inside the toUpperCase method that changes all the lower case letters in the input variable s into upper case. Do not change characters that are not lower case letters.

After converting the characters, the method must return the result.

Algorithm: it is given as the comment inside the toUpperCase method

I want to show you an example here on how it will work:

Suppose the input string is: s = "Hello4" The variable output will contain the result. We start with: output = "";
s = "Hello4" ^ | i=0 c = s.charAt(i) (= 'H') it is not a lower case letter, simply append it to output with: output = output + c; // output will be equal to "H" s = "Hello4" ^ | i=1 c = s.charAt(i) (= 'e') it IS a lower case letter, append the uppercase to output with: output = output + (char) (c-'a'+'A); // output will be equal to "HE" and so on...

Turnin your work.

Enter these terminal command to turn in your lab:

cd ~/cs170/lab6 /home/cs170002/turnin-lab Mystery.java lab6 /home/cs170002/turnin-lab IsPrime.java lab6a /home/cs170002/turnin-lab UpperCase.java lab6b

For the Lead TA:

Make sure that when a student leaves the lab that he/she has turned in his/her work !!!