1. Debugging “Why you were up till 2AM”
Dan Fleck
Fall 2007

2. Debugging
Definition: The process of finding problems (bugs) in programs and removing them

3. Why do we call it debugging?
1947, one of the first computers the Harvard University Mark II Aiken Relay Calculator
On the 9th of September, 1947, when the machine was experiencing problems, an investigation showed that there was a moth trapped between the points of Relay #70, in Panel F.
The operators removed the moth and affixed it to the log. The entry reads: "First actual case of bug being found." -

4. General Steps Recognize that a bug exists
Isolate the source of the bug
Identify the cause of the bug
Determine a fix for the bug
Apply the fix and test it

5. Isolating source of the bug
Once found, and understood, most bugs are easy to fix. We will concentrate only on finding the bug

6. Confirm your beliefs
Finding your bug is a process of confirming the many things you believe are true, until you find one which is not true
You believe that at a certain point in your source file, a certain variable has a certain value
You believe that in a given if-then-else statement, the “else” part is the one that is executed
You believe that when you call a certain function, the function receives its parameters correctly

7. Confirm your beliefs
So the processing of finding the location of a bug consists of confirming all these things!
Check everything!
Okay… How?

8. One way
Put lots of print statements in at each point that says “I am in method X with parameters set to Y,Z”
Note: If you do this, be able to turn them on/off easily!
Boolean DEBUG = true;
If (DEBUG) System.out.println…..

9. Often a better way Use the debugger
Set a breakpoint at a point in the code
Step through the code
Look at the variables (local and class variables) to see ones that are not as you expected
The debugger is usually better/faster… but print statements are also easy to turn on/off as a whole to make a DEBUG mode for your program. Which is often useful.

10. The Most Important Thing
To do ANY debugging you must understand what the program should do
What each method should do
What each if statement should do
If your beliefs are wrong, confirming your beliefs will not help!

11. The Most Important Thing
Add beliefs into your code… beliefs are otherwise known as COMMENTS!

12. Lets work some examples
See Netbeans debugging project
Fibonacci - Compile time problems
DebugMe - Use the debugger to find the problems
WordAnalyzer -
Test 1 - Use print statements
Test 2 - Use print statements
Test 3 - Use the debugger

13. My Presidents aren’t sorted by party
What are your beliefs? Or more specifically, what are some hypothesis that you should confirm?
You’re not calling the sort routine at all
CLASS ENDED HERE.

14. My Presidents aren’t sorted by party
What are your beliefs? Or more specifically, what are some hypothesis that you should confirm?
You’re not calling the sort routine at all
The sort is not “seeing” every president
The sort is sorting by something other than party
The array is not getting updated by the sort
The array is getting sorted, then resorted by something else

15. My Presidents aren’t sorted by party Confirming your beliefs
You’re not calling the sort routine at all:
Confirm: put in a print statement inside the sort routine saying “DEBUG: sorting begun”
The sort is not “seeing” every president
Confirm: put in a print statement to print out every president the sort loop “sees”
The sort is sorting by something other than party
Write code that says “sorted president (Reagan, Rebulican) after (Clinton, Democrats)”

16. When I print I see duplicate Presidents
During the sort I don’t add any new data
Confirm: Check the length of the data before/after the sort
During the print, I only print each value once
Confirm: compare the length of the list to the number of times you issue a print statement
assert (length == countPrints) : “Too many ”+length+”:”+countPrints;

17. Suduko cannot find a solution
Confirm there is a solution
Confirm you’re reading the values in correctly from the grid
Confirm you’re able to access the correct value using index -> row, col conversion
Confirm your List can move both forward and backward correctly
Confirm you are setting / clearing BitSets appropriately (at various points print out the BitSets for a row/col/box and the grid and compare

18. Suduko cannot find a solution
Confirm you’re able to find the “next” value correctly
Confirm you’re updating the SudokuList appropriately ….

19. COMMENT YOUR CODE! COMMENT! COMMENT! COMMENT! COMMENT! COMMENT!

20. Whoa there… I can’t even compile!
First, try never to get here. Compile early, compile often. If you can’t compile at any point STOP… fix the code before adding any new code.
Review the compiler error, read it carefully
Check your book and online resources for examples and see what may be wrong
Ask questions on WebCT, to your TA or Professor.

21. The Most Important Thing (again)
Add beliefs into your code… beliefs are otherwise known as COMMENTS!
Or if you can… java assertions!

22. More info: http://java.sun.com/j2se/1.4.2/docs/guide/lang/assert.html
Java Assertions
assert Expression1 : Expression2 ;
If Expression 1 is not true, throw an error with String in Expression2
assert x<4 : “Invalid x value ”+x ;
Assertions can be enabled during debugging and disabled for production use (increasing performance)
java -ea edu.gmu.MyClass // enable
java -da edu.gmu.MyClass // disable
More info:

23. References