CSci 2101 Debugging Tips.

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Tips on how to get your program to work correctly

• Start thinking about your problem set assignment as soon as possible. Draw diagrams, write pseudocode and/or method descriptions, but don't start coding right away. The first approach (or two) to a problem is often incorrect or inefficient. By catching it before you start writing a program you can save yourself a few hours a week. And if your first approach is right, you will have a much better idea of what you are doing by the time you start coding.
• When you are ready to write a program, don't try to write the whole program on paper. Programs on paper often have tons of programming mistakes and are impossible to debug. Write down the detailed description of your program (in English or in pseudocode or in diagrams), check it to make sure that it makes sense, and start writing code (see below).
• Create a file with the class name (or open the file that you are supposed to modify). Sketch out your program by writing comments for each method or a part of the program (don't write any code yet!)
• Use incremental approach to programming: write a line or two of code, compile, test. This way you know exactly where the mistake is: it's in the newly added line of code!

  If you are writing a Java class, start by writing stubs for all methods (the initial version of a method that has the right type, but doesn't do anything. Remember to return a dummy value from a method that has to return a value). Compile and test the class with the stubs.

• If you get 55 compiler errors for your program (or any other number of errors greater or equal than 1):
  • Don't panic!
  • Start from the top one. Chances are the other 54 errors are induced by the first one (or, more likely, all 55 errors are induced by 3 mistakes that you made in a recently added 2 lines of code).
  • If the top error doesn't make sense, check the curly braces (use jEdit to do this: click on the opening brace and it will show you the matching closing one).
  • If you don't understand what's causing an error, look in the book or read the APIs: this may give you a hint.
  • If nothing helps, try commenting out suspected lines of code and see if the error messages go away or change. Then uncomment the lines one-by-one, read the error messages.
  • Once you have discovered an error, stop and think of how you should fix it. Don't rush into correcting your code: you will make it worse by randomly trying to get it to compile. Fix the mistake only when you understand the problem.
• When working on your program, take breaks, especially if you get stuck. Have dinner or a snack, take a walk (the best!), call a friend. It's amazing how a mistake that was escaping you for an hour is just staring you in the face when you come back after a 10-minute break!
• If at any time of working on your program you feel that the program is too large and messy, rewrite it right away! Don't consider it a waste of time: if you don't, it will only get larger and messier, and, chances are, you will still have to rewrite it later, only it will be more painful. When rewriting a program:
  • Save the file with a different name, for instance MyClassVersion1.java instead of MyClass.java.
  • Open a new file MyClass.java.
  • Copy the parts of the program that you are happy with, as if you are doing incremental programming; compile and test them as often as you can.
  • Rewrite the parts that you are not happy with. Look at the old code, and see what you can clear or optimize. Creating helper methods might be helpful. Sometimes you might even want to write additional classes. Compile and test the new file often!
• If your program doesn't seem to react to any changes you make...
  • Save your file, recompile, and run (maybe your forgot to save or to recompile?) Make sure that the compilation was successful, otherwise you are running the old code (the result of the last successful compilation).
  • If this doesn't help, you might be running a different program with the same name. Check your path to make sure that you are in the correct directory. Do ls -l, look at the last modification time on java files (maybe you are modifying a file in a different directory?).
  • If this doesn't help, remove all the .class files, recompile the program, and run it again. This should help. If it doesn't, ask the instructor or the TA (but see advice for applets below).
  • If you are working with an applet, quit all the appletviewer windows, recompile the program, and start the appletviewer again. If you are viewing an applet in a browser, empty the browser cache (Tools -> Internet Options -> General -> Delete Files on Internet Explorer), save and recompile the program, and open the files in the browser again. Quitting the browser before re-displaying the applet is not necessary, but you might want to do it just in case.
• Write comments as you are writing your code (or, better yet, before!), not afterwards.
• If you have Java questions, ask in class, at the lab, post your questions on Wiki, ask your instructor or a TA, or other students. The more you ask, the more you learn!