How to debug

I see an awful lot of posts on StackOverflow that show that the person asking the question hasn’t got the slightest clue how to go about debugging their problems. So here’s a few specifics for a few extremely common situations:

1. It’s not a bug in the compiler
It’s not a bug in the compiler, it’s never a bug in the compiler. Stop making that your default assumption. I’ve been a programmer for over 25 years, and the only time I saw a bug in the compiler was in the early versions of cfront, which was AT&T’s way to convert C++ programs into C programs so you could compile and link them with C tools. If you think there is even the slightest possibility that it’s a bug in the compiler, you’re going to stop looking before you see what you did wrong. And yes, you did something wrong. Similarly…

2. It’s probably not a bug in the library routines
The probability of a bug in the library routines depends a lot on the number of people using it. If it’s a core part of Java, chances are you’re not the first person to notice something the other 25 million Java developers somehow overlooked. If it’s a project that you found on SourceForge that hasn’t been updated in 4 years and only had one developer, it’s a possibility, but one you should discount until you’ve made sure you’re calling it right.

3. Null Pointer Exceptions happen for a reason
If you got a NullPointerException in your code, or any type of exception in library code, you did something wrong. Look at the stack trace. Look for the top-most entry that is your code. Look at the line there. Think about what you see on that line. Can one of those variables be null? Did you initialize all the variables in every possible way through the code to that point? Are you giving the correct arguments to whatever library code you’re calling? If necessary, put a breakpoint there or throw in some debugging statements and print out what you’re using in that line to make sure they’re what you expected.

4. Debugging AJAX calls is hard, but it’s easier than trying to explain it on StackOverflow
A large number of questions are of the nature “I do this ajax call, and it doesn’t work”. What doesn’t work? Are you making the call? Is the server receiving it? Is the server doing the right thing with it? Is it passing back what you’re expecting?

The first thing you need to do is use a good debugger. If the problem happens on Firefox, then you’re in luck because you can use Firebug + Firequery.

If you’re unlucky, and the problem only happens in IE (and face it, those are your only two alternatives because if a problem happens in any non-IE browser, it happens in all of them, whereas if you code works in IE8 you’re not 100% sure it works in IE7 or IE9), then you need to use whatever debugger options are available to you. I found some useful information here and I end up using a combination of Firebug Lite and IE Developer Toolbar. Fortunately most of the IE8 and IE7 problems I’ve encountered happen in IE9 with the Browser Mode and Document Mode set appropriately.

Once you’ve got your debugger up, you want to set a breakpoint on the actual ajax call (to verify that you’re actually getting to the call and not missing it for some other reason), on the success callback (to verify that the server has sent a response) and on the failure callback (to verify that the server didn’t throw up its hands and give up). It also helps if you’ve got access to the server side logs and can see what’s going on there as well, but that’s often not possible like when you’re calling somebody else’s web API. In the IE debugger, you need to go to the Network tab and “Start Capturing”, and in Firebug you just need to look at the “Console” tab. After the ajax call returns, you can look at the appropriate tab and see what was sent to the server and what came back. And in the success callback you can look at the returned response and single step through the logic to see if you’re doing the right thing with it. And you can do all that in less time than it would take to write a question to StackOverflow. If you’re still stumped, you also have a lot more information you can put in your question, which will help all the eager question answerers out there who don’t have the ability to step through your code.

5. A question and answer site is not the place to learn the syntax of a language
If you code doesn’t even compile, then you don’t know enough to ask a question that is useful to either you or the other users of StackOverflow. Pick up a book and learn the basics.

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4 Responses to How to debug

  1. Tina Marie says:

    I’ve not used FireBug, but the Chrome debugger never ceases to amaze me. Every time I say “I wish the debugger did “, I find a way to do it.

    Also, WebStorm/RubyMine rocks. It also has a JS debugger (although I use the Chrome one), but also allows you to set breakpoints in your HAML rendering, step into the ruby side of your AJAX calls, and generally feel like you have a real debugger.

    I argue that the most important feature of any language to me is the quality of their debugging tools.

  2. Paul Tomblin says:

    I know the Chrome debugger is getting better, but last time I looked it had a long way to go before it beats Firebug + FireQuery.

  3. Stephen Harris says:

    I love that your LJ reposting says “-1 comments” :-)

  4. Rich Lafferty says:

    This reminds me of (and thus you might appreciate) Mark-Jason Dominus’s old Good Advice and Maxims for Programmers, from comp.lang.perl.misc Back In The Day.

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