How to reduce the number of bugs when coding?

Junit , run code coverage, find bugs tool.

Avoid fancy coding. The more complicated the code, the more likely there's bugs. Usually on modern systems, clearly written code will be fast and small enough.

Use available libraries. The easiest way to not have bugs writing a utility routine is to not write it.

Learn a few formal techniques for the more complicated stuff. If there's complicated conditions, nail them down with pen and paper. Ideally, know some proof techniques. If I can prove code correct, it's almost always good except for big, dumb, obvious bugs that are easy to fix. Obviously, this only goes so far, but sometimes you can formally reason about small but complicated things.

Beyond that, set the highest warning level your compiler offers, and make sure warnings are treated as errors. Bugs often hide in those "erroneous" errors.

• Don't ignore error codes - e.g. don't assume that you got a valid result, that a file has been successfully created, etc... Because some day, something will happen.
• Don't assume that your code will never enter some condition and that therefore "it's safe to ignore that condition".
• Test your code, then have it tested by someone else. I find I'm the worst person to test my own code.
• Take a break, then re-read your code and see if you "missed the obvious". Often happens to me.

if you're so sure that condition X will never happen... use an assert to make sure that when condition X happens, you'll know about it (through an exception, or logging, or whatever). –

I found that if I pass all of the context to a function (or method) that that function needs to do its job, and return the meaningful data that I'm looking for, that my code has become much more robust.

Implicit state is the enemy and in my experience is the #1 source of bugs. This state can be global variables or member variables, but if results are dependent on something that's not passed to the function you're asking for trouble. Clearly it is not feasible to eliminate state, but minimizing it has huge positive effects on program reliability.

Involve your testers as early as you can

http://programmers.stackexchange.com/questions/7927/how-to-reduce-the-number-of-bugs-when-coding