How to Debug an Exception

1. Read the stack trace carefully. In particular, look for:
  a) The name and text of the exception
  b) Where the trace enters your code
2. Go to the lowest level in your code that the stack trace showed
3. Look carefully at the line of code throwing the error, and the preceeding lines, and see if you can see how they relate to the exception you got.
4. Try to imagine what values the variables might have had at this point, to generate the exception you saw.
5. If you have a debugger, set a breakpoint on this line and inspect all relevant variables. If you don’t have access to a debugger, print out the values of the variables.
6. If you can see a value that looks bad, follow that variable up through the program to see where it came from and why it’s incorrect.
7. If you can’t see any values that look bad, rethink you assumptions about what caused the error.