Using a Debugger

SWC What is ”debugging”? An error in a computer program is often called a ”bug”… …so, to ”debug” is to find and get rid of errors in a program Applies to logical errors, syntax errors are handled by the compiler Logical errors can be extremely hard to find…

SWC Strategies for debugging Write error-free programs! Ideal, but practically impossible  Write ”defensive” code. Do up-front checks for e.g. null references, illegal values, etc.. Read the code. Many errors can actually be caught just by reading the code Read the code (again). Many errors can actually be caught just by reading the code (really) Explain the code to others. Trying to verbalise the code can often reveal errors

SWC Strategies for debugging Create proper tests. Will help you detect errors and pinpoint their location (if the test is detailed enough…) Logging/printouts. This is often a beginners approach to debugging. Can work, but tedious Use a debugger. Only realistic approach in larger programs

SWC What is a ”debugger” A special program which can ”manage” execution of another program –Pause program execution at a specific point –Start the program from that point again –Let a user run the program one line at a time –Inspect the value of variables In modern integrated development environments – like NetBeans – the debugger is an integral part of the environment.

SWC Keep it simple… Modern debugges can do a lot of stuff – you only need to know a little bit to get started –Breakpoints –Variable inspection –Stepping over –Stepping into –Stepping out of

SWC Breakpoints A breakpoint is set at a line in a program The breakpoint makes the debugger pause execution of the program at this line This enables us to see the values of variables at this point in the program! We run a program in ”debug mode” by pressing Ctrl + F5

SWC Breakpoints in Netbeans Breakpoint (Ctrl + F8, or click in left margin)

Breakpoints in Netbeans SWC Breakpoint Program is paused here!

Breakpoints in Netbeans SWC See current value of a variable by hovering mouse cursor over it

Breakpoints in Netbeans SWC See current values of all variables, instance fields, parameters, etc. in the ”Variables” debug window

SWC The debugging toolbar This little toolbar actually contains all we need to know (and use) about the debugger at this point Various actions to take, now that the program is paused at a breakpoint

SWC Finish Debugger Session Finishes the debugging ”session” In other words – stop the program, we are done for now! Shortcut: Shift + F5

SWC Pause Pauses the program… …how is that different than pausing at a breakpoint?? Could e.g pause a long iteration Shortcut: None…

SWC Continue Continue execution of the program, until the next breakpoint is met If no more breakpoints, program simply runs to the end Shortcut: F5

SWC Step Over Move one line ahead, staying in the current method (what else?) Often referred to as ”single-stepping” Executes all method calls in the line of code Shortcut: F8

SWC Step Over Expression Almost like Step Over, but only executes one of the method calls in the line of code Invoke again to execute next method call Useful to analyse complex lines of code, without actually stepping into the called code Shortcut: Shift + F8

SWC Step Into If the line of code contains a method call, the debugger ”steps into” that code Useful for following the details of a method call Can not step into library methods… Shortcut: F7

SWC Step Out The debugger returns to the method which called the method we are currently inside Note that remaining lines of code in the called method are executed Shortcut: Ctrl + F7

SWC Run to Cursor Runs the program to the current position of the mouse cursor Like an ad-hoc breakpoint Useful for e.g breaking out of a loop, while staying inside a method Shortcut: F4

SWC Debugging in a nutshell Set breakpoints where needed Run program in debug mode (Ctrl + F5) Inspect variable values at breakpoints (mouse hovering, the ”Variables” debug window) Use the Step… facilities to continue execution from the breakpoint Rewrite the code as appropriate Repeat from the top…