How to find Error in the application

Well...first check for compilation errors: these are usually syntax or spelling problems and can be solved pretty quickly, but which will prevent you from running your code.

Then, check for compiler warnings: these are usually something silly that is likely to bite you when you run the application, but won't stop you doing so. However, you should look very closely at them, and try to get rid of them - I run with "treat warnings as errors" simply because the compiler does know what it is talking about, and a warning generally means I forgot something and it's likely to be important.

Then when you run your application there are two types of problems you might get:

Exceptions: the application finds a critical error and cannot continue properly. You need to look at the exception in some detail using the debugger and work out exactly what caused it - and that may be some distance from the point at which the error became obvious to the application.

Logic problems: The application does something you didn't intend it to, or doesn't do something you did mean it to.

Both of these are pretty much only fixable by careful use of the debugger to work out exactly what is going on in your code: we can't run your code, so we can't tell you what to do. Use the debugger, and look at how your code runs, and what your variables are doing.