Standalone Java Windows software with password

Question

I'm trying to develop a standalone software for windows in java. My goal is to have a completely standalone software which can encrypt some data and decrypt them when the user inserts a correct password. Following the standalone mindset I would love to avoid using some DB to store the data (mandatory installation of a MySQL DB sounds horrible to me), so I decided to store the data in a simple txt file, converting my java storing-data class to JSON and saving this string in the file. Obviously I would encrypt the string before saving it. The most weak part of my project (or at least in my opinion) is the login. I've come up with this idea:

1. using PBE from java.crypto to encrypt and decrypt data
2. a check string for checking the user password like "this is my check string"
3. I encrypt my check string using a particular algorithm, a particular salt and a particular first time password like "admin". Then I store my encrypted check string in a separate txt file. The first time the user execute the application he can log with "admin" password and then change it.
4. the authentication process is this: the user inputs his password, and my program tries to decrypt my check string using that password. If the decrypted string equals the original check string the log in is successful. On the contrary it gives an error message to retry cause the password is wrong.

My idea seems somehow solid to me (at least for a non professional point of view) but I've read that you can extract the source code from an exe file and this would mean that anyone can spoil my secret check string, my salt and my encryption algorithm (cause this data are all written explicitly in one of my java classes).

Since this is my first time with java.crypto and in general with a standalone authentication problem, I would love to receive some advices from someone more experienced. Anyway I have some experience with MYSQL and db in general, but I would love to have a light and not that expensive way to store data, cause i know that storing in txt files is quite "naive". Anyway I'm accepting any type of advice, especially cause I've no experience with this type of problem.