Send secret messages. What do you think?

Question

I made a Java message encoder to encode and decode messages with a custom key using an alphabet rearrangement. Encode your message and send it to a friend using a key that you both know. Your friend can decode the message using the same program with the code.

I appreciate your comments and suggestions!
https://code.sololearn.com/cYE9clWB1v5b/?ref=app