a$.$ The CaesarCipher and XORCipher classes couple two concerns, the business logic of encrypting data and the mechanics of how to get the input from a file on the filesystem and writing the output to a file. This makes it harder to test just the business logic. Using the Hexagonal Architecture pattern we can refactor the encrypt$()$ and decrypt$()$ methods to use an existing Java type, byte$[],$ as an abstract port for the input argument and return a byte$[]$ for the result.

b$.$ Using the Dependency Inversion pattern we can do even better by creating an abstract interface called EncryptionMethod that will become the interface between high level client code and the lower level encryption$/$decryption methods. In addition to encrypt$()$ and decrypt$()$ methods we could also put the setPassword$()$ method into the interface and add a getPassword$()$ method that would return a string representation of the internal password data to make testing setPassword$()$ easier.