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For a desktop Java app, some options for implementing authentication include:

Local authentication: You could use a database management system such as MySQL or SQLite to store and retrieve user credentials. The Java DataBase Connectivity (JDBC) API can be used to connect to the database and execute SQL statements for storing and retrieving the credentials.
Single sign-on (SSO) authentication: You could use the OAuth 2.0 authorization framework to allow users to log in with an existing account (e.g., Google, Facebook, etc.). The OAuth 2.0 Client Library for Java can be used to facilitate the authentication process.
Two-factor authentication (2FA): You could use a time-based one-time password (TOTP) library or application programming interface (API) to generate and verify the additional codes. The Java TOTP library can be used to implement TOTP in your Java app.
It's also worth considering using a pre-existing Java library or framework to handle authentication for you. Some popular options include:

Apache Shiro: A powerful and flexible open-source security framework that can handle authentication, authorization, and session management.
Spring Security: A widely-used security framework that provides a range of authentication options, including local, SSO, and 2FA.
I hope this helps! Let me know if you have any other questions.