java api for authentication

No, Dropbox API apps should use the OAuth app authorization flow to let the user authorize the app in their browser. This way, apps never handle the user's username and password.

There's an example of this in the Java SDK here:

https://github.com/dropbox/dropbox-sdk-java/tree/master/examples/authorize

Note that once your app has an access token for a user, it doesn't need any further manual user intervention. It can store and re-use that access token for future calls automatically.

If the device your app runs on doesn't have a browser though, feel free to open a ticket with more details and we'll see if there's any help we can offer:

https://www.dropbox.com/developers/contact