To receive the token with which to authenticate against specific user can use web app:
http://apps.camilolopes.com.br/dpboxapiweb/#/
or can be run Maven project
https://github.com/ITSStartup/dropboxapi-example
When the token is received it has to be saved on file and to be used for future uploads and downloads. Examples for downloading and uploading can be found in example section of dropbox-sdk
https://github.com/dropbox/dropbox-sdk-java/tree/master/examples