The official Java SDK for API v2 helps you to integrate Dropbox into your apps using Java.
Dropbox for Java - Dropbox for Java is open source on GitHub.
You can add the Java SDK to your project via Maven or Gradle.
To get started with the Java SDK, check out the README in the GitHub repo, under the Setup section.
For a complete tutorial, check out the README in the GitHub repo, under the Dropbox for Java tutorial section.
Here's the full documentation for the Java SDK.