I found a free plugin that can generate class diagrams with android studio.
It’s called SimpleUML.
Update Android Studio 2.2+:
To install the plugin, follow steps in this answer: https://stackoverflow.com/a/36823007/1245894
Older version of Android Studio
On Mac: go to Android Studio
-> Preferences
-> Plugins
On Windows: go to Android Studio
-> File
-> Settings
-> Plugins
Click on Browse repositories...
and search for SimpleUMLCE
(CE means Community Edition, this is what android studio is based on).
Install it, restart, then you can do a right click on the folder containing the classes you want to visualize, and select Add to simpleUML Diagram
.
That’s it; you have you fancy class diagram generated from your code!