When Android Studio starts treating Java files as plain text

Several students have somehow accidentally made Android Studio start treating specific Java source files as plain text. This can be fixed by removing the association:

1. Open the Android Studio settings, File | Settings (or Android Studio | Preferences on Mac)
2. Go to the Editor | File Types section
3. Select Text in the Recognized File Types pane (top)
4. In the Registered Patters pane (bottom), select and remove any entries ending in .java