Keep single class with Proguard?

I'm trying to "keep" a single class in when using Proguard but it only works when putting it in a directory and "keeping" the entire directory. When trying to match only a file it won't work.

When File.java is put in com.example.a.directory this works:

-keep class com.example.a.directory.** { *; } 

But when putting it directly in com.example.a this wont work:

-keep class com.example.a.File { *; } 

Why?