kaeppler/maven-android-plugin — comment on commit 07ac43bc64247ac36f857a5527b257d74de0ffbf
File: src/main/java/com/jayway/maven/plugins/android/phase04processclasses/ProguardMojo.java line 231
By kaeppler:

Not sure I understand. The if-block checks for MacOS by probing the path to Java home. If true, it uses classes.jar (and it should be safe to not use File.separator here, since MacOS always uses forward slashes).
The else block would then apply only for Unix/Linux type OSs and Windows, and I believe on all of these it's rt.jar, and here the File.separator makes sure the path will work on both *nix and Windows type platforms.