According to the documentation you are only allowed to use the characters 'r','w','x' and '-'. But in your code snippet you are using 's', which is illegal of course. It should be like this to work:
Set<PosixFilePermission> posixPermissions = PosixFilePermissions.fromString("rwxrwxr-x");
Files.setPosixFilePermissions(someDir, posixPermissions);
Setuid (+s) ist not possible in pure Java, because it's a OS-specific feature.