How to cast the (FileInfo) type to (VmDiskFileInfo) type objects? VI SDK Java

I am trying to get virtual disk size and type from datastore. After getting result, to get the size and type, I have to cast  (FileInfo) type to (VmDiskFileInfo) type objects but I got a casting error. It is VI SDK Java.

1) Do you know how to cast  (FileInfo) type to (VmDiskFileInfo)?

2) If it is not working, how to get vdisk size and type on a datastore. I need to get all vdisks including free vdisks which are not asssigned to any vms.

Thanks -- Sung