The odd-looking chmod 755 name.class command sets the permissions. The permissions are encoded in binary, 111 101 101, then in octal 755, one bit for each permission, 1=ok 0=not ok to do that. 755 means I give myself permission to do everything, but I don’t let others write, though I let them read and execute.

There is a high order bit that tells if a "file" is actually a directory: d rwx rwx rwx. In that case:

• The read bit gives you to right to get a list of files in the directory.
• The write bit lets you create new files in that directory, change the attributes of files in that directory or write/delete files in that directory.
• The execute bit lets you access files/directories in that directory that you know by name and lets you change into that directory.