Difference between File.separator and slash in paths

Question

What is the difference between using File.separator and a normal / in a Java Path-String?

In contrast to double backslash \\\\

Answer 1

Using File.separator made Ubuntu generate files with "\" on it's name instead of directories. Maybe I am being lazy with how I am making files(and directories) and could have avoided it, regardless, use "/" every time to avoid files with "\" on it's name