It is necessary to assemble a single path from several lines depending on the OS.

It is logical to expect behavior in Java.
https://docs.oracle.com/javase/8/docs/api/java/nio/file/Paths.html
Converts a path string, or a sequence of strings that when joined form a path string, to a Path. If more does not specify any elements then the value of the first parameter is the path string to convert. If more specifies one or more elements then each non-empty string, including first, is considered to be a sequence of name elements (see Path) and is joined to form a path string. The details as to how the Strings are joined is provider specific but typically they will be joined using the name-separator as the separator. For example, if the name separator is "/" and getPath("/foo","bar","gus") is invoked, then the path string "/foo/bar/gus" is converted to a Path. A Path representing an empty path is returned if first is the empty string and more does not contain any non-empty strings.

My temporary fix is something like this:
print("\\".join(["C:/123\\345", "\\", "folder///filename.bin"]).replace("/", "\\").replace("\\\\", "\\").replace("\\\\", "\\"))