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Another possibility is the XMLEncoder class.

By "disk file" I meant a file that was written on a disk. I suppose it could also have been written to a memory stick. Basically I meant the file was written to a place out side of the program where...

Yes. You will need to write special code to write out the contents of the ArrayList as readable text.
What do you want to do with the disk file?

The file contains binary data not text.