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The code in post#1 was having the problem. You need to find out where the program was looking for the file referenced there:
FileInputStream file = new FileInputStream("class.dat");

You want to...

Something like this:
File aFile = new File<THEPathHERE>); // create a File object
System.out.println("an ID "+ aFile.getabolutepath()); // print its absolute path

check the spelling.

Create a File object using the filename and print its absolute path to see where the program is looking for the file.