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Use the command line to create the jar file. Search for a jar tutorial. Or search for how your IDE can be configured to create the jar file.

Did you create a manifest file?
Did the manifest file have the correct information?
Did you include the manifest file in the jar file?

Try running it from the command prompt or wrap all your files up in a jar file.

How are you launching the application?

That is not going to work because that is not where the file is. Where in my previous reply did I suggest doing that?

You can either place the file in the location that the JVM is looking or you can include the full path.

String filename = "C:/full/path/to/file/items.txt";

By the way you can just use a JFileChooser.

System.out.println(System.getProperty("user.dir"));

Add that as the first line in your program and it will display where it is looking for the file.

File not found is self evident. Your program cannot find a file called items.txt in the current working directory. Where is it?

Which is line 33?
Where is the rest of your code? If it is too long then create a SSCCE (google it).
In the catch statement put e.printStackTrace() as this will give you more information than...