If I understand correctly when you print out the records they appear in readible form on the console (command prompt), so you are able to process your database file correctly. But when trying to show them in the JTable you get the stack trace from some posts above. And you are not able to interpret this stack trace. I can easily tell you that there is a NullPointerException on line 137 of your ServerImpl class. So you are probably invoking a method on a data member or variable which is uninitialized.
I would suggest first finishing your Data class completely using some test cases (
JUnit,
TestNG,...) and only continue with your Server implementation when your Data class is completely finished and you know it's working as expected. And when you later make a change to the Data class these test cases will tell you if you have broken existing functionality. Then the same story applies to your Server implementation: finish that one before starting with the GUI. That's how I developed my assignment and so you don't lose valuable time deciding if it's a GUI, Server or Data error.
For Data class the only resource I can think of is documentation of RandomAccessFile (because that's the class which I used to access the data file).