I have a file with nested classes, but it\'s becoming long enough to be unreadable. Is there a way I can break out the nested classes into separate files?
Use the right tools.
Write unit-tests to cover the code you are going to refactor.
Then use refactoring tools, e.g. Eclipse/JDT to extract each class. Run the tests for each refactoring job.
For extra fun, you can do this without the unit tests.
I'm afraid not.
As an alternative, you could consider converting some of them to plain Java classes: although nested classes increase encapsulation, you can certainly create good OO design without them.
My two cents.