The question about migrating Forms to Java (or ADF or APEX) comes up time and time again. I wanted to pull some core information together in a single blog post to address this question.
The first question I always ask is "WHY" - Forms may still be a viable option for you so "if it ain't broke don't fix it". Bottom line is whatever anyone tells you, its going to be a considerable effort and cost to migrate from Forms to something else so the business is going to want to know WHY you spend all those hard earned dollars switching from something that might have been serving you quite adequately.
Second point, if you are going to switch, I would encourage you NOT to look at building a Forms clone. So many times I see people trying to build an ADF application and EXACTLY mimic the Forms model - ADF is NOT a Forms clone. You should be building to the sweet spot of your target technology, not your 20 year old client/server technology. This is also the chance for the business to embrace change, so maybe look at new processes, channels and technology options that weren't available when you first developed your Forms applications.
To help you understand what is involved, I've put together a number of resources.