Hi Sagar
Disclaimer - I'm not a developer and I avoided Java at university so will not answer all of your questions.
What you might like to do is run up on OpenUI5 and SAPUI5. This might be an area you could transition into SAP development based on your Java experience. ABAP is the development language for core systems, however Java is now being considered a skill to develop for SAP developers.
If you are a strong developer you should know the principals of programming. Therefore, with dedication you might find it easier to transition to a SAP background and learn the other languages. If you search Google and other areas you will find ways to get demo lessons and might be able to self teach yourself initially, although training would have benefits.
Regards
Colleen