... Assume the incoming string is a big number. ... The DecimalFormat in Java has a limitation to format a big number.
95% of problems are solved by clarifying your requirements. Let's filter out the things that are definitely not requirements. "incoming string is a big number" - that statement can never hold true. Is it a String? If yes, then it's a String, and never a number. This distinction is important. I'll assume that the answer to this question is "yes".
Now that you have a String, why are you using a DecimalFormat? What does a String have to do with a decimal number formatter? Nothing related to the problem that you are trying to solve. I'll leave the determination of the correct approach to solve your problem up to you - after all, my speculations at your requirements may not be correct.
To reiterate, here are the things that are definitely incorrect: - You have a String, that is a big number" - DecimalFormat has an issue with "big numbers" - an unsubstantiated conclusion drawn on a false basis of understanding.