Thank you very much for the hint. The downvoting is my misoperation in fact, sorry about it. Finnally fixed the issue haha.

Thanks a lot for your reply and help. I googled a lot and cannot find a way to build such a huge string in memory to return. I tried ByteArrayInputStream and FileInputStream and FileOutputStream. Could you please give me a hint on what class or api I could use to solve the issue? Thank you very much.

In performace test, there is a test input string which length is 18 000 000. and the output string's length is 164 000 000. In java, trying to build a string with a length of 164 000 000 will cause out of memory since the return type of test method is string. Thanks for any advice to pass the test? Thank you!