Why is part of the string lost? How to avoid it?

Question

Java

The answer to the problem contains many (300,000) six-digit numbers that need to be printed quickly, for this I use BufferedWriter, but on one of the tests part of the string is lost: wrong answer 298290th numbers differ - expected: '78442', found: '78' . I wrote the answer to a file and did not find a separate number 78 there. What is the problem?

Answer 1

Need to flush

Answer 2

The fact that the code is written crookedly. And the fact that you think that all the Wangs are here and will guess what is going wrong without a description at all

Answer 3

Use a debugger.