What does “ StringBuilders are not thread-safe” mean?

Question

I have read some articles about the pros and cons of String and StringBuilder in the Java Programming language. In one of the articles, the author

Answer 1

If multiple threads are modifying the same instance of a StringBuilder, the result can be unexpected - i.e. some of the modifications may be lost. That's why you should use StringBuffer in such situations. If, however, each thread StringBuilder instance can be modified by only one thread, it is better to use StringBuilder, since it would be more efficient (thread safety comes with a performance cost).