What is the difference between StringBuilder and StringBuffer in Java?

The main differences between StringBuffer and StringBuilder lie in their thread safety and performance, both of which are mutable string classes.

1. Thread safety: StringBuffer is thread-safe, as its methods are synchronized and can be accessed and modified by multiple threads safely. In contrast, StringBuilder is not thread-safe, as its methods are not synchronized and cannot guarantee safety in a multi-threaded environment.
2. Performance: StringBuilder performs better than StringBuffer because StringBuilder’s methods are not synchronized, making it more efficient in a single-threaded environment. On the other hand, StringBuffer’s methods are synchronized, requiring thread synchronization, which can result in some performance loss.

Therefore, it is recommended to use StringBuilder when thread safety is not a concern, as it has better performance; whereas in a multi-threaded environment or when thread safety is needed, StringBuffer should be used.