How to understand StringBuffer is thread-safe and StringBuilder is non-thread-safe in Java?

StringBuffer(Thread-safe)

• StringBuffer is thread-safe meaning that they have synchronized methods to control access so that only one thread can access StringBuffer object's synchronized code at a time.
• StringBuffer objects are generally safe to use in a multi-threaded environment where multiple threads may be trying to access the same StringBuffer object at the same time.

StringBuilder(Non-thread-safe)

• StringBuilder is not synchronized so that it is not thread-safe. By not being synchronized, the performance of StringBuilder can be better than StringBuffer.
• If we are working in a single-threaded environment, using StringBuilder instead of StringBuffer may result in increased performance. This is also true of other situations such as a StringBuilder local variable where only one thread will be accessing a StringBuilder object.