What is the difference between String,StringBuffer & StringBuilder?

Question

Hard to learn.

Answer 1

1. String is Immutable and Synchronised, I.e unchangeable n thread safe.
2. String Buffer is mutable and Synchronised, I.e changable and thread safe.
3. String Builder is mutable and unsynchronised, I.e changable and not thread safe.

Answer 2

Thread safe means at a time only one thread can access the data type.

Answer 3

What does thread safe means ?