String vs StringBuffer vs StringBuilder

String:

• A string is immutable.
• Stored in the Constant String Pool.
• Every immutable object in Java is ThreadSafe.

StringBuffer: 

• StringBuffer is mutable, it can be changed after it is defined. 
• Its object is stored in heap. 
• It is also Thread Safe. 
• Performance is slow because of thread safety. (Note: Thread Safe is nothing but no two threads can simultaneously access the same method).

StringBuilder: 

• StringBuilder is same as StringBuffer. Value can be changed and the object is stored in heap. 
• The only difference is that, it is not Thread Safe. 
• It is better in terms of performance because it is not thread-safe and there are no checks done to verify thread safety.