class String

Important Note: What class to use and when?

StringBuffer is synchronized, hence, thread-safe. It should be used when a string is to be modified from multiple threads.
StringBuilder should be preferred over StringBuffer when there is a single thread to mutate the string.
String should be used when frequent modification to the string is not expected. Because everytime a change is made to its object, a new object gets created and referenced rather than modifying the original.

String StringBuilder StringBuffer

immutable mutable mutable

thread-safe not thread-safe thread-safe
The figure shows the StringBuilder (fourth) is mutable unlike String (first, second, third).

String	StringBuilder	StringBuffer
immutable	mutable	mutable
thread-safe	not thread-safe	thread-safe

Note

System.identityHashCode(Object) does not return the actual memory address. Java does not provide any such direct mechanism.