String StringBuffer StringBuilder

String

 is immutable,
if you try to alter their values, another object gets created, whereas 

StringBuffer

 and 

StringBuilder

 are mutable so
they can change their values.

Thread-Safety Difference:

The difference between 

StringBuffer

 and 

StringBuilder

 is
that 

StringBuffer

 is
thread-safe. So when the application needs to be run only in a single thread then it is better to use 

StringBuilder

. 

StringBuilder

 is
more efficient than 

StringBuffer

.

Situations:
If your string is not going to change use a String class because a 

String

 object
is immutable.
If your string can change (example: lots of logic and operations in the construction of the string) and will only be accessed from a single thread, using a 

StringBuilder

 is
good enough.
If your string can change, and will be accessed from multiple threads, use a 

StringBuffer

because 

StringBuffer

 is
synchronous so you have thread-safety.