I know this question has been answered many times in many sites many years ago.:P Still I have some doubt so thought of posting this. The basic difference is, String is immutable and each time we do any operation on String it creates a new String Object.
Ex:-
String str = "some";
str = str + " text"
In above case 2 new Strings get created instead of modifying the existing str which can be avoided by using StringBuffer.
Ex:-
StringBuffer str = new StringBuffer();
str.append("try");
str.append("this");
My question here is, to append method we are passing a String again. Do new String objects not get created for "try" and "this" in String pool in above case.
Yes, new String objects get created for "try" and "this". The benefit here is that the StringBuffer stores the string of characters internally as a dynamically resized array.
It's more obviously beneficial if we were to concatenate more than two Strings:
"try" + "this" + "test"
This would potentially create 5 String objects because you need intermediate values. (Technically concatenation of literals is performed at compile time so this is just a conceptual example.) It would also be typical for a compiler to refactor the above snippet in to using StringBuilder anyway if they were not literals.
StringBuilder is a newer and non-synchronized version of StringBuffer. In general, you should prefer StringBuilder. The difference between these two classes is covered in "StringBuilder and StringBuffer in Java".