String, Stringbuffer and StringBuilder in java basic knowledge Video

We should always use StringBuilder append() method rather than String "+" operator to avoid cost. 

Note that what "+" operator does is:

1. create a new string;
2. make a copy of the left string;
3. append the right char/string to the end.

So, a series of "+" operation is O(n^2)-time, not O(n).

StringBuilder is smarter and you can think it as a dynamically resizing array of char.

No copies for append() and it can output a String by calling toString() method.

Note: StringBuilder has no synchronization, i.e. it is not thread-safe.

If synchronization is required, it is recommend to use StringBuffer.

http://sophie-notes.blogspot.com/2013/02/java-stringbuilder-vs-string.html