Once you have assigned a value to a "String Object" (i am 
saying an "object" not the "object reference")that value 
can never change. This is immutability.

by defining "s = s+Technologies" we are pointing the 
REFERENCE to newly created "string object" with the 
modified string. So the previous string object is 
considered to be lost.

This concept has explained very well in String, I/Os 
chapter of Kethy Siera, SCJP 5.

All the best