This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
First of all, you should write where have you found this question.
when you are using setHeader you are deleting the old contents of the header, and you are puting into it a new value.
when you are using addHeader you are NOT deleting old contents, you just add a new value to this header.