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.
addHeader & setHeader work like this... If "a" DOES NOT exist then "a" is set (NOTE here I have used SET... simple...). If "a" DOES exist then addHeader ADDS (menas add the existing value) but the setHeader SETS (means overwrite existing value). So the answer will be "c", since last statement is setHeader("a", "c"); Had it been like addHeader("a", "c"); then outcome would be "abc".