This week's book giveaway is in the OCAJP forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide 1Z0-808 and have Jeanne Boyarsky & Scott Selikoff on-line! See this thread for details.
basically it says in assignment conversions, the type of expression must be converted to the type of the variable. also if you look in the return type section in JLS , you will see that they use assignment conversion. so whatever you return must be assignable to the return type mentioned in your method using assignment conversion rules/contexts given in the book. since null can be assigned to any object reference variable, null is a valid return type from any method returning any object.