Hi,
After the append, the object where "sb" references to has a value of "bbbaaabbb". "sb = null;" removes the reference to that object, but the original "sb" (that one in the main method) still refers to that object. The whole clue is, that when you pass an object as an argument to a method, you pass a copy of the reference to that object. So by setting the copy reference to null, doesn't mean that the original will be set to null..
Greetz,