;; -?>> can be used if intermediate forms may potentially return nil. 
;; This is to prevent 

user=> (use '[clojure.contrib.core :only (-?>>)])
nil

;; create function that will return nil if the passed value is "Hello"

user=> (defn return-nil-for-hello [str-value] 
		(if (= str-value "Hello") 
			nil 
			str-value))
#'user/return-nil

user=> (->> "Greetings" return-nil-for-hello .toUpperCase)
"GREETINGS"

;; null pointer exception is thrown because return-nil-for-hello
;; returns nil and method .toUpperCase cannot process nil

user=> (->> "Hello" return-nil-for-hello .toUpperCase)
NullPointerException   clojure.lang.Reflector.invokeNoArgInstanceMember (Reflector.java:314)

;; using -?>> will prevent null exception thrown
user=> (-?>> "Hello" return-nil-for-hello .toUpperCase)
nil