15 min. Null

C/C++ Legacy
'not initialized pointer'
in java/C# we don't work with pointers,
objects are not pointers

Null is Dishonest
it can be a valid instance of any reference type
it is not an object
it has no methods,
it cannot be a valid substitute for objects (types)
null is always a 'special case'
Liskov's Substitution Principle (LSP)
if S is a subtype of T, then objects of type T in a
program may be replaced with objects of type S
without altering any of the desirable properties of
that program

Null in the API
as a parameter
please use method overloading
as a result
needs special care
needs extra documentation/naming convention
do it in code (self-documented)
@NotNull/@Nullable (findbugs, IntelliJ)

Null Object
a Null Object is an object with defined neutral (\"null\")
behavior
opportunity for 'Tell don't Ask'
can be extended by the use of a Visitor

Option (Some/None)
'generic' Null Object
originated (!?) in ML, it is also present in its derivatives
ocaml, F#, scala) and even java
ideal for API usage
parameter & result
intention clear, self-documented
smaller granularity (generic functions),
ability to compose those ('maybe monad')

Navigating over nulls
a.getB().getC()
need to handle null on the receiver for every '.'
Comega
groovy: Safe Navigation Operator
java7
ruby: andand
DISCLAIMER Extensive object navigation violates the
Law of Demeter making future refactorings and testing
more difficult.
Tell Don't Ask

Defensive Programming
Don't accept nulls
NotNullAdvice for services (beans)
ValidatedAs.notNull(param,...) for other data
throws IllegalArgumentException

References
Richard Cobbe:'Much Ado About Nothing: Putting Java's
Null in its Place'
http://delicious.com/andrei.pamula/null
http://delicious.com/andrei.pamula/option