Libraries - Java 8 Lambdas - page 59

NOTE
There are other methods on Stream and its primitive specialized variants. Specifically,
range and iterate give us other ways of generating our own streams.
Optional
Something I've glossed over so far is that reduce can come in a couple of forms: the one
we've seen, which takes an initial value, and another variant, which doesn't. When the initial
value is left out, the first call to the reducer uses the first two elements of the Stream . This is
useful if there's no sensible initial value for a reduce operation and will return an instance of
Optional .
Optional is a new core library data type that is designed to provide a better alternative to
null . There's quite a lot of hatred for the old null value. Even the man who invented the
concept, Tony Hoare, described it as “my billion-dollar mistake.” That's the trouble with be-
ing an influential computer scientist—you can make a billion-dollar mistake without even
seeing the billion dollars yourself!
null is often used to represent the absence of a value, and this is the use case that Optional
is replacing. The problem with using null in order to represent absence is the dreaded
NullPointerException . If you refer to a variable that is null , your code blows up. The
goal of Optional is twofold. First, it encourages the coder to make appropriate checks as to
whether a variable is null in order to avoid bugs. Second, it documents values that are ex-
pected to be absent in a class's API. This makes it easier to see where the bodies are buried.
Let's take a look at the API for Optional in order to get a feel for how to use it. If you want
to create an Optional instance from a value, there is a factory method called of . The Op-
tional is now a container for this value, which can be pulled out with get , as shown in
Example 4-22 .
Example 4-22. Creating an Optional from a value
Optional < String > a = Optional . of ( "a" );
assertEquals ( "a" , a . get ());
Because an Optional may also represent an absent value, there's also a factory method
called empty , and you can convert a nullable value into an Optional using the ofNullable