Streams - Java 8 Lambdas - page 28

Example 3-13 provides some code that finds the shortest track on an album. In order to make
it easier to see that we've got the right result, I've explicitly listed the tracks on this album in
the code snippet; I'll admit that it's not the best-known album.

Example 3-13. Finding the shortest track with streams
List < Track > tracks = asList ( new new Track ( "Bakai" , 524 ),

new

new Track ( "Violets for Your Furs" , 378 ),
new new Track ( "Time Was" , 451 ));

Track shortestTrack = tracks . stream ()
. min ( Comparator . comparing ( track -> track . getLength ()))
. get ();

assertEquals ( tracks . get ( 1 ), shortestTrack );

When we think about maximum and minimum elements, the first thing we need to think
about is the ordering that we're going to be using. When it comes to finding the shortest
track, the ordering is provided by the length of the tracks.

In order to inform the Stream that we're using the length of the track, we give it a Compar-
ator . Conveniently, Java 8 has added a static method called comparing that lets us build a
comparator using keys. Previously, we always encountered an ugly pattern in which we had
to write code that got a field out of both the objects being compared, then compare these
field values. Now, to get the same element out of both elements being compared, we just
provide a getter function for the value. In this case we'll use length , which is a getter func-
tion in disguise.

It's worth reflecting on the comparing method for a moment. This is actually a function that
takes a function and returns a function. Pretty meta, I know, but also incredibly useful. At
any point in the past, this method could have been added to the Java standard library, but the
poor readability and verbosity issues surrounding anonymous inner classes would have made
it impractical. Now, with lambda expressions, it's convenient and concise.

It's now possible for max to be called on an empty Stream so that it returns what's known as
an Optional value. An Optional value is a bit like an alien: it represents a value that may
exist, or may not. If our Stream is empty, then it won't exist; if it's not empty, then it will.
Let's not worry about the details of Optional for the moment, since we'll be discussing it in
detail in Optional . The only thing to remember is that we can pull out the value by calling the
get method.