Advanced Collections and Collectors - Java 8 Lambdas - page 82

public public Function < StringCombiner , String > finisher () {

return

return StringCombiner: : toString ;

}

We create our final value from the one remaining container.

There's one aspect of collectors that I haven't described so far: characteristics . A character-
istic is a Set of objects that describes the Collector , allowing the framework to perform
certain optimizations. It's defined through a characteristics method.

At this juncture, it's worth reminding ourselves that this code has been written as an educa-
tional exercise and differs a little bit from the internal implementation of the joining col-
lector. You may also be thinking that the StringCombiner class is looking quite useful.
Don't worry—you don't need to write that either! Java 8 contains a
java.util.StringJoiner class that performs a similar role and has a similar API.

The main goals of going through this exercise are not only to show how custom collectors
work, but also to allow you to write your own collector. This is especially useful if you have
a domain class that you want to build up from an operation on a collection and none of the
standard collectors will build it for you.

In the case of our StringCollector , the container that we were using to collect values was
different from the final value that we were trying to create (a String ). This is especially
common if you're trying to collect immutable values rather than mutable ones, because oth-
erwise each step of the collection operation would have to create a new value.

It's entirely possible for the final value that you're collecting to be the same as the container
you've been folding your values into all along. In fact, this is what happens when the final
value that you're collecting is a Collection , such as with the toList collector.

In this case, your finisher method needs to do nothing to its container object. More form-
ally, we can say that the finisher method is the identity function: it returns the value
passed as an argument. If this is the case, then your Collector will exhibit the
IDENTITY_FINISH characteristic and should declare it using the characteristics method.

Reduction as a Collector

As you've just seen, custom collectors aren't that hard to write, but if you're thinking about
writing one in order to collect into a domain class it is worth examining the alternatives. The
most obvious is to build one or more collection objects and then pass them into the construct-