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StringCombiner: : merge )
. toString ();
At this stage, we have some code that looks vaguely sane, but it's quite hard to reuse this
same combining operation in different parts of our code base. So we're going to refactor our
reduce operation into a Collector , which we can use anywhere in our application. I've
called our Collector the StringCollector . Let's refactor our code to use it in
Example 5-24 .
Example 5-24. Collecting strings using a custom StringCollector
String result =
artists . stream ()
. map ( Artist: : getName )
. collect ( new
new StringCollector ( ", " , "[" , "]" ));
Now that we're delegating the whole of the String -joining behavior to a custom collector,
our application code doesn't need to understand anything about the internals of StringCol-
lector . It's just another Collector like any in the core framework.
We begin by implementing the Collector interface ( Example 5-25 ). Collector is generic,
so we need to determine a few types to interact with:
▪ The type of the element that we'll be collecting, a String
▪ Our accumulator type, StringCombiner , which you've already seen
▪ The result type, also a String
Example 5-25. How to define a collector over strings
public class
class StringCollector
StringCollector implements
implements Collector < String , StringCombiner , String > {
A Collector is composed of four different components. First we have a supplier , which is
a factory for making our container—in this case, a StringCombiner . The analogue to this is
the first argument provided to the reduce operation, which was the initial value of the re-
duce (see Example 5-26 ) .
Example 5-26. A supplier is a factory for making our container
public Supplier < StringCombiner > supplier () {
return () -> new
new StringCombiner ( delim , prefix , suffix );
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