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operation can be very suitable for this purpose. Sometimes you want to go further than re-
duce allows, though.
Enter the collector , a general-purpose construct for producing complex values from streams.
These can be used with any Stream by passing them into the collect method.
The standard library provides a bunch of useful collectors out of the box, so let's look at
those first. In the code examples throughout this chapter the collectors are statically imported
from the class.
Into Other Collections
Some collectors just build up other collections. You've already seen the toList collector,
which produces java.util.List instances. There's also a toSet collector and a toCollec-
tion collector, which produce instances of Set and Collection . I've talked a lot so far
about chaining Stream operations, but there are still times when you'll want to produce a
Collection as a final value—for example:
▪ When passing your collection to existing code that is written to use collections
▪ When creating a final value at the end of a chain of collections
▪ When writing test case asserts that operate on a concrete collection
Normally when we create a collection, we specify the concrete type of the collection by call-
ing the appropriate constructor:
List < Artist > artists = new
new ArrayList <>();
But when you're calling toList or toSet , you don't get to specify the concrete implementa-
tion of the List or Set . Under the hood, the streams library is picking an appropriate imple-
mentation for you. Later in this topic I'll talk about how you can use the streams library to
perform data parallel operations; collecting the results of parallel operations can require a
different type of Set to be produced than if there were no requirement for thread safety.
It might be the case that you wish to collect your values into a Collection of a specific
type if you require that type later. For example, perhaps you want to use a TreeSet instead
of allowing the framework to determine what type of Set implementation you get. You can
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