Streams - Beginning Java 8 Language Features - page 635

Recall that the toCollection() method takes a Supplier as an argument that is used to collect the data. In this
case, you have used the constructor reference TreeSet::new as the Supplier . This has an effect of using a TreeSet ,
which is a sorted set, to collect the data.
You can also sort the list of names using the sorted operation. The sorted() method of the Stream interface
produces another stream containing the same elements on a sorted order. The following snippet of code shows how
to collect sorted names in a list:
List<String> sortedName = Person.persons()
.stream()
.map(Person::getName)
.sorted()
.collect(Collectors.toList());
System.out.println(sortedName);
[Chris, Donna, Jeff, Ken, Laynie, Li]
Note that the code applies the sorting before it collects the names. The collector notices that it is collecting an
ordered stream (sorted names) and preserves the ordering during the collection process.
You will find many static methods in the Collectors class that return a Collector meant to be used as a nested
collector. One of these methods is the counting() method that returns the number of input elements. Here is an
example of counting the number of people in the streams:
long count = Person.persons()
.stream()
.collect(Collectors.counting());
System.out.println("Person count: " + count);
Person count: 6
You may argue that you could have achieved the same result using the count() method of the Stream interface
as follows:
long count = Person.persons()
.stream()
.count();
System.out.println("Persons count: " + count);
Persons count: 6
When do you use the Collectors.counting() method instead of the Stream.count() method to count the number
of elements in a stream? As mentioned before, collectors can be nested. You will see examples of nested collectors
shortly. These methods in the Collectors class are meant to be used as nested collectors, not in this case just to count
the number of elements in the stream. Another difference between the two is their type: the Stream.count() method
represents an operation on a stream whereas the Collectors.counting() method returns a Collector . Listing 13-7
shows the complete program to collect sorted names in a list.