The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.
forEach
aggregate operation differs from the enhanced
for
statement
or iterators.
forEach
aggregate operation lets the system decide "how" the iteration takes place.
Using aggregate operations lets
you focus on "what" instead of "how."
double average = roster .stream() .filter(p -> p.getGender() == Person.Sex.MALE) .mapToInt(Person::getAge) .average() .getAsDouble();
filter
, mapToInt
average
average
returns an OptionalDouble
. The
getAsDouble
method is then invoked on that returned object. It is always a good
idea to consult the
API Specification for information about whether an operation is intermediate
or terminal.
p -> p.getGender() == Person.Sex.MALE
is an example of what?
Person::getAge
is an example of what?
Stream.reduce
method
and the Stream.collect
method.
Stream.reduce
always creates a new value when it processes an element.
Stream.collect
modifies (or mutates) the existing value.
List
, would Stream.reduce
or
Stream.collect
be the
most appropriate operation to use?
List
.
List<String> namesOfMaleMembersCollect = roster .stream() .filter(p -> p.getGender() == Person.Sex.MALE) .map(p -> p.getName()) .collect(Collectors.toList());
parallelStream()
instead of stream()
.
for
statement as a
pipeline with lambda expressions. Hint: Use the
filter
intermediate operation and the forEach
terminal
operation.
for (Person p : roster) { if (p.getGender() == Person.Sex.MALE) { System.out.println(p.getName()); } }
roster .stream() .filter(e -> e.getGender() == Person.Sex.MALE) .forEach(e -> System.out.println(e.getName());
for
loops. Hint: Make a pipeline that invokes the filter
, sorted
, and
collect
operations, in that order.
List<Album> favs = new ArrayList<>(); for (Album a : albums) { boolean hasFavorite = false; for (Track t : a.tracks) { if (t.rating >= 4) { hasFavorite = true; break; } } if (hasFavorite) favs.add(a); } Collections.sort(favs, new Comparator<Album>() { public int compare(Album a1, Album a2) { return a1.name.compareTo(a2.name); }});
List<Album> sortedFavs = albums.stream() .filter(a -> a.tracks.anyMatch(t -> (t.rating >= 4))) .sorted(Comparator.comparing(a -> a.name)) .collect(Collectors.toList());
anyMatch
), the sorting, and the collection of albums matching our criteria into a List
. The Comparator.comparing()
method takes a function that extracts a Comparable
sort key, and returns a Comparator
that compares on that key.