Java 8: Language enhancements

Skelia

Agenda

Lambda Method References Default methods Static methods Streams (bulk collection operations)

Lambda - ?

Expression describing the

anonymous function, the result of the

execution of which is the object of

unknown origin, which implements the

required functional interface

Lambda. Examples

Java < 8Comparator<Integer> cmp = new Comparator<Integer>() { @Override public int compare(Integer x, Integer y) { return (x < y) ? -1 : (x > y) ? 1 : 0; }};

Java 8Comparator<Integer> cmp = (x, y) -> (x < y) ? -1 : (x > y) ? 1 : 0;

Lambda. More examples

//interface Function<T, R> - R apply(T t);Function<String, Integer> f0 = (String s) -> Integer.parseInt(s);

//interface Supplier <T> - T get();Supplier<Integer> answerFactory = () -> 42;

// interface Supplier<T> - T get();Supplier<Integer> deepThought = () -> { long millis = TimeUnit.DAYS.toMillis(2737500000L); Thread.sleep(toThinkInMillis); return 42;};

Method References

Easy-to-read lambda expressions for methods that already have a name

Method references. Precondition

public class Member { public static int compareByAge(Member a, Member b) { return a.getBDay().compareTo(b.getBDay()); }}

Member[] rosterAsArray = /* init array of Members */;

Method references. Java < 8

class MemberAgeComparator implements Comparator<Member> { public int compare(Member a, Member b) { return Member.compareByAge(a, b); }}

Arrays.sort(rosterAsArray, new MemberAgeComparator());

Method references. Java 8 with Lambda

Arrays.sort(rosterAsArray, (a, b) -> Member.compareByAge(a, b));

Method reference

Arrays.sort(rosterAsArray, Member::compareByAge);

Method References. Types

Type Example

Reference to a static method ContainingClass::staticMethodNameReference to an instance method of a particular object

ContainingObject::instanceMethodNamememberInstance::compareByName;(memberInstance.compareByName(a, b))

Reference to an instance method of an arbitrary object of a particular type

ContainingType::methodNameString::compareToIgnoreCase(a.compareToIgnoreCase(b))

Reference to a constructor ClassName::newHashSet::new(() -> { return new HashSet<>(); })

Default methods. The problem

interface Collection<T> {/* @since 1.8 */void removeAll(Predicate<T> p);

}

Default methods. Solution

interface Collection<T> {/* @since 1.8 */default void removeAll(Predicate<T> p) {

// fallback implementation goes here}

}

Default methods. Behavior

Static methods

public interface Ticket { String qDublin (); static Ticket random () { return () -> " toDublin "; } }

assertEquals ("toDublin", Ticket.random().qDublin ());

Streams. Problempublic void printGroups(List<People> people) { Set<Group> groups = new HashSet<>(); for (Person p : people) { if (p.getAge() >= 65) groups.add(p.getGroup()); }

List<Group> sorted = new ArrayList <>(groups); Collections.sort(sorted, new Comparator <Group>() { public int compare (Group a, Group b) { return Integer.compare(a.getSize(), b.getSize()); } });

for (Group g : sorted) System.out.println(g.getName());}

Streams. Solution

public void printGroups (List<People> people) { people.stream() .filter(p -> p.getAge() > 65) .map(p -> p.getGroup()) .distinct() .sorted(comparing(g -> g.getSize()) .map(g -> g.getName()) .forEach(n -> System.out.println(n));}

Stream → sequence of elements.

Streams. Design

source op op … sink→ → → →

sources: collections, iterators, channels, ... operations: filter, map, reduce, ... sinks: collectors, forEach, iterator...

Streams. Parallelism

int v = list.parallelStream() .reduce(Math::max) .get ();

Implicit invocation of parallelStream() instead of stream()

Implementation is hidden Uses ForkJoinPool

Java 8 is coming...

Scheduled to release in March 2014

Questions?

Thank you!