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Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
Java 8 Stream API. A different way to process collections.
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Java 8 Stream API. A different way to process collections.

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A look on one of the features of Java 8 hidden behind the lambdas. A different way to iterate Collections. You'll never see the Collecions the same way. …

A look on one of the features of Java 8 hidden behind the lambdas. A different way to iterate Collections. You'll never see the Collecions the same way.

These are the slides I used on my talk at the "Tech Thursday" by Oracle in June in Madrid.

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  • 1. Java8 Stream API A different way to process collections David Gómez G. @dgomezg dgomezg@autentia.com
  • 2. Streams? What’s that?
  • 3. A Stream is… An convenience method to iterate over collections in a declarative way List<Integer>  numbers  =  new  ArrayList<Integer>();
 for  (int  i=  0;  i  <  100  ;  i++)  {
   numbers.add(i);
 }   List<Integer> evenNumbers = new ArrayList<>();
 for (int i : numbers) {
 if (i % 2 == 0) {
 evenNumbers.add(i);
 }
 } @dgomezg
  • 4. A Stream is… An convenience method to iterate over collections in a declarative way List<Integer>  numbers  =  new  ArrayList<Integer>();
 for  (int  i=  0;  i  <  100  ;  i++)  {
   numbers.add(i);
 }   List<Integer> evenNumbers = numbers.stream()
 .filter(n -> n % 2 == 0)
 .collect(toList()); @dgomezg
  • 5. So… Streams are collections? Not Really Collections Streams Sequence of elements Computed at construction In-memory data structure Sequence of elements Computed at iteration Traversable only Once External Iteration Internal Iteration Finite size Infinite size @dgomezg
  • 6. Iterating a Collection List<Integer> evenNumbers = new ArrayList<>();
 for (int i : numbers) {
 if (i % 2 == 0) {
 evenNumbers.add(i);
 }
 } External Iteration - Use forEach or Iterator - Very verbose Parallelism by manually using Threads - Concurrency is hard to be done right! - Lots of contention and error-prone - Thread-safety@dgomezg
  • 7. Iterating a Stream List<Integer> evenNumbers = numbers.stream()
 .filter(n -> n % 2 == 0)
 .collect(toList()); Internal Iteration - No manual Iterators handling - Concise - Fluent API: chain sequence processing Elements computed only when needed @dgomezg
  • 8. Iterating a Stream List<Integer> evenNumbers = numbers.parallelStream()
 .filter(n -> n % 2 == 0)
 .collect(toList()); Easily Parallelism - Concurrency is hard to be done right! - Uses ForkJoin - Process steps should be - stateless - independent @dgomezg
  • 9. Lambdas & Method References
  • 10. @FunctionalInterface @FunctionalInterface
 public interface Predicate<T> { 
 boolean test(T t); ! ! ! ! ! } An interface with exactly one abstract method ! ! @dgomezg
  • 11. @FunctionalInterface @FunctionalInterface
 public interface Predicate<T> { 
 boolean test(T t); ! default Predicate<T> negate() {
 return (t) -> !test(t);
 } 
 ! } An interface with exactly one abstract method Could have default methods, though! ! @dgomezg
  • 12. Lambda Types Based on abstract method signature from @FunctionalInterface: (Arguments) -> <return type> @FunctionalInterface
 public interface Predicate<T> { 
 boolean test(T t); } T -> boolean @dgomezg
  • 13. Lambda Types Based on abstract method signature from @FunctionalInterface: (Arguments) -> <return type> @FunctionalInterface
 public interface Runnable { 
 void run(); } () -> void @dgomezg
  • 14. Lambda Types Based on abstract method signature from @FunctionalInterface: (Arguments) -> <return type> @FunctionalInterface
 public interface Supplier<T> { 
 T get(); } () -> T @dgomezg
  • 15. Lambda Types Based on abstract method signature from @FunctionalInterface: (Arguments) -> <return type> @FunctionalInterface
 public interface BiFunction<T, U, R> { 
 R apply(T t, U t); } (T, U) -> R @dgomezg
  • 16. Lambda Types Based on abstract method signature from @FunctionalInterface: (Arguments) -> <return type> @FunctionalInterface
 public interface Comparator<T> { 
 int compare(T o1, T o2); } (T, T) -> int @dgomezg
  • 17. Method References Allows to use a method name as a lambda Usually better readability ! Syntax: <TargetReference>::<MethodName> ! TargetReference: Instance or Class @dgomezg
  • 18. Method References phoneCall -> phoneCall.getContact() Method ReferenceLambda PhoneCall::getContact () -> Thread.currentThread() Thread::currentThread (str, c) -> str.indexOf(c) String::indexOf (String s) -> System.out.println(s) System.out::println @dgomezg
  • 19. From Collections to Streams
  • 20. Characteristics of A Stream • Interface to Sequence of elements • Focused on processing (not on storage) • Elements computed on demand (or extracted from source) • Can be traversed only once • Internal iteration • Parallel Support • Could be Infinite @dgomezg
  • 21. Anatomy of a Stream Source Intermediate Operations filter map order function Final operation pipeline @dgomezg
  • 22. Anatomy of Stream Iteration 1. Start from the DataSource (Usually a collection) and create the Stream List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10); 
 Stream<Integer> numbersStream = numbers.stream();
 @dgomezg
  • 23. Anatomy of Stream Iteration 2. Add a chain of intermediate Operations (Stream Pipeline) Stream<Integer> numbersStream = numbers.stream()
 .filter(new Predicate<Integer>() {
 @Override
 public boolean test(Integer number) {
 return number % 2 == 0;
 }
 }) ! .map(new Function<Integer, Integer>() {
 @Override
 public Integer apply(Integer number) {
 return number * 2;
 }
 }); @dgomezg
  • 24. Anatomy of Stream Iteration 2. Add a chain of intermediate Operations (Stream Pipeline) - Better using lambdas Stream<Integer> numbersStream = numbers.stream()
 .filter(number -> number % 2 == 0)
 .map(number -> number * 2); @dgomezg
  • 25. Anatomy of Stream Iteration 3. Close with a Terminal Operation List<Integer> numbersStream = numbers.stream()
 .filter(number -> number % 2 == 0)
 .map(number -> number * 2) .collect(Collectors.toList()); •The terminal operation triggers Stream Iteration •Before that, nothing is computed. •Depending on the terminal operation, the stream could be fully traversed or not. @dgomezg
  • 26. Stream operations
  • 27. Operation Types Intermediate operations • Always return a Stream • Chain as many as needed (Pipeline) • Guide processing of data • Does not start processing • Can be Stateless or Stateful Terminal operations • Can return an object, a collection, or void • Start the pipeline process • After its execution, the Stream can not be revisited
  • 28. Intermediate Operations // T -> boolean Stream<T> filter(Predicate<? super T> predicate); ! //T -> R
 <R> Stream<R> map(Function<? super T, ? extends R> mapper); 
 //(T,T) -> int
 Stream<T> sorted(Comparator<? super T> comparator); Stream<T> sorted(); ! //T -> void
 Stream<T> peek(Consumer<? super T> action); ! Stream<T> distinct();
 Stream<T> limit(long maxSize);
 Stream<T> skip(long n); @dgomezg
  • 29. Final Operations Object[] toArray(); void forEach(Consumer<? super T> action); //T -> void
 <R, A> R collect(Collector<? super T, A, R> collector);
 ! ! java.util.stream.Collectors.toList(); java.util.stream.Collectors.toSet(); java.util.stream.Collectors.toMap(); java.util.stream.Collectors.joining(CharSequence); ! ! ! @dgomezg
  • 30. Final Operations (II) //T,U -> R Optional<T> reduce(BinaryOperator<T> accumulator); //(T,T) -> int
 Optional<T> min(Comparator<? super T> comparator);
 //(T,T) -> int Optional<T> max(Comparator<? super T> comparator);
 long count();
 ! @dgomezg
  • 31. Final Operations (y III) //T -> boolean boolean anyMatch(Predicate<? super T> predicate);
 boolean allMatch(Predicate<? super T> predicate);
 boolean noneMatch(Predicate<? super T> predicate);
 ! @dgomezg
  • 32. Usage examples - Context public class Contact {
 private final String name;
 private final String city;
 private final String phoneNumber;
 private final LocalDate birth;
 
 
 public int getAge() {
 return Period.between(birth, LocalDate.now())
 .getYears();
 }
 //Constructor and getters omitted
 ! }
 @dgomezg
  • 33. Usage examples - Context public class PhoneCall {
 private final Contact contact;
 private final LocalDate time;
 private final Duration duration;
 ! //Constructor and getters omitted }
 Contact me = new Contact("dgomezg", "Madrid", "555 55 55 55", LocalDate.of(1975, Month.MARCH, 26));
 Contact martin = new Contact("Martin", "Santiago", "666 66 66 66", LocalDate.of(1978, Month.JANUARY, 17));
 Contact roberto = new Contact("Roberto", "Santiago", "111 11 11 11", LocalDate.of(1973, Month.MAY, 11));
 Contact heinz = new Contact("Heinz", "Chania", "444 44 44 44", LocalDate.of(1972, Month.APRIL, 29));
 Contact michael = new Contact("michael", "Munich", "222 22 22 22", LocalDate.of(1976, Month.DECEMBER, 8));
 
 List<PhoneCall> phoneCallLog = Arrays.asList(
 new PhoneCall(heinz, LocalDate.of(2014, Month.MAY, 28), Duration.ofSeconds(125)),
 new PhoneCall(martin, LocalDate.of(2014, Month.MAY, 30), Duration.ofMinutes(5)),
 new PhoneCall(roberto, LocalDate.of(2014, Month.MAY, 30), Duration.ofMinutes(12)),
 new PhoneCall(michael, LocalDate.of(2014, Month.MAY, 28), Duration.ofMinutes(3)),
 new PhoneCall(michael, LocalDate.of(2014, Month.MAY, 29), Duration.ofSeconds(90)),
 new PhoneCall(heinz, LocalDate.of(2014, Month.MAY, 30), Duration.ofSeconds(365)),
 new PhoneCall(heinz, LocalDate.of(2014, Month.JUNE, 1), Duration.ofMinutes(7)),
 new PhoneCall(martin, LocalDate.of(2014, Month.JUNE, 2), Duration.ofSeconds(315))
 ) ; @dgomezg
  • 34. People I phoned in June phoneCallLog.stream()
 .filter(phoneCall -> phoneCall.getTime().getMonth() == Month.JUNE)
 .map(phoneCall -> phoneCall.getContact().getName())
 .distinct()
 .forEach(System.out::println);
 ! @dgomezg
  • 35. Seconds I talked in May Long total = phoneCallLog.stream()
 .filter(phoneCall -> phoneCall.getTime().getMonth() == Month.MAY)
 .map(PhoneCall::getDuration)
 .collect(summingLong(Duration::getSeconds)); @dgomezg
  • 36. Seconds I talked in May Optional<Long> total = phoneCallLog.stream()
 .filter(phoneCall -> phoneCall.getTime().getMonth() == Month.MAY)
 .map(PhoneCall::getDuration)
 .reduce(Duration::plus); 
 total.ifPresent(duration -> {System.out.println(duration.getSeconds());} );
 ! @dgomezg
  • 37. Did I phone to Paris? boolean phonedToParis = phoneCallLog.stream()
 .anyMatch(phoneCall -> "Paris".equals(phoneCall.getContact().getCity()))
 ! ! @dgomezg
  • 38. Give me the 3 longest phone calls phoneCallLog.stream()
 .filter(phoneCall -> phoneCall.getTime().getMonth() == Month.MAY)
 .sorted(comparing(PhoneCall::getDuration))
 .limit(3)
 .forEach(System.out::println); @dgomezg
  • 39. Give me the 3 shortest ones phoneCallLog.stream()
 .filter(phoneCall -> phoneCall.getTime().getMonth() == Month.MAY)
 .sorted(comparing(PhoneCall::getDuration).reversed())
 .limit(3)
 .forEach(System.out::println); @dgomezg
  • 40. Creating Streams
  • 41. Streams can be created from Collections Directly from values Generators (infinite Streams) Resources (like files) Stream ranges @dgomezg
  • 42. From collections use stream() List<Integer> numbers = new ArrayList<>();
 for (int i= 0; i < 10_000_000 ; i++) {
 numbers.add((int)Math.round(Math.random()*100));
 } Stream<Integer> evenNumbers = numbers.stream(); or parallelStream() Stream<Integer> evenNumbers = numbers.parallelStream(); @dgomezg
  • 43. Directly from Values & ranges Stream.of("Using", "Stream", "API", "From", “Java8”); can convert into parallelStream Stream.of("Using", "Stream", "API", "From", “Java8”) .parallel();
 @dgomezg
  • 44. Generators - Functions Stream<Integer> integers = Stream.iterate(0, number -> number + 2); This is an infinite Stream!, will never be exhausted! Stream fibonacci = Stream.iterate(new int[]{0,1}, t -> new int[]{t[1],t[0]+t[1]}); 
 fibonacci.limit(10)
 .map(t -> t[0])
 .forEach(System.out::println); @dgomezg
  • 45. Generators - Functions Stream<Integer> integers = Stream.iterate(0, number -> number + 2); This is an infinite Stream!, will never be exhausted! Stream fibonacci = Stream.iterate(new int[]{0,1}, t -> new int[]{t[1],t[0]+t[1]}); 
 fibonacci.limit(10)
 .map(t -> t[0])
 .forEach(System.out::println); @dgomezg
  • 46. From Resources (Files) Stream<String> fileContent = Files.lines(Paths.get(“readme.txt”)); Files.lines(Paths.get(“readme.txt”))
 .flatMap(line -> Arrays.stream(line.split(" ")))
 .distinct()
 .count());
 ! Count all distinct words in a file @dgomezg
  • 47. Parallelism
  • 48. Parallel Streams use stream() List<Integer> numbers = new ArrayList<>();
 for (int i= 0; i < 10_000_000 ; i++) {
 numbers.add((int)Math.round(Math.random()*100));
 } //This will use just a single thread Stream<Integer> evenNumbers = numbers.stream(); or parallelStream() //Automatically select the optimum number of threads Stream<Integer> evenNumbers = numbers.parallelStream(); @dgomezg
  • 49. Let’s test it use stream() ! for (int i = 0; i < 100; i++) {
 long start = System.currentTimeMillis();
 List<Integer> even = numbers.stream()
 .filter(n -> n % 2 == 0)
 .sorted()
 .collect(toList()); 
 System.out.printf( "%d elements computed in %5d msecs with %d threadsn”,
 even.size(), System.currentTimeMillis() - start, Thread.activeCount());
 } 5001983 elements computed in 828 msecs with 2 threads 5001983 elements computed in 843 msecs with 2 threads 5001983 elements computed in 675 msecs with 2 threads 5001983 elements computed in 795 msecs with 2 threads @dgomezg
  • 50. Let’s test it use stream() ! for (int i = 0; i < 100; i++) {
 long start = System.currentTimeMillis();
 List<Integer> even = numbers.parallelStream()
 .filter(n -> n % 2 == 0)
 .sorted()
 .collect(toList()); 
 System.out.printf( "%d elements computed in %5d msecs with %d threadsn”,
 even.size(), System.currentTimeMillis() - start, Thread.activeCount());
 } 4999299 elements computed in 225 msecs with 9 threads 4999299 elements computed in 230 msecs with 9 threads 4999299 elements computed in 250 msecs with 9 threads @dgomezg
  • 51. Enough, for now, But this is just the beginning Thank You. @dgomezg dgomezg@gmail.com www.adictosaltrabajlo.com

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