Java8 Stream API
A different way to process collections
David Gómez G.
@dgomezg
dgomezg@autentia.com
Streams?
What’s that?
A Stream is…
An convenience method to iterate over
collections in a declarative way
List<Integer>  numbers  =  new  ArrayL...
A Stream is…
An convenience method to iterate over
collections in a declarative way
List<Integer>  numbers  =  new  ArrayL...
So… Streams are collections?
Not Really
Collections Streams
Sequence of elements
Computed at construction
In-memory data s...
Iterating a Collection
List<Integer> evenNumbers = new ArrayList<>();

for (int i : numbers) {

if (i % 2 == 0) {

evenNum...
Iterating a Stream
List<Integer> evenNumbers = numbers.stream()

.filter(n -> n % 2 == 0)

.collect(toList());
Internal It...
Iterating a Stream
List<Integer> evenNumbers = numbers.parallelStream()

.filter(n -> n % 2 == 0)

.collect(toList());
Eas...
Lambdas
&
Method References
@FunctionalInterface
@FunctionalInterface

public interface Predicate<T> {


boolean test(T t);
!
!
!
!
!
}
An interface w...
@FunctionalInterface
@FunctionalInterface

public interface Predicate<T> {


boolean test(T t);
!
default Predicate<T> neg...
Lambda Types
Based on abstract method signature from
@FunctionalInterface:
(Arguments) -> <return type>
@FunctionalInterfa...
Lambda Types
Based on abstract method signature from
@FunctionalInterface:
(Arguments) -> <return type>
@FunctionalInterfa...
Lambda Types
Based on abstract method signature from
@FunctionalInterface:
(Arguments) -> <return type>
@FunctionalInterfa...
Lambda Types
Based on abstract method signature from
@FunctionalInterface:
(Arguments) -> <return type>
@FunctionalInterfa...
Lambda Types
Based on abstract method signature from
@FunctionalInterface:
(Arguments) -> <return type>
@FunctionalInterfa...
Method References
Allows to use a method name as a lambda
Usually better readability
!
Syntax:
<TargetReference>::<MethodN...
Method References
phoneCall -> phoneCall.getContact()
Method ReferenceLambda
PhoneCall::getContact
() -> Thread.currentThr...
From Collections
to
Streams
Characteristics of A Stream
• Interface to Sequence of elements
• Focused on processing (not on storage)
• Elements comput...
Anatomy of a Stream
Source
Intermediate
Operations
filter
map
order
function
Final
operation
pipeline
@dgomezg
Anatomy of Stream Iteration
1. Start from the DataSource (Usually a
collection) and create the Stream
List<Integer> number...
Anatomy of Stream Iteration
2. Add a chain of intermediate Operations
(Stream Pipeline)
Stream<Integer> numbersStream = nu...
Anatomy of Stream Iteration
2. Add a chain of intermediate Operations
(Stream Pipeline) - Better using lambdas
Stream<Inte...
Anatomy of Stream Iteration
3. Close with a Terminal Operation
List<Integer> numbersStream = numbers.stream()

.filter(num...
Stream operations
Operation Types
Intermediate operations
• Always return a Stream
• Chain as many as needed (Pipeline)
• Guide processing o...
Intermediate Operations
// T -> boolean
Stream<T> filter(Predicate<? super T> predicate);
!
//T -> R

<R> Stream<R> map(Fu...
Final Operations
Object[] toArray();
void forEach(Consumer<? super T> action); //T -> void

<R, A> R collect(Collector<? s...
Final Operations (II)
//T,U -> R
Optional<T> reduce(BinaryOperator<T> accumulator);
//(T,T) -> int

Optional<T> min(Compar...
Final Operations (y III)
//T -> boolean
boolean anyMatch(Predicate<? super T> predicate);

boolean allMatch(Predicate<? su...
Usage examples - Context
public class Contact {

private final String name;

private final String city;

private final Str...
Usage examples - Context
public class PhoneCall {

private final Contact contact;

private final LocalDate time;

private ...
People I phoned in June
phoneCallLog.stream()

.filter(phoneCall ->
phoneCall.getTime().getMonth() == Month.JUNE)

.map(ph...
Seconds I talked in May
Long total = phoneCallLog.stream()

.filter(phoneCall ->
phoneCall.getTime().getMonth() == Month.M...
Seconds I talked in May
Optional<Long> total = phoneCallLog.stream()

.filter(phoneCall ->
phoneCall.getTime().getMonth() ...
Did I phone to Paris?
boolean phonedToParis = phoneCallLog.stream()

.anyMatch(phoneCall ->
"Paris".equals(phoneCall.getCo...
Give me the 3 longest phone calls
phoneCallLog.stream()

.filter(phoneCall ->
phoneCall.getTime().getMonth() == Month.MAY)...
Give me the 3 shortest ones
phoneCallLog.stream()

.filter(phoneCall ->
phoneCall.getTime().getMonth() == Month.MAY)

.sor...
Creating Streams
Streams can be created from
Collections
Directly from values
Generators (infinite Streams)
Resources (like files)
Stream r...
From collections
use stream()
List<Integer> numbers = new ArrayList<>();

for (int i= 0; i < 10_000_000 ; i++) {

numbers....
Directly from Values & ranges
Stream.of("Using", "Stream", "API", "From", “Java8”);
can convert into parallelStream
Stream...
Generators - Functions
Stream<Integer> integers =
Stream.iterate(0, number -> number + 2);
This is an infinite Stream!,
wi...
Generators - Functions
Stream<Integer> integers =
Stream.iterate(0, number -> number + 2);
This is an infinite Stream!,
wi...
From Resources (Files)
Stream<String> fileContent =
Files.lines(Paths.get(“readme.txt”));
Files.lines(Paths.get(“readme.tx...
Parallelism
Parallel Streams
use stream()
List<Integer> numbers = new ArrayList<>();

for (int i= 0; i < 10_000_000 ; i++) {

numbers....
Let’s test it
use stream()
!
for (int i = 0; i < 100; i++) {

long start = System.currentTimeMillis();

List<Integer> even...
Let’s test it
use stream()
!
for (int i = 0; i < 100; i++) {

long start = System.currentTimeMillis();

List<Integer> even...
Enough, for now,
But this is just the beginning
Thank You.
@dgomezg
dgomezg@gmail.com
www.adictosaltrabajlo.com
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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.

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

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

  1. 1. Java8 Stream API A different way to process collections David Gómez G. @dgomezg dgomezg@autentia.com
  2. 2. Streams? What’s that?
  3. 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. 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. 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. 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. 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. 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. 9. Lambdas & Method References
  10. 10. @FunctionalInterface @FunctionalInterface
 public interface Predicate<T> { 
 boolean test(T t); ! ! ! ! ! } An interface with exactly one abstract method ! ! @dgomezg
  11. 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. 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. 13. Lambda Types Based on abstract method signature from @FunctionalInterface: (Arguments) -> <return type> @FunctionalInterface
 public interface Runnable { 
 void run(); } () -> void @dgomezg
  14. 14. Lambda Types Based on abstract method signature from @FunctionalInterface: (Arguments) -> <return type> @FunctionalInterface
 public interface Supplier<T> { 
 T get(); } () -> T @dgomezg
  15. 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. 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. 17. Method References Allows to use a method name as a lambda Usually better readability ! Syntax: <TargetReference>::<MethodName> ! TargetReference: Instance or Class @dgomezg
  18. 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. 19. From Collections to Streams
  20. 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. 21. Anatomy of a Stream Source Intermediate Operations filter map order function Final operation pipeline @dgomezg
  22. 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. 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. 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. 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. 26. Stream operations
  27. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 37. Did I phone to Paris? boolean phonedToParis = phoneCallLog.stream()
 .anyMatch(phoneCall -> "Paris".equals(phoneCall.getContact().getCity()))
 ! ! @dgomezg
  38. 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. 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. 40. Creating Streams
  41. 41. Streams can be created from Collections Directly from values Generators (infinite Streams) Resources (like files) Stream ranges @dgomezg
  42. 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. 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. 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. 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. 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. 47. Parallelism
  48. 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. 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. 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. 51. Enough, for now, But this is just the beginning Thank You. @dgomezg dgomezg@gmail.com www.adictosaltrabajlo.com
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