Introduction to Java 8 Stream API with examples.

1. Stream API
Valdas Žigas · kaunas.jug@gmail.com · www.kaunas-jug.lt
{ λ }
Java 8

2. Valdas Žigas
● 13.6 x JAVA
● KTU Programų inžinerija
● SCJP 1.5, PL/SQL OCA
● Oracle University Delivery Instructor
● LKSoft, BPI, Infor, Affecto. PSE, uTrack

3. Presentation Source Code
https://github.com/valdasz/kaunasjug3streamapi.git

4. Java 8 { λ }. Impression
List<Long> idList = new ArrayList<>();
...
idList.stream().distinct().map(EmployeeStreamMain::
findById).filter(e -> e != null).filter(e -> e.getSalary() >
40000).findFirst().orElse(null);

5. Java 8 { λ }. Impression
12 Years Without Lambdas ...

6. java.util.stream.Stream<T>
A sequence of elements supporting sequential
and parallel bulk operations
public interface Stream<T> extends BaseStream<T,
Stream<T>>
public interface BaseStream<T, S extends BaseStream<T,
S>> extends AutoCloseable

7. java.util.stream.BaseStream<T>

8. Simple example. forEach
List<String> pronouns = Arrays.asList("i", "you", "he", "she", "it", "we", "you", "they");
pronouns.stream().forEach(p ->
System.out.println(p));
StreamFromListSimpleForEach.java

9. Simple example. forEach. Old school
List<String> pronouns = Arrays.asList("i", "you", "he", "she", "it", "we", "you",
"they");
pronouns.stream().forEach(new Consumer<String>() {
public void accept(String p) {
System.out.println(p);
}
});
StreamFromListSimpleForEachOldStyle.java

10. Simple example. filter
List<String> pronouns = Arrays.asList("i", "you", "he", "she", "it", "we", "you",
"they");
pronouns.stream().distinct().filter(p -> p.length() == 3).forEach(p ->
System.out.println(p));
StreamFromListSimpleFilter.java

11. Simple example. filter. Old school
pronouns.stream().distinct().filter(new Predicate<String>() {
public boolean test(String p) {
return p.length() == 3;
}
}).forEach(new Consumer<String>() {
public void accept(String p) {
System.out.println(p);
}}); StreamFromListSimpleFilterOldStyle.java

12. ● Collection.stream(),
Collection.parallelStream()
● Arrays.stream(T[])
● Stream.of(T...)
● IntStream.range(int, int)
● Stream.iterate(T,
UnaryOperator<T>)
Creating streams

13. Creating streams. Arrays.stream
Arrays.stream(new String[] { "This", "is", "Java8", "Stream" })
.forEach(System.out::println);
Arrays.stream(new Integer[] { 1, 2, 3 }).forEach(System.out::println);
Arrays.stream(new int[] { 1, 2, 3 }).forEach(System.out::println);
StreamFromArraysStream.java

14. Creating streams. Stream.of
Stream.of("This", "is", "Java8", "Stream").forEach(System.out::println);
Stream.of(new Integer[] { 1, 2, 3 }).forEach(System.out::println);
Stream.of(new int[] { 1, 2, 3 }).forEach(System.out::println);
StreamFromStreamOf.java

15. Creating streams. IntStream
IntStream.of(new int[] { 1, 2, 3 }).forEach(System.out::println);
IntStream.range(1, 3).forEach(System.out::println);
IntStream.rangeClosed(1, 3).forEach(System.out::println);
IntStreamFromIntStream.java

16. Creating streams. Infinite. Iterate
static <T> Stream<T> iterate(T seed, UnaryOperator<T> f)
IntStream.iterate(1, p -> p + 2).limit(10).forEach(System.out::
println);
StreamFromStreamInfiniteIterate.java

17. Creating streams. Infinite. Generate
static <T> Stream<T> generate(Supplier<T> s)
IntStream.generate(() -> (int) (System.nanoTime() % 100)).
limit(10)
.forEach(System.out::println);
StreamFromStreamInfiniteGenerate.java

18. Creating streams
● BufferedReader.lines
● Random.ints()
● File.list
● Pattern.splitAsStream
● JarFile.stream()
● ………..
StreamFromBufferedReader.java
StreamFromPatternSplitAsStream.java

19. Stream features
● No Storage
● Functional in nature
● Laziness-seeking
● Possibly unbounded
● Consumable
StreamFunctional.java
StreamLaziness.java
StreamConsumable.java

20. Stream Operators
● Intermediate (filter, map, limit, sorted ..)
● Terminal (forEach, reduce, findFirst, sum,
collect..)
● Short-circuiting (intermediate/terminal: limit ..
/findFirst..)

21. Stream Operators. Intermediate
map (mapToInt, flatMap, ..), filter, distinct, sorted, peek, limit, substream, parallel,
sequential, unordered ..
● Return new Stream
● Always lazy
● Stateless/stateful
filter,map/distinct,sorted
● Some short-circuiting (limit)

22. Stream Operators. Terminal
forEach, forEachOrdered, toArray, reduce, collect,
min, max, count, findFirst...
● Consumes pipeline/terminates
● Eager (except iterator,
spliterator)
● some short-circuiting (findFirst,
findAny)

23. Parallel Streams
● Collection.parallelStream()
BaseStream.parallel()
● same results as stream() apart
nondeterministic ops (findAny)
ParallelStreamFromListSimpleForEach.java
ParallelStreamLaziness.java

24. Parallel Streams. Non-interference
seq: terminal op starts -> do not update
source -> terminal op ends.
List<String> l = new ArrayList<String>(Arrays.asList("kaunas-jug", "meeting"));
Stream<String> sl = l.parallelStream();
l.add("#3");
String s = sl.collect(Collectors.joining(" "));
System.out.println(s);
NonInterference.java

25. Parallel Streams. Stateless behaviours
Best approach: avoid stateful behaviour
Set<Integer> seen = Collections.synchronizedSet(new HashSet<>());
List<Integer> numbers = Arrays.asList(1, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3);
List<Integer> result = numbers.parallelStream().map(e -> {return seen.add(e) ? 0 : e;
}).collect(Collectors.toList());
StatefulBehaviour.java

26. Parallel Streams. reduce. Associativity
(a op b) op c == a op (b op c)
a op b op c op d == (a op b) op (c op d)
IntStream stream = IntStream.rangeClosed(1, 4).
parallel();
OptionalInt rez = stream.reduce((x, y) -> x - y)
ReduceAssociativity.java

27. System.exit(0)
Thank You !