T h e Da r k S i d e of Jav a 8 - doag.org · T h e Da r k S i d e of Jav a 8 G r zego r z P i wow a rek @pivovarit. 23/03/2019 The Dark Side of Java 8 https:

23/03/2019 The Dark Side of Java 8

https://pivovarit.github.io/talks/java-8-dark-side/?print-pdf#/ 1/90

The Dark Side of Java 8The Dark Side of Java 8Grzegorz PiwowarekGrzegorz Piwowarek

@pivovarit@pivovarit

http://pivovarit.github.io/

http://twitter.com/pivovarit



{ { } }Lead Backend Engineer @ Lead Backend Engineer @

Trainer @ Trainer @ , ,

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Java 8Java 8



https://twitter.com/mariofusco/status/957540001146986496

https://twitter.com/mariofusco/status/957540001146986496



Feature-driven -> Cadence-drivenFeature-driven -> Cadence-driven



Feature-driven:Feature-driven:Java 7 - Strings in Switch StatementsJava 8 - Project LambdaJava 9 - Project JigSaw...



Cadence-driven:Cadence-driven:Java 10 - 3.18Java 11 - 9.18Java 12 - 3.19...



Java 12 - 3.19 �Java 12 - 3.19 �



LTS Releases:LTS Releases:Java 7Java 7Java 8Java 8

Java 9Java 9Java 10Java 10

Java 11Java 11Java 12Java 12

......

Java 17Java 17





#1 Conditional Stream#1 Conditional StreamTerminationTermination



List<LocalDate> getDates()

[2012-02-02, 2012-03-02, 2012-04-02, (...)]



Task: take all dates before XXXTask: take all dates before XXX



Works properly but evaluates the whole streamDoesn't work with in�nite streams

getDates().stream() .filter(date -> date.isBefore(LocalDate.of(2012, 4, 4))) .map(...) .forEach(System.out::println);



2012-02-02, 2012-03-02, 2012-04-02,

-- POTENTIAL CUT OFF POINT --

2012-05-02, 2012-06-02, 2012-07-02, 2012-08-02, 2012-09-02, 2012-10-02, 2012-11-02, 2012-12-02, 2013-01-02, 2013-02-02, 2013-03-02, 2013-04-02, 2013-05-02, 2013-06-02, 2013-07-02, 2013-08-02, 2013-09-02, 2013-10-02, 2013-11-02, 2013-12-02,

...

getDates().stream() .peek(System.out::println) .filter(date -> date.isBefore(LocalDate.of(2012, 4, 4))) .map(...) .forEach(...);



Solution: migrate to JDK9+Solution: migrate to JDK9+Stream#takeWhile/dropWhileStream#takeWhile/dropWhile

[JDK8] don't use Stream API if you want to be able to break out of it[JDK8] use limit() to cap the number of fetched elements (if applicable)



getDates().stream() .filter(date -> date.isBefore(LocalDate.of(2014, 4, 4))) .map(...) .forEach(System.out::println);



The stream terminates as soon the �rst mismatch is encountered

getDates().stream() .takeWhile(date -> date.isBefore(LocalDate.of(2014, 4, 4))) .map(...) .forEach(System.out::println);



#2 Stream#�atMap()#2 Stream#�atMap()



Streams ~ Lazy SequencesStreams ~ Lazy SequencesAs performant as imperative equivalents (time complexity)As performant as imperative equivalents (time complexity)

In theory.In theory.



return getUsers().stream() .flatMap(u -> u.getAddresses().stream()) .filter(address -> address.contains("1")) .findAny()



List<String> addresses = new ArrayList<>(); for (User u : getUsers()) { for (String address : u.getAddresses()) { if (address.contains("1")) { return address; // short-circuts the ongoing iteration } } }



Not so true in practiceNot so true in practice



Task: take a �rst encountered address of a �rstTask: take a �rst encountered address of a �rstencountered userencountered user

static List<User> getUsers() { // db fetch stub return List.of(new User(List.of("a1","a2","a3","a4","a5"))); }



getUsers().stream() .flatMap(u -> u.getAddresses().stream()) .findAny();



getUsers().stream() .flatMap(u -> u.getAddresses().stream()) .peek(System.out::println) .findAny();



a1 a2 a3 a4 a5

https://bugs.openjdk.java.net/browse/JDK-8075939https://bugs.openjdk.java.net/browse/JDK-8075939https://bugs.openjdk.java.net/browse/JDK-8189234https://bugs.openjdk.java.net/browse/JDK-8189234

https://bugs.openjdk.java.net/browse/JDK-8075939

https://bugs.openjdk.java.net/browse/JDK-8189234



getUsers().stream() .flatMap(u -> getAllAddresses(u)) // large/infinite stream .peek(System.out::println) .findAny();



It gets worse once we start adding more operations:It gets worse once we start adding more operations:getUsers().stream() .flatMap(u -> u.getAddresses().stream()) .map(a -> { System.out.println("making a super expensive call now"); return 42; }) .findAny();

making a super expensive call now making a super expensive call now making a super expensive call now making a super expensive call now making a super expensive call now



The causeThe cause





Solution: implement your own Spliterator-based lazySolution: implement your own Spliterator-based lazy�atMap()�atMap()



https://stackover�ow.com/a/32767282/2229438https://stackover�ow.com/a/32767282/2229438

https://stackoverflow.com/a/32767282/2229438



Practical Solution: migrate to JDK10+Practical Solution: migrate to JDK10+JDK-8 Solution: don't use Stream API if you need nested collections to be evaluated lazilyJDK-9 Solution: see above

http://hg.openjdk.java.net/jdk/jdk10/rev/fca88bbbafb9http://hg.openjdk.java.net/jdk/jdk10/rev/fca88bbbafb9

http://hg.openjdk.java.net/jdk/jdk10/rev/fca88bbbafb9



#3 JDK9 Stream#takeWhile#3 JDK9 Stream#takeWhile



List<List<String>> list = List.of( List.of("1", "2"), List.of("3", "4", "5", "6", "7"));



list.stream() .flatMap(Collection::stream) .forEach(System.out::println);

1 2 3 4 5 6 7



Stream.of("1", "2", "3", "4", "5", "6", "7") .takeWhile(i -> !i.equals("4")) .forEach(System.out::println);

1 2 3



List<List<String>> list = List.of( List.of("1", "2"), List.of("3", "4", "5", "6", "7")); list.stream() .flatMap(Collection::stream) .takeWhile(i -> !i.equals("4")) .forEach(System.out::println);

1 2 3 5 6 7





Solution: migrate to JDK10+Solution: migrate to JDK10+



#4 Lambda Expressions vs.#4 Lambda Expressions vs.Checked ExceptionsChecked Exceptions



java.util.function.Functionjava.util.function.Function@FunctionalInterface public interface Function<T, R> { /** * Applies this function to the given argument. * * @param t the function argument * @return the function result */ R apply(T t); }







Solution: use helper wrappersSolution: use helper wrappersgetUsers().stream() .map(unchecked(u -> new URL(u.webpage))) .collect(Collectors.toList());

https://github.com/pivovarit/throwing-functionhttps://github.com/pivovarit/throwing-function

https://github.com/pivovarit/throwing-function





A bound of the form “throws α” is purely informational: it directsresolution to optimize the instantiation of “α” so that, if

possible, it is not a checked exception type. (…)

Otherwise, if the bound set contains “throws αi”, and the properupper bounds of “αi” are, at most, Exception, Throwable, and

Object, then Ti = RuntimeException.https://docs.oracle.com/javase/specs/jls/se8/html/jls-18.htmlhttps://docs.oracle.com/javase/specs/jls/se8/html/jls-18.html

https://docs.oracle.com/javase/specs/jls/se8/html/jls-18.html



Simply put:Simply put:Every T in “<T extends Throwable>” is generously inferred to be

a RuntimeException if a more speci�c type can't be inferred.http://mail.openjdk.java.net/pipermail/lambda-dev/2013-February/007981.htmlhttp://mail.openjdk.java.net/pipermail/lambda-dev/2013-February/007981.html

http://mail.openjdk.java.net/pipermail/lambda-dev/2013-February/007981.html



static void rethrow(Exception t) throws Exception { throw t; }



static <T extends Exception> void rethrow(Exception t) throws T { throw t; // Unhandled exception: java.lang.Exception }



static <T extends Exception> void rethrow(Exception t) throws T { throw (T) t; // Just cast it, #YOLO }



public static void main(String[] args) { rethrow(new Exception(":>")); // no try-catch needed :) }



Let's put that into practiceLet's put that into practice



public interface ThrowingFunction<T, R> { R apply(T t) throws Exception; }

static <T, R> Function<T, R> sneaky(ThrowingFunction<T, R> f)



static <T, R> Function<T, R> sneaky(ThrowingFunction<T, R> f) { return t -> { try { return f.apply(t); } catch (Exception ex) { return ThrowingFunction.sneakyThrow(ex); } }; }



getUsers().stream() .map(sneaky(u -> new URL(u.webpage))) .map(sneaky(url -> url.openConnection()) .map(sneaky(c -> c.getInputStream())));



http://4comprehension.com/sneakily-throwing-exceptions-in-lambda-expressions-in-java/http://4comprehension.com/sneakily-throwing-exceptions-in-lambda-expressions-in-java/

http://4comprehension.com/sneakily-throwing-exceptions-in-lambda-expressions-in-java/



Just because you don’t like the rules, doesn’t mean its a goodidea to take the law into your own hands. Your advice is

irresponsible because it places the convenience of the codewriter over the far more important considerations of

transparency and maintainability of the program. by Brian Goetz

https://stackover�ow.com/questions/19757300/java-8-lambda-streams-�lter-by-method-with-exception#comment54437053_19757456

https://stackoverflow.com/questions/19757300/java-8-lambda-streams-filter-by-method-with-exception#comment54437053_19757456



#5 Parallel Streams#5 Parallel Streams



Parallel processing made easy™Parallel processing made easy™



Parallel processing made easy (?)™Parallel processing made easy (?)™



SequentialSequentialgetDates().stream() .map(...) .forEach(System.out::println);



ParallelParallelgetDates().parallelStream() .map(...) .forEach(System.out::println);



Ctrl + RCtrl + R

Not a very good idea.Not a very good idea.



Multiple questions arise:Multiple questions arise:...but where are those tasks run?...but where are those tasks run?

...in the cloud?...in the cloud?...in the blockchain?...in the blockchain?

...how to provide a custom thread pool?...how to provide a custom thread pool?...what's the maximum number of tasks executed in parallel?...what's the maximum number of tasks executed in parallel?

......



Code inspection reveals:Code inspection reveals:

Implementation detailA thread pool that is used for execution of parallel streams is unspeci�ed

static final ForkJoinPool common;



public final ForkJoinTask<V> fork()

Arranges to asynchronously execute this task in the pool thecurrent task is running in (...)



The size of the computation will be in proportion to the common pool and notThe size of the computation will be in proportion to the common pool and notthe custom pool.the custom pool.

Fixed in JDK10 - https://bugs.openjdk.java.net/browse/JDK-8190974Fixed in JDK10 - https://bugs.openjdk.java.net/browse/JDK-8190974

ForkJoinPool customPool = new ForkJoinPool(42); customPool.submit( () -> getDates().parallelStream().forEach(e -> {}));



ForkJoinPool Settings:ForkJoinPool Settings:The parallelism levelForkJoinWorkerThreadFactoryUncaughtExceptionHandlerasyncMode

Since JDK 9:Since JDK 9:corePoolSizemaximumPoolSizeminimumRunnablesaturation allowedkeepAliveTime



Note, however, that this technique of submitting a task to afork-join pool to run the parallel stream in that pool is an

implementation “trick” and is not guaranteed to work. by Stuart Marks

https://stackover�ow.com/questions/28985704/parallel-stream-from-a-hashset-doesnt-run-in-parallel/29272776#29272776

https://stackoverflow.com/questions/28985704/parallel-stream-from-a-hashset-doesnt-run-in-parallel/29272776#29272776





Which is faster? Sequential or Parallel?Which is faster? Sequential or Parallel?SequentialSequential

Random rand = new Random(); int[] array = new int[90000000]; Arrays.stream(array) .map(i -> rand.nextInt()) .boxed() .collect(Collectors.toList());



Which is faster? Sequential or Parallel?Which is faster? Sequential or Parallel?ParallelParallel

Random rand = new Random(); int[] array = new int[90000000]; Arrays.stream(array).parallel() .map(i -> rand.nextInt()) .boxed() .collect(Collectors.toList());



Parallel - doesn't always mean fasterParallel - doesn't always mean faster



#6 Stream#generate#6 Stream#generate



/** * Returns an infinite sequential unordered stream where each element is * generated by the provided {@code Supplier}. This is suitable for * generating constant streams, streams of random elements, etc. * * @param <T> the type of stream elements * @param s the {@code Supplier} of generated elements * @return a new infinite sequential unordered {@code Stream} */ public static<T> Stream<T> generate(Supplier<T> s)



Stream.generate(() -> 42) .forEach(System.out::println); // 42 // 42 // 42 // ...



Stream.generate(rand::nextInt) .forEach(System.out::println); // ${some.random.value1} // ${some.random.value2} // ${some.random.value3} // ...



Stream.generate(new Supplier<Integer>() { private int value; @Override public Integer get() { return value++; }}) .limit(8) .forEach(System.out::println);

1 2 3 4 5 6 7 8



Stream.generate(new Supplier<Integer>() { private int value; @Override public Integer get() { return value++; }}).parallel() .limit(8) .forEach(System.out::println);

0 2 4 7 6 8 1 3



Stream.generate(new Supplier<Integer>() { private AtomicInteger value = new AtomicInteger(); @Override public Integer get() { return value.getAndIncrement(); }}).parallel() .limit(20) .forEach(System.out::println);

0 1 2 3 4 5 7 6 ...



List<Integer> result = Stream.generate(new Supplier<Integer>() { private final AtomicInteger value = new AtomicInteger(); @Override public Integer get() { return value.getAndIncrement(); }}) .parallel() .peek(System.out::println) .limit(4) .collect(Collectors.toList());

i " "// 0 // 6 // 7 // 5 // 3 // 4 // 2 // 1 // 8 // 9 results: [5, 6, 0, 7]



/** * Returns an in�nite sequential UNORDERED stream whereeach element is * generated by the provided Supplier. */



#7 Optional vs Option#7 Optional vs Option



Nulls are interpreted by Optional as Optional.empty()

User user = ... Optional.ofNullable(user) .map(o -> o.getAddress()) // getAddress() returns null .ifPresent(System.out::println); //

scala> Some(null)



Two different results!

users.stream() .map(o -> o.getAddress()) .map(this::handleAddress) // handles null properly .findAny() ... users.stream() .findAny() .map(o -> o.getAddress()) .map(this::handleAddress) // null never makes it here ...



Key takeaway:Key takeaway:Java sucks, let's PHPJava sucks, let's PHP

It's worth paying attention and migrating to new minor JDK releases. SmallIt's worth paying attention and migrating to new minor JDK releases. Smallthings matter.things matter.

Also, read the docs.Also, read the docs.


https://pivovarit.github.io/talks/java-8-dark-side/?print-pdf#/ 90/90Twitter/GitHub: @pivovarit

Thank You!Thank You!

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https://pivovarit.github.io/talks/java-8-dark-sidehttps://pivovarit.github.io/talks/java-8-dark-side

@[email protected]


https://pivovarit.github.io/talks/java-8-dark-side


http://4comprehension.com/

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T h e Da r k S i d e of Jav a 8 - doag.org · T h e Da r k S i d e of Jav a 8 G r zego r z P i wow a rek @pivovarit. 23/03/2019 The Dark Side of Java 8 https: