Devoxx 2014: Conference day 3 Notes (14/11/2014)

50 new things we can do with Java 8

José Paumard

Date

• Instant.now()
  • Duration.between(start, end).toMillis()
• LocalDate.now() — .of() —
  • Period: time between 2 dates
  • now.with(TemporalAdjuster.next(DayOfWeek.SUNDAY));
• LocalTime.now() — of(10,20) //10:20
• ZoneTime / ZoneId.of(“Europe/London”)
  • ZonedDateTime — Period
• Formatter (DateTimeFormatter.ISO_DATE_TIME etc.)
• Bridge with java.util.Date:
  • Date.from(instant)
  • TimeStamp.from(instant)
  • Date.from / toLocalDate()

Stream

• IntStream
• Patter.compile(…).splitAsStream(txt)

String concat

• compiler does stringbuilder conversion
• StringJoiner: concat with delimeters + pre-/postfix
  • String.join(…)

Numbers

• Long.max(), Long.sum()
• lambda: .reduce(…,Long::max)
• static Long.hashCode(l)

I/O reading text files

• Files.lines(Path)
  • path=Paths.get(…);
  • autoclosable with try(resource){} pattern
• Files.list(path)
• Files.walk(path): recursive find

Iterable

• forEach consumes (e.g; strings.forEacht(System.out::println))
• boolean b = list.removeIf(s-> s.length() > 4)
• list.replaceAll(String::toUpperCase)
• list.sort(Comparator.naturalOrder())

Comparator

• Comparator.naturalOrder()
• Comparator.comparingBy(Person::getLastName) .thenComparing(Person::getFirstName).thenComparing(..)
• comparator.reversedOrder() / comp.reversed()
• Comparator.nullsFirst() /nullsLast()

Optional

• Optional.empty();
• Optional.of(“one”)
• opt.isPresent() /.get() / .orElse(“”) / .orElseThrow()
• Stream integration ifPresent / filter / map / flatMap
  • flatMap().collect(…)

Map

• map.forEach( (key, value) -> …)
• better Compare-And-Swap functions for concurrency:
  • map.get(key) -> map.getOrDefault(key, default)
  • map.putIfAbsent(key, object)
  • map.replace(key, old, new) -> extra test on old value
  • map.remove(key, old) - extra test on old value
  • map.compute() / computeIfAbsent() / computeIfPresent()

Type Annotations

• @NonNull

Parallel Arrays

• Arrays.parallelPrefix
• parallelSort

Completable Future -> graphs of tasks (“reactive programming”)

• CompletableFuture.supplyAsync()
• thenApply() thenAccept()
• thenCompose()
• allof(): combine multiple futures -> join()
• thenCombine() / applyToEither: get the first result

Atomic Variables

• AtomicLong -> updateAndGet(lambda) / accumulateAndGet(lambda)
• multithreaded ;increment() / .sum() computes final result

StampedLock

• s.writeLock() s.readLock()
• tryOptimisticRead()

ConcurrentHashMap

• completely replaced, no locking
• size(): do not use! -> map.mappingCount()
• search(threshold, lambda) : threshold before going parallel
• reduce() / reduceKey() / reduceEntries()
• ConcurrentHashMap.newKeySet() // values are Boolean.TRUE

Books

• Java SE8 for the Really Impatient - Cay Horstman

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Introduction to Android Wear - A Glimpse Into the Future

Cyril Mottier

principles:

• contextual
• glanceable
• low interaction

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In Full Flow: Java 8 Lambdas in the Stream

Paul Sandoz

Stream Basics

• Stream:
  • abstract view over data
  • aggregate ops
• not RxJava, not CompletableFuture, not Distributed Stream
• e.g.;
  • list.stream().filter(s -> s.length() > 0).count()
  • mapToLong(e -> 1L)
  • reduce(0, Long::sum);
  • reduce(“”, (a, b) -> a + “ “ + b)
  • Optional
• performance:
  • parallel can go slower for small datasets (due to overhead)
  • reduce can generate a lot of extra classes -> .collect()

Parallel Game

• a stream is sequential by default
• can be made parallel()
• Greedy -> takes more work -> multicore
• implementation under the hood:
  • forkjoinpool -> split problem in parts
• when to use?
  • http://gee.cs.oswego.edu/dl/html/StreamParallelGuidance.html
  • minimum threshold (~100µs)
  • workload ‘components’
  • measure
  • JMH — http://openjdk.java.net/projects/code-tools/jmh/
  • switch off HyperThreading
  • visualize
    • -> DOT file -> SVG
    • adapt() / peek(): to get feedback
    • datamodels:
      • ArrayList / Range: best
      • HashSet: not completely balanced
      • LinkedList / iterator: poor choice (size unknown)
  • boxing:
    • gc works harder
    • pointer chasing/ cache issues
    • -> Int/Long Streams

The future

• near term:
  • better parallel resource control
  • improved splitting implementations
    • Files.lines()
    • unordered input to limit()
  • more operations:
    • takeWhile()
    • skipWhile()
    • Matcher.stream()
  • helper methods
    • Optional.stream()
    • Stream.ofNullable(T t)
  • longer term:
  • Value types
  • extending generics over values
  • IntStream extends Stream
  • Tuples