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Legacy lambda code
The document discusses the complexities and patterns associated with using lambda expressions in Java, particularly in the context of porting a legacy C# application to Java. It highlights common anti-patterns and best practices for utilizing Java 8 streams and lambdas effectively, including various methods for counting, validating, sorting, and grouping elements. The presentation also delves into performance considerations and lambda internals, showcasing the benefits of type inference and the runtime generation of lambda classes.
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Legacy lambda code
- 1. Javaland 2015 Peter Lawrey LegacyLambda Code in Java
- 2. Agenda • Lambdas: Acomplicated way to do something simple? • Lambda Patterns we used. • Lambda Patterns we shouldn’t have used. • Lambda Patterns we have used since.
- 3. Counting elements ina Stream long count = list.stream().count();
- 4. Counting elements ina Stream long count = list.stream().count(); // Stream.count() @Override public final long count() { return mapToLong(e -> 1L).sum(); }
- 5. Counting elements ina Stream long count = list.stream().count(); // LongStream.sum() @Override public final long sum() { // use better algorithm to compensate for intermediate overflow? return reduce(0, Long::sum); }
- 6. Counting elements ina Stream long count = list.parallelStream().count(); When the examples get more complex, lambdas become much more interesting.
- 7. Porting a legacyC# application For the last 8 months, Higher Frequency Trading ported a legacy C# application with over 25K lines of code to Java. We have translated many LINQ statements into Java 8 Stream + Lambda. What are some common patterns and anti-patterns we have seen?
- 8. Summing BigDecimal getResults().stream() .reduce(BigDecimal.ZERO, (bd, t)-> bd.add(t.getRequirement()), BigDecimal::add);
- 9. Validate all entries positions.stream().forEach(Position::validate); Validatethrows an exception.
- 10. Sorting by multiplefields. setTrades(trades.stream() .sorted(comparing(t -> t.getInfo().getDate()) .thenComparing(Position::getCUSIP) .thenComparing(Position::getQuantity) .reversed()) .collect(toList()));
- 11. Sorting by multiplefields, Quantity reversed setTrades(trades.stream() .sorted(comparing(t -> t.getInfo().getDate()) .thenComparing(Position::getCUSIP) .reversed() .thenComparing(Position::getQuantity) .reversed()) .collect(toList()));
- 12. Group By Map<String, List<Position>>positionBySymbol = positions.values().stream() .filter(p -> p.getQuantity() != 0) .collect(groupingBy(Position::getSymbol));
- 13. Streaming Maps pos.entrySet().stream() .filter(p ->p.getValue().getQuantity() != 0.0) .forEach(p -> pos2.put(p.getKey(), p.getValue()));
- 14. Contains 2.0 if (list.stream() .anyMatch(p-> p.getType() == Type.Cash)) {
- 15. Deep copy List<Position> newPositions= classPos.stream() .map(Position::clone) .collect(toList())
- 16. To collect ornot to collect (anti-pattern) getTrades().stream() .filter(t -> getDate().equals(t.getInfo().getDate())) .collect(toList()) .forEach(t -> trades.add(t.getInfo()));
- 17. To collect ornot to collect (solution) List<TradeInfo> trades = getTrades().stream() .filter(t -> getDate().equals( t.getInfo().getDate())) .map(Trade::getInfo) .collect(toList());
- 18. Sort of sorted(anti-pattern) Map<Date, List<Trade>> groupTrades = trades.stream() .sorted(comparing(Trade::getDate)) .collect(groupingBy(Trade::getDate));
- 19. Sorting (solution) Map<Date, List<Trade>>groupTrades = trades.stream() .collect(groupingBy( TradeDetail::getTradeDate, TreeMap::new, toList()));
- 20. Multi-sorted (anti-pattern) return trade.stream() .filter(t-> !isExcluded(t)) .sorted(comparing(Trade::getDate)) .sorted(comparing(Trade::getCUSIP)) .sorted(comparing(Trade::getNetAmount)) .collect(toList()); See slide 2 example for solution.
- 21. Concurrent removal ? anti-pattern? input.stream() .filter(t -> t.getParent() == p.getParent()) .forEach(input::remove);
- 22. Optional Denial Position todayPos= newPos.stream() .filter(pos -> pos.getCUSIP() .equals(p.getCUSIP())) .findFirst().orElse(null); if (todayPos != null) {
- 23. Optional Denial Optional<MTrade> otodayTrade= trades.stream() .filter(t -> t.getCUSIP().equals(p.getCUSIP())) .findFirst(); MTrade todayTrade = null; if (otodayTrade.isPresent()) todayTrade = otodayTrade.get(); if (todayTrade != null && todayTrade.getClosingPrice()!=null){
- 24. Optional for equals publicboolean equals(Object obj) { return Optional.ofNullable(obj) .filter(that -> that instanceof Test) .map(that -> (Test)that) .filter(that -> Objects.equals(this.s1, that.s1)) .filter(that -> Objects.equals(this.s2, that.s2)) .isPresent(); } Posted by Marko Topolnik
- 25. Find the twentymost frequent words in a file To use parallel or not? List<String> words = Files.lines(path).parallel() .flatMap(line -> Arrays.asList(line.split("b")).stream()) .collect(groupingBy(w -> w, counting())) .entrySet().stream() .sorted(comparing(Map.Entry<String,Long>::getValue).reversed()) .limit(20) .map(Map.Entry::getKey) .collect(Collectors.toList());
- 26. Lambdas and templates(before Java 8) long value; this.lock.lock(); try { value = doSomething(); } finally { this.lock.unlock(); }
- 27. Lambdas and templates(with Java 8) public static <R> R with(Lock lock, Callable<R> work) { lock.lock(); try { return work.call(); } finally { lock.unlock(); } }
- 28. Lambdas and templates(with Java 8) long value = with(lock, this::doSomething); Note: with inlining, the temporary “Long” object can be eliminated.
- 29. Lambdas and performance publicvoid readMarshallable(Wire wire) { wire.read(Fields.I).int32(x -> i = x) .read(Fields.J).int32(x -> j = x) .read(Fields.K).int32(x -> k = x) .read(Fields.L).int32(x -> l = x) .read(Fields.M).int32(x -> m = x) .read(Fields.N).int32(x -> n = x) .read(Fields.O).int32(x -> o = x) .read(Fields.P).int32(x -> p = x); }
- 30. Lambdas and performance garbagein bytes per lambda
- 31. Lambdas and performance -XX:BCEATraceLevel=3 Printsmessages like Skipping method because: code size (271) exceeds MaxBCEAEstimateSize (150). So I raised the -XX:MaxBCEAEstimateSize=300
- 32. Lambda type inference •Java 8 uses type inference much more than before. • Inference can appear to be as a cast. // error type is not known. Object o = () -> System.out::println; // type is inferred. Runnable o = () -> System.out::println; // type is inferred not cast at runtime. Object o = (Runnable & Serializable) () -> System.out::println;
- 33. Lambda internals • Classesfor Lambdas are generated at runtime. lambda.getClass() still works. • The contents of code in a lambda is added as a static method, except … • When a lambda expression refers to a method e.g. this::method or Class::new no additional method is created. • Provided your code is inlined, the lambda “object” can be eliminated. This also works for anonymous inner classes.
- 34. End randomly (Don’ttry this at home) IntStream.range(0, 128).parallel() .forEach(System::exit);
- 35. Q & A http://vanillajava.blogspot.com/ PeterLawrey @PeterLawrey