Uploaded byHenri Tremblay
DevoxxFR 2013: Lambda are coming. Meanwhile, are you sure we've mastered the generics?
The document discusses the implementation and understanding of lambda expressions and generics in programming, specifically using Java. It covers topics such as covariance, type safety warnings, and methods to handle generic types effectively. The content also includes practical examples and links for further learning about lambda expressions and generics.
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DevoxxFR 2013: Lambda are coming. Meanwhile, are you sure we've mastered the generics?
- 1. Les lambda arrivent! Enattendant, êtes-vous sûr d'avoir compris les génériques? Henri Tremblay Architecte Senior OCTO Technology @henri_tremblay 27 au 29 mars 2013
- 2. Henri Tremblay OCTOTechnology Open Source Responsable R&D EasyMock Performance Objenesis Calcul parallèle
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- 7. What do Ineed to know? Why © OCTO 2011 7
- 8. What do Ineed to know? Why Covariance © OCTO 2011 8
- 9. What do Ineed to know? Why Covariance Capture © OCTO 2011 9
- 10. What do Ineed to know? Why Covariance Capture Erasure © OCTO 2011 10
- 11. What do Ineed to know? Why Covariance Capture Erasure Bridge © OCTO 2011 11
- 12. What do Ineed to know? Why Covariance Capture Erasure Bridge © OCTO 2011 12
- 13. Lambda return Tweet.TWEETS.stream() .collect(Collectors .groupingBy(t->t.containsHashTag("#lambda"), HashMap::new, ArrayList::new));
- 14. Lambda = Funwith generics Stream<Tweet> stream = Tweet.TWEETS.stream(); Function<Tweet, Boolean> lambda = t -> t.containsHashTag("#lambda"); Collector<Tweet, Map<Boolean, List<Tweet>>> collector = Collectors.<Tweet, Boolean, List<Tweet>, Map<Boolean, List<Tweet>>> groupingBy(lambda, HashMap::new, ArrayList::new); return stream.collect(collector);
- 15. Lambda = Inférence return Tweet.TWEETS.stream() .collect( Collectors .groupingBy(t->t.containsHashTag("#lambda"), HashMap::new, ArrayList::new));
- 16. Dreaded warnings Type safety: The expression of type List needs unchecked conversion to conform to List<String> Type safety: Unchecked cast from List<capture#1-of ?> to List<String> © OCTO 2011 16
- 17. Ostrich defense @SuppressWarnings("unchecked") © OCTO 2011 17
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- 19. Rule #1 A code compiling without warning should never ever cause a ClassCastException © OCTO 2011 19
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- 21. Capture usually is Type List<?> bar(); <T> IExpectationSetters<T> expect(T value); void andReturn(T value); // Method of IExpectationSetters Returns a capture#6 expect(bar()).andReturn(new ArrayList<String>()); And you get The method andReturn(List<capture#6-of ?>) in the type IExpectationSetters<List<capture#6-of ?>> is not applicable for the arguments (ArrayList<String>) © OCTO 2011 21
- 22. Only solution We need to cast expect((List<String>) bar()).andReturn(new ArrayList<String>()); Tell to expect we want a List<String> But still a warning Type safety: Unchecked cast from List<capture#6-of ?> to List<String> Framework coder tip: Try to never return a wildcard unless necessary © OCTO 2011 22
- 23. How the compilertells the type Determine The parameter the return value type <T> T anyObject(T clazz) © OCTO 2011 23
- 24. How the compilertells the type The assigned Determine the variable return type MyType var = <T> T anyObject() © OCTO 2011 24
- 25. But watch outwith overloading public void foo(String s) public void foo(Object o) foo(anyObject()); Can’t guess the type © OCTO 2011 25
- 26. Trick #1 Determine the return Artificially give the type with a value type dedicated parameter <T> T anyObject(Class<T> clazz) © OCTO 2011 26
- 27. But how topass a generic? public void foo(String s) public void foo(Object o) foo(anyObject(List<String>.class)); Examples: Illegal ToXXXFunction © OCTO 2011 27
- 28. Example • Collectors.mapping params • Function, Collector • ToIntFunction, Collector.OfInt • ToLongFunction , Collector.OfLong • ToDoubleFunction, , Collector.OfDouble © OCTO 2011 28
- 29. Some solutions This would work foo((String) anyObject()); foo((List<String>) anyObject()); // Warning But still doesn’t work for generics © OCTO 2011 29
- 30. Trick #2 So the only solution is foo(EasyMock.<List<String>> anyObject()); … which sadly doesn’t support static imports foo(.<List<String>> anyObject()); // Illegal © OCTO 2011 30
- 31. Trick #2 appliedto Lambda Return type: Map<Boolean, List<Tweet>> return Tweet.TWEETS.stream() .collect( Collectors.<Tweet, Boolean, List<Tweet>, Map<Boolean, List<Tweet>>> groupingBy(t->t.containsHashTag("#lambda"), HashMap::new, ArrayList::new));
- 32. Trick #3: Lambdainference Object o = (Runnable) () -> { System.out.println("hi"); }; Collections.sort(strings, (String a, String b) -> a.compareTo(b));
- 33. Conclusion Who has learned something today? © OCTO 2011 33
- 34. Useful links Nicelambda tutorial (in French): http://lambda.ninjackaton.ninja-squad.com/ Description of inference types on lambda: http://cr.openjdk.java.net/~briangoetz/lambda/lambda-state-4.html Everything on generics: http://www.angelikalanger.com/GenericsFAQ/JavaGenericsFAQ.html
- 35. Conclusion Questions? © OCTO 2011 35