1. Les lambda arrivent!
En attendant, êtes-vous sûr d'avoir
compris les génériques?
Henri Tremblay
Architecte Senior
OCTO Technology
@henri_tremblay
27 au 29 mars 2013

2. Henri Tremblay
OCTO Technology Open Source
Responsable R&D EasyMock
Performance Objenesis
Calcul parallèle

3. September 2013

4. September 2013

5. September 2013

7. What do I need to know?
Why
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8. What do I need to know?
Why
Covariance
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9. What do I need to know?
Why
Covariance
Capture
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10. What do I need to know?
Why
Covariance
Capture
Erasure
© OCTO 2011 10

11. What do I need to know?
Why
Covariance
Capture
Erasure
Bridge
© OCTO 2011 11

12. What do I need 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 = Fun with 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>
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17. Ostrich defense
@SuppressWarnings("unchecked")
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18. Why
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19. Rule #1
A code compiling
without warning should
never ever cause a
ClassCastException
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20. Capture
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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>)
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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
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23. How the compiler tells the type
Determine The parameter
the return
value type
<T> T anyObject(T clazz)
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24. How the compiler tells the type
The assigned Determine the
variable return type
MyType var = <T> T anyObject()
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25. But watch out with overloading
public void foo(String s)
public void foo(Object o)
foo(anyObject());
Can’t
guess the
type
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26. Trick #1
Determine the return Artificially give the type with a
value type dedicated parameter
<T> T anyObject(Class<T> clazz)
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27. But how to pass a generic?
public void foo(String s)
public void foo(Object o)
foo(anyObject(List<String>.class));
Examples:
Illegal ToXXXFunction
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28. Example
• Collectors.mapping params
• Function, Collector
• ToIntFunction, Collector.OfInt
• ToLongFunction , Collector.OfLong
• ToDoubleFunction, , Collector.OfDouble
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29. Some solutions
This would work
foo((String) anyObject());
foo((List<String>) anyObject()); // Warning
But still doesn’t work for
generics
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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
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31. Trick #2 applied to 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: Lambda inference
Object o = (Runnable) () -> {
System.out.println("hi");
};
Collections.sort(strings,
(String a, String b) -> a.compareTo(b));

33. Conclusion
Who has learned
something today?
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34. Useful links
Nice lambda 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?
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