Lambda expressions are coming soon. They promise to be a nice complexity cocktail with the 9 years old generics. So, before confronting the lambdas, are you sure you're mastered the generics? This quick talk will give you some tips to learn to swim before the flood.

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
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11. What do I need to know?
Why
Covariance
Capture
Erasure
Bridge
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12. What do I need to know?
Why
Covariance
Capture
Erasure
Bridge
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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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