Lambda expressions are coming soon. They promise to be a nice complexity cocktail with the 9 years old generics. Lets go deep into the generics to get a better understanding. Mix them with lambdas and wait calmly for Java 8 to arrive.

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. 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

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

4. September 2013

5. September 2013

6. March 2014

7. March 2014

8. September 2004

9. Lambda
return Tweet.TWEETS.stream()
.collect(Collectors
.partitioningBy(
t->t.containsHashTag("#lambda")));

10. Lambda = Fun with generics
Stream<Tweet> stream = Tweet.TWEETS.stream();
Predicate<Tweet> lambda = t -> t.containsHashTag("#lambda");
Collector<Tweet, ?, Map<Boolean, List<Tweet>>> collector =
Collectors.partitioningBy(lambda);
return stream.collect(collector);

11. 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);

12. What do I need to know?
Why
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13. What do I need to know?
Why
Covariance
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14. What do I need to know?
Why
Covariance
Capture
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15. What do I need to know?
Why
Covariance
Capture
Inference
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16. What do I need to know?
Why
Covariance
Capture
Inference
Erasure
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17. What do I need to know?
Why
Covariance
Capture
Erasure
© OCTO 2011
Inference
Bridge
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18. Faire compiler
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19. 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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20. Ostrich defense
@SuppressWarnings("unchecked")
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21. Why
© OCTO 2011
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27 au 29 mars 2013

22. Rule #1
A code compiling
without warning should
never ever cause a
ClassCastException
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23. Covariance
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27 au 29 mars 2013

24. Arrays
Arrays are covariant:
Number n = Integer.MAX_VALUE;
Number[] list = new Integer[0];
Generics are not:
List<Number> l =
new ArrayList<Integer>(); // Illegal
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25. Why not?
List<Integer> li = new ArrayList<Integer>();
List<Number> ln = li; // illegal
ln.add(new Float(3.1415));
int i = li.get(0); // ClassCastException
Would work if covariant
And allow to break rule #1
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26. Why for array?
Integer[] list = // ...
foo(list);
public void foo(Object[] o) {
// ...
}

27. Arrays and generics don’t mix well
Can’t have an array of generics
List<String>[] lsa = new List<String>[10];// illegal
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28. Because
If it was allowed
List<String>[] lsa = new List<String>[10]; // illegal
Object[] oa = lsa; // OK (covariant)
oa[0] = new ArrayList<Integer>(); // OK
String s = lsa[0].get(0); // bad
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29. Exception
List<?>[] l = new ArrayList<?>[3];
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30. Capture
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27 au 29 mars 2013

31. Capture usually is
Type
List<?> bar();
<T> IExpectationSetters<T> expect(T value);
void andReturn(T value); // Method of IExpectationSetters
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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32. Detail
List<?> bar();
<T> IExpectationSetters<T> expect(T value);
void andReturn(T value);
expect(bar()).andReturn(new ArrayList<String>());
List<Capture#6> bar = bar();
IExpectationSetters<List<Capture#6>> es =
expect(bar());
es.andReturn(List<Capture#6> value);
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33. Only solution
We need to cast
expect((List<String>) bar()).andReturn(new
Tell to expect we want a
ArrayList<String>());
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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34. Inference
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35. Diamonds are a programmer best friend
List<String> l = new ArrayList<>();

36. How the compiler tells the type
Determine
the return
value type
The parameter
<T> T anyObject(T clazz)
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37. How the compiler tells the type
The assigned
variable
Determine the
return type
MyType var = <T> T anyObject()
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38. 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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39. Trick #1
Determine the return
value type
Artificially give the type with a
dedicated parameter
<T> T anyObject(Class<T> clazz)
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40. But how to pass a generic?
public void foo(String s)
public void foo(Object o)
foo(anyObject(List<String>.class));
Illegal
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41. Some solutions
This would work
foo((String) anyObject());
foo((List<String>) anyObject()); // Warning
But still doesn’t work for
generics
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42. 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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43. 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));

44. Lambda = Inference
return Tweet.TWEETS.stream()
.collect(Collectors
.partitioningBy(
t->t.containsHashTag("#lambda"));

45. How did he do it?
List<Tweet> list = Tweet.TWEETS;
Stream<Tweet> stream = list.stream();
R result = stream.collect(Collector<? super T, ?, R> collector);
R result = stream.collect(Collector<? super Tweet, ?, R> collector);
Collector<T, ?, Map<Boolean, List<T>>> collector =
Collectors.partitioningBy(Predicate<? super T> predicate);
Collector<Tweet, ?, Map<Boolean, List<Tweet>>> collector =
Collectors.partitioningBy(Predicate<? super Tweet> predicate);
Map<Boolean, List<Tweet>> result = stream.collect(Collector<? super Tweet, ?,
Map<Boolean, List<Tweet>>> collector);
Predicate<? super Tweet> lambda = (Tweet t) -> t.containsHashTag("#lambda");

46. Trick #3: Lambda inference
Object o = (Runnable) () -> {
System.out.println("hi");
};
Collections.sort(strings,
(String a, String b) -> a.compareTo(b));

47. Erasure
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48. Erasure…
public void foo() {
List<String> l = new ArrayList<String>();
for (String s : l) {
No type
System.out.println(s);
}
public void foo() {
}
List l = new ArrayList();
for (String s : l) {
System.out.println(s);
Compilation
}
}

49. … or not erasure
public class A extends ArrayList<String> {}
public static void main(final String[] args) {
ParameterizedType type = (ParameterizedType)
A.class.getGenericSuperclass();
System.out.println(
type.getActualTypeArguments()[0]);
}
 prints class java.lang.String
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50. Type class
java.lang.reflect.Type
•
•
•
•
•
GenericArrayType
ParameterizedType
TypeVariable
WildcardType
Implemented by Class
New powers unleashed!
java.lang.reflect.GenericDeclaration
Implemented by Class, Method, Constructor
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51. Useful!
class A {}
abstract class BaseDao<T> {
public T load(final long id) {
// …
}
}
class ADao extends BaseDao<A> {}
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52. Useful!
@SuppressWarnings("unchecked")
public T load(final long id) {
ParameterizedType type =
(ParameterizedType) getClass()
.getGenericSuperclass();
ADao
BaseDao<A>
A
Type actualType = type.getActualTypeArguments()[0];
return em.find((Class<T>) actualType, (Long) id);
}
Unsafe cast

53. Bridge
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54. Everything seems normal…
class A<T> {
abstract void set(T value);
}
class B extends A<String> {
String value;
@Override
void set(final String value) {
this.value = value;
}
}

55. But is not
class B extends A {
void set(String value) {
this.value = value;
}
volatile void set(Object o){
set((String)o);
}
}

56. Example
Raw type warning
Perfectly compiling
A a = new B();
a.set(new Object());
But at runtime:
java.lang.ClassCastException

57. The actual problem being
B.class.getDeclaredMethods()
This
Returns that
volatile void set(java.lang.Object)
void B.set(java.lang.String)
And gives you no way to find out
which method is bridged

58. What about lambdas?
public class A {
public static void main(String[] args) {
Method[] methods = A.class.getDeclaredMethods();
Arrays.stream(methods).forEach(m ->
System.out.println(m + " "
+ m.isBridge() + " " + m.isSynthetic()));
}
}
Prints this
public static void A.main(java.lang.String[]) false false
private static void A.lambda$0(java.lang.reflect.Method) false true

59. Conclusion
Who has learned
something today?
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60. 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-final.html
Everything on generics:
http://www.angelikalanger.com/GenericsFAQ/JavaGenericsFAQ.html

61. Conclusion
Questions?
http://perfug.github.io/
+Henri
@henritremblay
htr@octo.com
© OCTO 2011
http://brownbaglunch.fr
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