We are in 2015 and Java 8 will slowly but surely become the new standard. Our world will be filled with lambdas. Generics, for themselves, have appeared in 2004. They brought benefits and complexity. Enough to blow up the complexity quota of Java according to Josh Bloch. But lambdas, whatever are sexy they are, are going to add even more complexity. Mix with generics your now playing with nitroglycerin. We will travel together through the bottom of the generics, through inference and through lambdas type resolution. To explain why. To provide solutions. To ease Java 8 adoption.

1. 27 au 29 mars 2013
Les lambda sont là!
Mais êtes-vous sûr d'avoir
compris les génériques?
Henri Tremblay
Architecte Senior
Pages Jaunes
@henri_tremblay

2. Amateur de
Stratégie TI
Performance
Productivité
Bons vins, bières, whiskies…
Fait de l’Open Source
Henri Tremblay
Aime être utile

3. March 18 2014

4. Java 7 End of Life

5. September 2004

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

7. 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);

8. 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);

9. What do I need to know?
9© OCTO 2011
Why

10. What do I need to know?
10© OCTO 2011
Why
Covariance

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

12. What do I need to know?
12© OCTO 2011
Why
Covariance
Capture
Inference

13. What do I need to know?
13© OCTO 2011
Why
Covariance
Capture
Inference
Erasure

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

15. Faire compiler
15

16. Dreaded warnings
16
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>

17. Ostrich defense
@SuppressWarnings("unchecked")
17

18. 27 au 29 mars 2013
Why
18© OCTO 2011

19. Rule #1
A code compiling
without warning should
never ever cause a
ClassCastException
19

20. 27 au 29 mars 2013
Covariance
20

21. Arrays
Arrays are covariant:
Number n = Integer.MAX_VALUE;
Number[] list = new Integer[0];
Generics are not:
List<Number> l =
new ArrayList<Integer>(); // Illegal
21

22. 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
22
Would work if covariant And allow to break rule #1

23. Why for array?
Integer[] list = // ...
foo(list);
public void foo(Object[] o) {
// ...
}

24. Arrays and generics don’t mix well
Can’t have an array of generics
List<String>[] lsa = new List<String>[10];// illegal
24

25. Exception
List<?>[] l = new ArrayList<?>[3];
25

26. Because
If it was allowed
List<String>[] lsa = new List<String>[10]; // illegal
Object[] oa = lsa; // OK (covariant)
oa[0] = new ArrayList<Integer>(); // OK
oa[0].add(42);
String s = lsa[0].get(0); // bad
26

27. 27 au 29 mars 2013
Capture
27

28. 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>)
28

29. 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);
29

30. Only solution
We need to cast
expect((List<String>) bar()).andReturn(new
ArrayList<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
30
Tell to expect we want a
List<String>

31. Inference
31

32. Diamonds are a programmer best friend
List<String> l = new ArrayList<>();

33. How the compiler tells the type
<T> T anyObject(T clazz)
33
The parameterDetermine
the return
value type

34. How the compiler tells the type
MyType var = <T> T anyObject()
34
Determine the
return type
The assigned
variable

35. But watch out with overloading
public void foo(String s)
public void foo(Object o)
foo(anyObject());
35
Can’t
guess the
type

36. Trick #1
<T> T anyObject(Class<T> clazz)
36
Artificially give the type with a
dedicated parameter
Determine the return
value type

37. But how to pass a generic?
public void foo(String s)
public void foo(Object o)
foo(anyObject(List<String>.class));
37
Illegal

38. Some solutions
foo((String) anyObject());
foo((List<String>) anyObject()); // Warning
38
This would work
But still doesn’t work for
generics

39. Trick #2
So the only solution is
foo(EasyMock.<List<String>> anyObject());
… which sadly doesn’t support static imports
foo(.<List<String>> anyObject()); // Illegal
39

40. Trick #2 applied to Lambda
return Tweet.TWEETS.stream()
.collect( Collectors.<Tweet, Boolean,
List<Tweet>, Map<Boolean, List<Tweet>>>
groupingBy(t->t.containsHashTag("#lambda"),
HashMap::new, ArrayList::new));
Return type: Map<Boolean, List<Tweet>>

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

42. How did it do it?
Tweet.TWEETS.stream()
List<Tweet> list = Tweet.TWEETS;
Stream<Tweet> stream = list.stream();

43. How did it do it?
Stream<Tweet> stream = list.stream();
R result = stream.collect(Collector<? super T, ?, R> collector);
R result = stream.collect(Collector<? super Tweet, ?, R> collector);

44. How did it do it?
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);

45. How did it do it?
Predicate<? super Tweet> lambda = t -> t.containsHashTag("#lambda");
We now know that
So the best t can be is a Tweet
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
47

48. Erasure…
public void foo() {
List<String> l = new ArrayList<String>();
for (String s : l) {
System.out.println(s);
}
}
No type
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
49

50. Type class
java.lang.reflect.Type
• GenericArrayType
• ParameterizedType
• TypeVariable
• WildcardType
• Implemented by Class
java.lang.reflect.GenericDeclaration
Implemented by Class, Method, Constructor
50
New powers unleashed!

51. Useful!
class A {}
abstract class BaseDao<T> {
public T load(final long id) {
// …
}
}
class ADao extends BaseDao<A> {}
51© OCTO 2011

52. Useful!
@SuppressWarnings("unchecked")
public T load(final long id) {
ParameterizedType type =
(ParameterizedType) getClass()
.getGenericSuperclass();
Type actualType = type.getActualTypeArguments()[0];
return em.find((Class<T>) actualType, (Long) id);
}
ADao
A
BaseDao<A>
Unsafe cast

53. Bridge
53© OCTO 2011

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

57. The actual problem being
B.class.getDeclaredMethods()
volatile void set(java.lang.Object)
void B.set(java.lang.String)
This
Returns that
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?
59© OCTO 2011

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
Hopefully everything on lambdas
http://www.lambdafaq.org/

61. Conclusion
Questions?
61© OCTO 2011
http://perfug.github.io/
http://brownbaglunch.fr
+Henri Tremblay
@henri_tremblay
henri@tremblay.pro