1. Covariance and
Contravariance.
Say what?!

3. Alin Pandichi

4. Alin Pandichi
Open space
Coding Dojo
Bucharest Java User Group
Global Day of Coderetreat

5. Alin Pandichi
Open space
Coding Dojo
Bucharest Java User Group
Global Day of Coderetreat
Software developer @
Mozaic Labs
Building eventriX

11. Covariance = the property of a function
of retaining its form when the variables
are linearly transformed.

13. Covariance = changing the input moves
the function in the same direction.

16. Contravariance = The basis and the vector
transform in an opposite way.

18. Contravariance = changing the input moves
the function in the opposite direction.

20. Covariance

21. Contravariance
Covariance

23. Variance refers to how
subtyping between complex
types (lists, arrays) relates to
subtyping between their
components (individual items).
Depending on variance, the
subtyping relation may be either
preserved, reversed, or ignored.

24. Integer is a subtype of Number
Integer i = 1;
Number n = i;
Number < Integer
Number ← Integer
< means “is a subtype of”
← means “can be assigned to”
*read from right to the left

25. Integer[] is a subtype of Number[]
Integer[] ints = {1, 2, 3};
Number[] nums = ints;
Number[] < Integer[]
Number[] ← Integer[]
< means “is a subtype of”
← means “can be assigned to”
*read from right to the left

26. Number ← Integer
Number[] ← Integer[]
A B← means “B can be assigned to A”
Array types can be assigned in the same order.
Arrays are covariant.

27. ArrayList<Integer> ints = new ArrayList<Integer>();
ArrayList<? extends Number> nums = ints;
Number ← Integer
Number[] ← Integer[]
<? extends Number> ← <Integer>
Can be assigned in the same order.
This generic list is covariant.
A B← means “B can be assigned to A”

28. ArrayList<Object> objs = new ArrayList<Object>();
ArrayList<? super Number> nums = objs;
Object ← Number
Object[] ← Number[]
<Object> → <? super Number>
Can be assigned in the reverse order.
This generic list is contravariant.
A B← means “B can be assigned to A”

29. Interesting properties of
covariance and contravariance:
With covariance you can only
take values out.
With contravariance you can
only put values in.

30. With covariance you can only
take values out.
ArrayList<? extends Error> errs =
new ArrayList<StackOverflowError>();
Error e = errs.get(0);
errs.add(new Error("")); // compiler error
errs.add(new StackOverflowError(""));
// compiler error

31. With contravariance you can only
put values in.
ArrayList<? super StackOverflowError> errs =
new ArrayList<Error>();
errs.add(new StackOverflowError("w00t"));
Error e = errs.get(0); //compiler-error
StackOverflowError soe = errs.get(0);
//compiler-error

32. public void copy(
List<? extends Number> source,
List<? super Number> destination) {
for(Number number : source) {
destination.add(number);
}
}
…
List<Integer> myInts = asList(1,2,3,4);
List<Integer> myDoubles = asList(3.14, 6.28);
List<Object> myObjs = new ArrayList<Object>();
copy(myInts, myObjs);
copy(myDoubles, myObjs);

33. In C#, covariant generic interfaces
are annotated with out:
interface IMessageRecieved <out T>
{
T RecievedData();
}
Covariance ­ take values out.

34. In C#, contravariant generic
interfaces are annotated with in:
interface IMessageSend <in T>
{
void SendData(T data);
}
Contravariance – put values in.

35. Covariance
Enables you to use a more derived
type than originally specified.
You can assign an instance of
IEnumerable<Derived> to a
variable of type
IEnumerable<Base>.

36. Contravariance
Enables you to use a more generic
(less derived) type than originally
specified.
You can assign an instance of
IEnumerable<Base> to a variable
of type IEnumerable<Derived>.

37. class A {
Object getSomething(){}
}
class B extends A {
String getSomething() {}
}
Covariance because it returns a String
which extends Object (more derived)

38. class A {
void doSomething(String param){}
}
class B extends A {
void doSomething(Object param){}
}
Contravariance because it takes an
Object which is a super class of String
(less derived)

41. Type systems that support
subtyping.

42. Programming language designers devising
typing rules for arrays, inheritance, and
generic datatypes

43. Programmers who use inheritance and
override methods.

44. Programmers who develop generic
interfaces, methods and code.

46. Subtyping
relation
Data
access
Overriding
methods
Wildcard
generics
Co
variance
Keep the
order
Take
values
out
More
derived
return
type
extends
Contra
variance
Reverse
the order
Put
values in
Less
derived
parameter
type
super

49. Resources
http://www.i­programmer.info/programming/theory/1632­covariance­and­c
ontravariance­a­simple­guide.html
https://dzone.com/articles/covariance­and­contravariance
https://msdn.microsoft.com/en­us/library/dd799517(v=vs.110).aspx
https://en.wikipedia.org/wiki/Covariance_and_contravariance_(computer_s
cience)#Covariant_arrays_in_Java_and_C.23
https://www.codeproject.com/articles/86419/variance­in­c­net­covariance­
and­contravarianc
http://stackoverflow.com/questions/8481301/covariance­invariance­and­con
travariance­explained­in­plain­english
http://stackoverflow.com/questions/2501023/demonstrate­covariance­and
­contravariance­in­java#2501513

50. Image resources
http://www.luchsinger­mathematics.ch/Var_Reduction.jpg
https://i.kinja­img.com/gawker­media/image/upload/s­­7lQpqRh­­­/c_scal
e,f_auto,fl_progressive,q_80,w_800/18ncmqm1vbv9vjpg.jpg
http://classrealm.com/blog/wp­content/uploads/2012/03/WAildmath­300
x270.jpg
http://www.i­programmer.info/images/stories/Core/Theory/covariance/ar
rowscontraandco.jpg
http://www.jonshawtrumpet.com/uploads/2/8/2/1/28211373/1670275_or
ig.jpg
https://www.memecreator.org/static/images/memes/3629822.jpg
http://i2.kym­cdn.com/photos/images/newsfeed/000/085/283/philosorapt
or.jpg
https://cdn.meme.am/cache/instances/folder757/500x/65401757/katt­willi
ams­shocked­let­me­rephrase­that­any­questions­i­can­answer.jpg
https://pixabay.com/en/puzzle­game­solution­connection­226743/
https://pixabay.com/en/blur­cellphone­close­up­design­1867748/
http://www.moneyloveandlegacy.com/images/HandsGivingCoins.jpg
http://hivhomekit.com/wp­content/uploads/2012/02/generic­stamp.png
https://i.ytimg.com/vi/uCpirskzaO8/hqdefault.jpg