Wildcards in the Java ™ Programming Language Neal Gafter with thanks to Mads Torgersen, University of Aarhus

1. Wildcards in the Java™ Programming Language Neal Gafter with thanks to Mads Torgersen, University of Aarhus

2. Wildcards • Genericity • Improves type system expressiveness • Based on parametric abstraction • The abstraction gap: • Clashes with object-oriented (subtype) abstraction • Wildcards • Mediate the two modes of abstraction • Address deep type-theoretic issues

3. The Wildcards Project • University of Aarhus • Mads Torgersen, Erik Ernst, Peter von der Ahé and Christian Plesner Hansen • Sun Microsystems • Gilad Bracha and Neal Gafter • Based on previous research by • Mads Torgersen & Kresten Krab Thorup • Mirko Viroli & Atsushi Igarashi

4. Project Results • Design of the wildcard mechanism • Mediation of the abstraction gap • Integration with other language features • Implementation • javac, the Java compiler • Java Platform APIs • Part of JDK1.5 (Tiger)

5. Java Genericity interface List extends Collection { void add(Object element); Object get(int index); ... }

6. Java Genericity interface List extends Collection { void add(Object element); Object get(int index); ... } List numbers = new ArrayList(); numbers.add(new Integer(7)); Number n = (Number)numbers.get(0);

7. Java Genericity interface List<T> extends Collection<T> { void add(T element); T get(int index); ... } List numbers = new ArrayList(); numbers.add(new Integer(7)); Number n = (Number)numbers.get(0);

8. Java Genericity interface List<T> extends Collection<T> { void add(T element); T get(int index); ... } List<Number> numbers = new ArrayList<Number>(); numbers.add(new Integer(7)); Number n = numbers.get(0);

9. Static typecheck List numbers = ... numbers.add(”Seven”); Number n = (Number)numbers.get(0); List<Number> numbers = ... numbers.add(”Seven”); Number n = numbers.get(0);

10. Static typecheck Runtime type error! List numbers = ... numbers.add(”Seven”); Number n = (Number)numbers.get(0); List<Number> numbers = ... numbers.add(”Seven”); Number n = numbers.get(0);

11. Static typecheck List numbers = ... numbers.add(”Seven”); Number n = (Number)numbers.get(0); Compile time error List<Number> numbers = ... numbers.add(”Seven”); Number n = numbers.get(0);

12. Object-Oriented Abstraction Common view on common properties Collection Set List

13. Object-Oriented Abstraction Common view on common properties Collection Set List List l = ...; Collection c = l;

14. Object-Oriented Abstraction Pointwise subtyping Collection<Number> Set<Number> List<Number> List<Number> nl = ...; Collection<Number> nc = nl;

15. Parametric Abstraction What is the common view? ? List<String> List<Number>

16. In the ”old” days... ...any List was just a List List List List List numbers = ...; List things = numbers; things.add(”Seven”); Number n = (Number)number.get(0);

17. Runtime type error! In the ”old” days... ...any List was just a List List List List List numbers = ...; List things = numbers; things.add(”Seven”); Number n = (Number)number.get(0);

18. The Array approach Runtime store check on every assignment Object[] String[] Number[] Number[] numbers = ...; Object[] things = numbers; things[0] = ”Seven”; Number n = numbers[0];

19. The Array approach Runtime store check on every assignment Object[] String[] Number[] Runtime type error! Number[] numbers = ...; Object[] things = numbers; Things[0] = ”Seven”; Number n = numbers[0];

20. Can we do the same for List? List<Object> ? List<String> List<Number> Runtime type error? List<Number> numbers = ...; List<Object> things = numbers; things.add(”Seven”); Number n = numbers.get(0);

21. Can we do the same for List? Erasure: No type argument info at runtime List<Object> ? List<String> List<Number> Runtime type error? List<Number> numbers = ...; List<Object> things = numbers; things.add(”Seven”); Number n = numbers.get(0);

22. Can we do the same for List? Erasure: No type argument info at runtime List<Object> List<String> List<Number> Runtime type error! List<Number> numbers = ...; List<Object> things = numbers; things.add(”Seven”); Number n = numbers.get(0);

23. Can we do the same for List? No type argument info at runtime List<Object> List<String> List<Number> Compile time error! List<Number> numbers = ...; List<Object> things = numbers; things.add(”Seven”); Number n = numbers.get(0);

24. The raw types approach List ? List<String> List<Number> List<Number> numbers = ...; List things = numbers; things.add(”Seven”); Number n = numbers.get(0);

25. The raw types approach Compile time type check undermined List ? List<String> List<Number> List<Number> numbers = ...; List things = numbers; things.add(”Seven”); Number n = numbers.get(0); Runtime type error!

26. The raw types approach Compile time type check undermined List List<String> List<Number> Compile time warning List<Number> numbers = ...; List things = numbers; things.add(”Seven”); Number n = numbers.get(0);

27. The raw types approach Compile time type check undermined List List<String> List<Number> Compile time warning List<Number> numbers = ...; List things = numbers; things.add(”Seven”); Number n = numbers.get(0);

28. The GJ approach Object List<String> List<Number> List<Number> numbers = ...; Object things = numbers; things.add(”Seven”); Object o = things.get(0);

29. The GJ approach Object List<String> List<Number> List<Number> numbers = ...; Object things = numbers; things.add(”Seven”); Object o = things.get(0); Compile time error!

30. The GJ approach Cannot use common interface Object List<String> List<Number> List<Number> numbers = ...; Object things = numbers; things.add(”Seven”); Object o = things.get(0); Compile time error!

31. The GJ approach Cannot use common interface Object List<String> List<Number> List<Number> numbers = ...; Object things = numbers; things.add(”Seven”); Object o = things.get(0); Compile time error!

32. Requirements for a common view • It should be some kind of List • so that we can safely get elements out of it • It should prevent insertion of wrong elements • to avoid heap pollution • It should do so at compile time • because we have no runtime type argument info • It must prevent insertion of any elements • because it is a share view of all Lists

33. Wildcards ”List of something ” ? List< > List<String> List<Number>

34. Wildcards ”List of something ” ? List< > List<String> List<Number>

35. Wildcards ”List of something ” List<?> List<String> List<Number> List<Number> numbers = ...; List<?> things = numbers; things.add(”Seven”); Object o = things.get(0);

36. Wildcards ”List of something ” List<?> List<String> List<Number> List<Number> numbers = ...; List<?> things = numbers; things.add(”Seven”); Object o = things.get(0);

37. Wildcards in Collections package java.util; public interface Collection<E> { boolean containsAll(Collection<?> c); boolean removeAll(Collection<?> c); ... }

38. Can we do better? Integer and Float are related List<?> List<Float> List<Integer>

39. Bounded Wildcards Numbers come out List<? extends Number> List<Float> List<Integer> List<Integer> ints = ...; List<? extends Number> numbers = ints; Number n = numbers.get(0); numbers.add(.7F);

40. Bounded Wildcards Adding still prohibited List<? extends Number> List<Float> List<Integer> List<Integer> ints = ...; List<? extends Number> numbers = ints; Number n = numbers.get(0); numbers.add(.7F);

41. Extends-bounds in Collections package java.util; public interface Collection<E> { boolean containsAll(Collection<?> c); boolean removeAll(Collection<?> c); boolean addAll(Collection<? extends E> c); ... }

42. What can we do about adding? Adding still prohibited List<? extends Number> List<Number> List<Integer>

43. Super-bounded Wildcards List<? super Integer> List<Number> List<Integer> List<Number> numbers = ...; List<? super Integer> ints = numbers; ints.add(7); Integer i = ints.get(0);

44. Super-bounded Wildcards Only Objects come out List<? super Integer> List<Number> List<Integer> List<Number> numbers = ...; List<? super Integer> ints = numbers; ints.add(7); Integer i = ints.get(0);

45. TreeSet Constructors package java.util; public class TreeSet<E> implements OrderedSet<E> { public TreeSet(OrderedSet<E> c); public TreeSet(Collection<? extends E> c); public TreeSet(Comparator<? super E> c); ... }

46. T Collections utility methods package java.util; public class Collections { public static void reverse(List<?> list); public static void shuffle(List<?> list); public static <T> void fill( List<? super T> list, T obj); public static <T> void copy( List<? super T> dest, List<? extends T> src); ... }

47. Subtyping with wildcards List<?> List<? extends Number> List<? extends Integer> List<Number> List<Integer>

48. Subtyping with wildcards List<?> List<? super Integer> List<? super Number> List<Integer> List<Number>

49. Type inference Better types to choose from <T>T choose(T fst, T snd) { ... } List<Number> numbers = ... Set<String> strings = ... Collection<?> c = choose(numbers,strings)

50. Type inference Built-in condition expression Boolean b = ... List<Number> numbers = ... Set<String> strings = ... Collection<?> c = b ? numbers : strings