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

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<?>Wildcards

in the Java™ Programming Language

Neal Gafter

with thanks to

Mads Torgersen, University of Aarhus

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<?>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

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<?>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

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<?>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)

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<?>Java Genericity

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

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<?>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);

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<?>Java Genericity

List numbers = new ArrayList();numbers.add(new Integer(7));Number n = (Number)numbers.get(0);

interface List<T> extends Collection<T> { void add(T element); T get(int index); ...}

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<?>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);

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<?>Static typecheck

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

List numbers = ...numbers.add(”Seven”);Number n = (Number)numbers.get(0);

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<?>Static typecheck



Runtime type error!

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<?>Static typecheck



Compile time error

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<?>Object-Oriented Abstraction

Common view on common properties

Collection

ListSet

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Common view on common properties

Collection

ListSet

List l = ...;Collection c = l;

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Pointwise subtyping

Collection<Number>

List<Number>Set<Number>

List<Number> nl = ...;Collection<Number> nc = nl;

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<?>Parametric Abstraction

What is the common view?

List<Number>List<String>

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<?>In the ”old” days...

...any List was just a List

List

ListList

List numbers = ...;List things = numbers;things.add(”Seven”);Number n = (Number)number.get(0);

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<?>In the ”old” days...

...any List was just a List

List

ListList

List numbers = ...;List things = numbers;things.add(”Seven”);Number n = (Number)number.get(0);

Runtime type error!

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Number[] numbers = ...;Object[] things = numbers; things[0] = ”Seven”;Number n = numbers[0];

The Array approach

Runtime store check on every assignmentObject[]

Number[]String[]

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Number[] numbers = ...;Object[] things = numbers; Things[0] = ”Seven”;Number n = numbers[0];

The Array approach

Runtime store check on every assignmentObject[]

Number[]String[]

Runtime type error!

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<?>Can we do the same for List?

List<Object>


List<Number> numbers = ...;List<Object> things = numbers; things.add(”Seven”);Number n = numbers.get(0);

Runtime type error?

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Erasure: No type argument info at runtime

List<Object>



Runtime type error?

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Erasure: No type argument info at runtime

List<Object>



Runtime type error!

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No type argument info at runtime

List<Object>



Compile time error!

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<?>The raw types approach

List


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

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Compile time type check undermined

List



Runtime type error!

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List



Compile time warning

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List



Compile time warning

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<?>The GJ approach

Object


List<Number> numbers = ...;Object things = numbers; things.add(”Seven”);Object o = things.get(0);

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<?>The GJ approach

Object



Compile time error!

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<?>The GJ approach

Cannot use common interface

Object



Compile time error!

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<?>The GJ approach

Cannot use common interface

Object



Compile time error!

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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

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<?>Wildcards

”List of something ”

List< >


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List< >


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<?>Wildcards


List<?>


List<Number> numbers = ...;List<?> things = numbers; things.add(”Seven”);Object o = things.get(0);

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<?>Wildcards


List<?>


List<Number> numbers = ...;List<?> things = numbers; things.add(”Seven”);Object o = things.get(0);

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<?>Wildcards in Collections

package java.util;

public interface Collection<E> { boolean containsAll(Collection<?> c); boolean removeAll(Collection<?> c); ...}

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<?>Can we do better?

Integer and Float are related

List<?>

List<Integer>List<Float>

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<?>Bounded Wildcards

Numbers come out

List<? extends Number>


List<Integer> ints = ...;List<? extends Number> numbers = ints;Number n = numbers.get(0);numbers.add(.7F);

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<?>Bounded Wildcards

Adding still prohibited



List<Integer> ints = ...;List<? extends Number> numbers = ints;Number n = numbers.get(0);numbers.add(.7F);

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<?>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);

...

}

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What can we do about adding?

Adding still prohibited


List<Integer>List<Number>

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<?>Super-bounded Wildcards

List<? super Integer>


List<Number> numbers = ...;List<? super Integer> ints = numbers;ints.add(7);Integer i = ints.get(0);

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<?>Super-bounded Wildcards

Only Objects come out



List<Number> numbers = ...;List<? super Integer> ints = numbers;ints.add(7);Integer i = ints.get(0);

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<?>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);

...

}

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<?>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); ...}

T

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<?>Subtyping with wildcards


List<? extends Integer> List<Number>

List<Integer>

List<?>

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<?>Subtyping with wildcards


List<? super Number> List<Integer>

List<Number>

List<?>

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<?>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)

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<?>Type inference

Built-in condition expression

Boolean b = ...List<Number> numbers = ...Set<String> strings = ...

Collection<?> c = b ? numbers : strings

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<?>Capture

package java.util;


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package java.util;


How is reverse() implemented?

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public static void reverse(List<?> list) { rev(list); } private static <T> void rev(List<T> list) { for (int i = 0; i < list.length/2; i++) { int j = list.length - i - 1; T tmp = list.get(i); list.set(i, list.get(j)); list.set(j, tmp); } }}

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public static void reverse(List<?> list) { rev(list); } private static <T> void rev(List<T> list) { for (int i = 0; i < list.length/2; i++) { int j = list.length - i - 1; T tmp = list.get(i); list.set(i, list.get(j)); list.set(j, tmp); } }}

Capture

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<?>Wildcard conclusions

● Bridges the gap between object-oriented and polymorphic abstraction

● Simpler and more precise signatures● Better type inference● All over the JDK 5 APIs

http://java.sun.com/j2se/1.5.0

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

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