1. Telecooperation/RBG Technische Universität Darmstadt Copyrighted material; for TUD student use only Introduction to Computer Science I Topic 17: Type Conversions, Generics Prof. Dr. Max Mühlhäuser Dr. Guido Rößling

2. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T18 Type Conversions With static typing, there are many contexts in which values of a specific type are expected –In a = expression, we expect expression to have a subtype of the type of a –In a + b, we expect that either a and b are both int, both float or double, or both Strings –In f(a, b), we expect the types of the arguments to match those of the formal parameters 2

3. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T18 Kinds of Type Conversions in Java Identity conversion – from a type to itself –It is OK to insert a redundant cast Widening primitive conversion – a primitive type to a wider type –e.g., byte to int (without loss of information) –e.g., int to double (possibly losing information) Narrowing primitive conversion – a primitive type to a narrower type –e.g., int to byte (discards higher-order bits) –e.g., float to int 3

4. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T18 Kinds of Type Conversions in Java Widening reference conversion –Given reference types A, B, A can be widened to B iff A is a subtype of B. –Widening conversions are always performed at compile time and never throw an exception. Narrowing reference conversion –Given reference types A, B, A can be narrowed to B iff B is a subtype of A. –Narrowing conversions are always checked at run-time and may throw a runtime exception. 4

5. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T18 Kinds of Type Conversions in Java II String conversion –Each type can be converted to String –Implicitly, the method toString() is called (from java.lang.Object ) –We have used this, e.g. for System.out.println(myObject) Boxing and unboxing conversion –From byte to Byte, int to Integer etc. and vice versa Unchecked conversion –A conversion that may lead to an error or issue a compiler warning –E.g., convert a raw type to a parameterized type 5

6. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T18 Conversion contexts Assignment conversion : v = expr –convert the type of expr to the type of v –Limited to conversions that do not raise exceptions Method invocation conversion : expr.method(expr) –convert the types of the arguments –Limited to conversions that do not raise exceptions Casting conversion: (T)expr –convert the type of expr to T –May also use narrowing reference conversion Numeric promotion –Brings the operands of a numeric operator to a common type so that an operation can be performed Allow identity, primitive widening and unboxing conversion e.g., 4 + 2.0 6.0 6

7. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T18 Narrowing Casting can cause (static) type information to be lost: // o refers to a Circle; widening is OK GraphicObject o = new Circle(5, 12, 4); Circle c1 = o; // compile-time error -- cannot narrow! Type information can be recovered at run-time by explicit tests and casts: if (o instanceof Circle) { // run-time test c1 = (Circle)o; // explicit run-time narrowing OK } 7

8. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T18 Collections Revisited A Collection should apply to more than one type –One implementation should be usable for several uses –Generic collection behavior does not depend on element type It should guarantee a specific element type –Assume we use a collection only for Person objects –It should be possible to obtain Person objects without narrowing from Object 8

9. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T18 Collections Revisited List myIntList = new LinkedList(); // 1 myIntList.add(new Integer(0)); // 2 Integer x = (Integer)myIntList.get(0); // 3 The cast in line 3 is annoying but essential –Java only guarantees the result to be of type Object –It may or may not be of type Integer Therefore, we could get a type error without the cast In our example, the cast is not checked by instanceof We could be mistaken and receive something else List myIntList = new LinkedList(); // 1 myIntList.add("Hello World"); // 2 Integer x = (Integer)myIntList.get(0); // 3 –We now receive a ClassCastException at run-time! 9

10. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T18 Expressing Intent Using Generics We need to tell Java more about our intention –This List will only contain elements conforming to Integer To do so, we need to: –Adapt the declaration of the List: List –Adapt the construction of the List: new LinkedList Benefits: –We can drop the type cast in line 3 –Non-matching types cannot be inserted or retrieved List myIntList = new LinkedList (); // 1 myIntList.add(new Integer(0)); // 2 Integer x = myIntList.get(0); // 3 myIntList.add("Hello World"); // compile-time error 10

11. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T18 What has Changed? myIntList is now a List –Not some List, but a List of Integer instances –We no longer need to cast the result to Integer We can also no longer store other types into the List The Java compiler (and Eclipse) will prevent this 11 myIntList.add("Hello World"); The method add(Integer) in the type List is not applicable for the arguments (String)

12. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T18 Whats the Big Deal? It may seem that we have not accomplished much –The declaration and construction of the list are longer –We have only dropped the cast However, there is a large difference! –The compiler can now check the type correctness –The declaration of myIntList specifies the variables type –The compiler will check that all accesses respect that type Why is this different from a simple cast? –With a cast, the programmer says the type should conform to the cast to X here Should be checked by instanceof –Generic declaration states this always conforms to type X 12

13. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T18 Lists Again Here is how List and Iterator are defined in java.util: 13 public interface List { void add(E x); Iterator iterator(); } public interface Iterator { E next(); boolean hasNext(); void remove(); } This should look familiar (see T15) Except for and E itself This is the declaration of the formal type parameter You can use these types almost everywhere

14. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T18 List vs. IntegerList We used List, but there is only a List ? The E is a formal parameter –Just like the formal parameters in method signatures E is substituted by Integer in all calls Intuitively, you might think of this as follows: 14 public interface IntegerList { void add(Integer x); IntegerIterator iterator(); void remove(); } This may help in understanding generics But it is also misleading There is only one List class, not one per type

15. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T18 Generics and Subtyping Is the following code legal? Line 1 is certainly legal –LinkedList is a subtype of List –The formal parameters (both String) match In line 2, does List conform to List ? –List is the same class in both operations –String is a subclass of Object –The intuitive answer is therefore yes 15 List ls = new LinkedList (); // 1 List lo = ls; // 2 lo.add(new Object()); // 3 String s = ls.get(0); // 4 The compiler will not accept this code!

16. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T18 Generics and Subtyping Let us first look at the next lines of code: 16 List ls = new LinkedList (); // 1 List lo = ls; // 2 lo.add(new Object()); // 3 String s = ls.get(0); // 4 Line 2 has aliased the Object and the String list We can insert an Object into a List (line 3) However, at the same time, we insert it into ls ls is still declared as a List … But ls would now also contain other object types If the compiler accepts line 2, it cannot detect this Therefore, the compiler will reject Line 2 type mismatch: cannot convert from List to List The compiler will not accept this code!

17. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T18 Generics, Subtyping, and Intuition Assume Y is a subclass of (interface or class) X G is some generic type declaration (e.g., List<>) G is not a subtype of G Why is this so hard to believe? Intuitively, we assume a collection does not change –Of course, this assumption is often wrong Assume the student office passes a list of students to the citys census office –A Student is a Person –Passing List where List is expected seems OK –But only if a copy of the list is passed, not a reference! –What happens if the census office adds a non-Student Person??? The shared (!) student list is now corrupted 17

18. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T18 Generics, Subtyping, and Intuition 18 Collection Set Collection Set Covariant Subtyping Pointwise Subtyping, safe not safe

19. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T18 Addressing Subtyping By Wildcards We want to print all elements in a collection Heres some (working) code without Generics: Now we adapt this to Generics: However, this will only accept Collection –It will not accept List, but only List –Remember that List is not a subtype of List 19 void printCollection(Collection c) { for (Object element: c) System.out.println(element); } void printCollection(Collection c) { for (Object element: c) System.out.println(element); }

20. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T18 Addressing Subtyping By Wildcards What do we have to do? –We need a common supertype of all kinds of collections –This is the Collection of unknown element type –In Java notation: Collection (here: List ) We can now adapt the code: The elements of the list are treated as Object –Because java.lang.Object is the superclass of all classes –Thus, any type – even the unknown type ? – fits to Object 20 void printCollection(Collection c) { for (Object element: c) System.out.println(element); }

21. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T18 The Unknown Type The Unknown Type seems very helpful –We can finally access the elements –If necessary, we can cast them to whatever we need Assuming the cast is legal, of course What about the following? c has the unknown element type ? The type to be inserted must conform to ? –The compiler does not know the actual type of ? –It cannot know if adding an Object will be OK –You cannot add an Object to a List ! –The only possible element to be added is null 21 Collection c = new ArrayList (); c.add(new Object()); The compiler will not accept this code!

22. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T18 A Generic Drawing Application Assume the following model of a drawing application: 22 abstract class Shape { abstract void draw(Canvas c); } class Circle extends Shape { private int x, y, radius; void draw(Canvas c) { //... } class Rectangle extends Shape { private int x, y, width, height; void draw(Canvas c) { //... } class Canvas { void draw(Shape s) { s.draw(this); }

23. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T18 A Generic Drawing Application Each shape extends class Shape and can draw itself We will usually have more than one shape to draw We can keep the shapes in a List We modify the code of Canvas accordingly: This works fine for any List What happens if we want to draw a List ? –Circle is a subtype of Shape –But List is not a subtype of List –Calling drawAll(List ) results in a compile error 23 void drawAll(List shapes) { for (Shape s: shapes) { s.draw(this); }

24. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T18 A Generic Drawing Application Our method drawAll can work on all Shape subtypes It should be able to handle List, List, List, …! Using ? will not work –The unknown type ? does not have a method draw(Canvas) –Casting to Shape is possible but dangerous We need to restrict (bound) the wildcard –It should accept Lists of all subtypes of Shape Java notation: 24 void drawAll(List shapes) { for (Shape s: shapes) { s.draw(this); }

25. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T18 Bounded Wildcards There is small but very important difference –List accepts only exactly List –List accepts lists of any subtype of Shape, including Shape itself List … ? extends X means: –We do not know the exact type ( ?) –But we know it must be a subtype of X –X is the upper bound of the wildcard 25

26. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T18 Bounded Wildcards What is wrong with the following code? We cannot add a Rectangle to a List … –We do not know the exact type of the elements in shapes –We know that they will be a subtype of Shape –But the type may not be a supertype of Rectangle E.g., we could call addRectangle(List ); We can not write to the list if the type is unknown –Bad news on the one hand –No type problems at run-time on the other hand –Java opts for the type-safe way 26 void addRectangle(List shapes) { shapes.add(0, new Rectangle()); } The compiler will not accept this code!

27. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T18 Writing Generic Methods We want to copy all array elements into a Collection We cannot use Collection as a parameter However, Collection will not work, either –The type is unknown, so any given type may not match 27 static void copyToCollection(Object[] array, Collection c) { for (Object o: array) { c.add(o); } The compiler will not accept this code!

28. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T18 Writing Generic Methods We use a generic method for this purpose: The declares this to be a generic method –T is introduced before the return type T is used as the formal parameter type The Java compiler can use this for checking accesses –The types of the array and the collection must conform –It uses the most specific type argument that matches In many cases, wildcards are sufficient –Wildcards are preferred over generic methods – clearer and more concise 28 static void copyToCollection(T[] array, Collection c) { for (T o: array) { c.add(o); }

29. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T18 Checking the Class Type We can query the basic type of a generic type But we cannot query the precise generic type –Remember that there is only one class List (see slide 15) –The compile-time information about the formal parameter is not available at runtime We adapt the statement to use getClass(): The output is true – getClass() returns java.util.List 29 List circles = new ArrayList (); List rects = new ArrayList (); System.out.println(circles instanceof List); // true System.out.println(circles.getClass() == rects.getClass()); System.out.println(circles instanceof List ); // error

30. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T18 Lower Bound Wildcards Assume that we want to implement a sink –It flushes all collection elements and returns the last element What is the generic type argument ? –There is no legal type, as the types do not match –The call in the last line is illegal 30 interface Sink { void flush(T t); } static T flushAll(Collection c, Sink sink) { T last = null; for (T t: c) { last = t; sink.flush(last); } return last; } Sink sink; Collection collection; String lastString = flushAll(collection, sink); The compiler will not accept this code!

31. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T18 Lower Bound Wildcards This seems easy to fix…: The method call now works fine However, T is now matched to Object –Because this matches the element type of the Sink –Therefore, the assignment of the return value to String fails We need to express unknown, but super type of T Java notation: 31 static T flushAll(Collection c, Sink sink) { static T flushAll(Collection c, Sink sink) {

32. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T18 Implementation Example 32 interface Sink { void flush(T t); } class ConcreteSink implements Sink { public void flush(T t) { System.err.println("Flushing " + t + ", type: " +t.getClass().getName()); } static T flushAll(Collection c, Sink sink) { T last = null; for (T t: c) { last = t; sink.flush(last); } return last; } Sink sink = new ConcreteSink (); Collection cs = Arrays.asList("a","bb2","cdf"); System.err.println(flushAll(cs, sink)); We have omitted the necessary class context to save space…

33. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T18 Static Typing Summary Static type systems are still topic of active research Due to generics and wildcards, the Java type system is relatively expressive but also pretty complicated Some programmers see static type systems as a restriction of their freedom ("I know what I am doing") Other programmers think that static type systems enforce a good structure of your code ("think before you write") The debate is not yet settled You should have an educated opinion about it! 33

34. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T18 Related Publications See the following materials for more on Generics: –Java Tutorial on Generics (by Gilad Bracha): http://java.sun.com/docs/books/tutorial/extra/genericshttp://java.sun.com/docs/books/tutorial/extra/generics/ –Gilad Bracha, Generics in the Java Programming Language Used as the main reference for these slides http://java.sun.com/j2se/1.5/pdf/generics-tutorial.pdf –Maurice Naftalin, Philip Wadler: Java Generics and Collections, OReilly, 2006 34