COMP-202. Generics. COMP Generics, 2013 Jörg Kienzle and others

Transcription

1 COMP-202 Generics

2 Objects and Casting Generics Instantiation Usage ArrayList<E> Iterators Iterator<E> Lecture Outline 2

3 Objects and Casting All instances of all classes in Java are also of the class Object That s why a class like ArrayList takes Objects as parameters, i.e. instances of any class But if you insert a String, how do you get back a String? ArrayList alist = new ArrayList(); alist.add("foo"); String s = alist.get(0); // error The result of get is of type Object, so we need to cast the result value back to its original class, i.e. String String s = (String)aList.get(0); 3

4 More on Objects and Casting If we insert a cast to the wrong type we get an error: ArrayList alist = new ArrayList(); alist.add( foo ); Cat c = (Cat)aList.get(0); // exception The above results in ClassCastException being thrown Casting problems are common runtime errors Wouldn t it be nicer if the compiler could check that only the same type of elements are inserted and taken out of this particular ArrayList? 4

5 Generics Generic types (also known as parameterized types, or templates) make it possible to statically customize an implementation of a generic abstraction to work with specific types The substitution of the generic element with the specific type uses a special syntax 5

6 Using Generics (1) So how can we configure an instance of ArrayList to contain only Strings? The (specific) element type goes in angle-brackets with the collection type ArrayList<String> listofstrings; The element type must also be specified in the new expression: listofstrings = new ArrayList<String>; 6

7 Separating Apples from Oranges (1) Define two ArrayList objects, one for containing apples and one for containing oranges ArrayList<Apple> listofapples; ArrayList<Orange> listoforanges; The element type must also be specified in the new expression: listofapples = new ArrayList<Apple>(); listoforanges = new ArrayList<Orange>(); Now the compiler knows listofapples only accepts Apple objects, and listoforanges only accepts Orange objects 7

8 Separating Apples from Oranges (2) No type-casts are needed: listofapples.add(new Apple); Apple firstapple = listofapples.get(0); Type safety is ensured at compile-time Easier to find bugs than with runtime exceptions listofapples.add(new Orange); 8

9 The ArrayList<E> Class - ArrayList<E>() - boolean add(e obj) - void add(int index, E obj) - E remove(int index) - E set(int index, E obj) - void clear() - boolean contains(e obj) - int indexof(e obj) - E get(int index) - boolean isempty() - int size() - Iterator<E> iterator() - ListIterator<E> listiterator() 9

10 Iterators All Java collection classes provide an iterator() method that returns an Iterator instance The iterator allows you to access each element in the collection, one at a time public interface Iterator<E> { boolean hasnext(); E next(); void remove(); } 10

11 Using an Iterator to Traverse a Collection ArrayList<String> names = new ArrayList<String>(); names.add( Mira Grace ); names.add( Enya Linnea ); names.add( Elian Fox ); Iterator<String> i = names.iterator(); while (i.hasnext()) { String element = i.next(); System.out.print(element); } 11

12 Exercise Implement the classes Student and Course int number String title Course Course(int n, String t) int getnumber() String gettitle() void registerstudent (Student s) 0..* classmember coursetaken 0..* Student String name Student(String n) String getname() void registerforcourse (Course c) 12