Software Engineering. Prof. Agostino Poggi

Transcription

1 Agent and Object Technology Lab Dipartimento di Ingegneria dell Informazione Università degli Studi di Parma Software Engineering Generics Prof. Agostino Poggi

2 public class ListOfIntegers { //... fields... public void add(int i) { public int get(int index) { //... other methods... public class ListOfStrings { //... fields... public void add(string s) { public String get(int index) { //... other methods... List Class Solution I public void main(string[] args) { ListOfIntegers integers = new ListOfIntegers(); ListOfStrings strings = new ListOfStrings(); //... method code... 2

3 public class List<T> { //... fields... public void add(t t) { public T get(int index) { //... other methods... List Class Solution II Generics parameters are added in class declaration public void main(string[] args) { List<Integer> integers = new List<Integer>(); List<String> strings = new List<String>(); //... method code... Generics parameters are replaced by the specific type 3

4 public void main(string[] args) { List<Integer> integers = new List<Integer>(); List<String> strings = new List<String>(); List<Object> objects = strings; objects.add(new Object()); String s = strings.get(0); g //... method code... Public void printlist(list<object> l) { for (Object e : l) { System.out.println(e); Public void printlist(list<t> l) { for (T e : l) { System.out.println(e); Public void printlist(list<?> l) { for (Object e: l) { System.out.println(e); Subtyping This assignment is not permitted!! Tries to read a string, but is an object! A list of strings is not a subtype of a list of objects This method cannot be applied to all the kinds of list! This method cannot be defined because a generic class cannot be a parameter type! This method can be applied to all the kinds of list! But (without a cast) the elements of the list can be only used as objects of the Object class 4

5 Generic Methods Public void printlist(list<?> l) { for (Object e : l) { System.out.println(e); e.hashcode(); Generics parameters must be inserted before return type Public <T> void printlist(list<t> l) { for (T e: l) { System.out.println(e); But also in this case, (without a cast) the elements of the list can be only used as objects of the Object class 5

6 Wildcards (1/2)? Unbounded wildcard: all types? extends Supertype Lower-bound: all types that are subtypes of Supertype? super Subtype Upper-bound: all types that are supertypes of fsubtype 6

7 Wildcards (2/3) Public void drawlist(list<shape> l) { for (Shape e : l) { e.draw(); Public void drawlist(list<?> l) { for (Shape e : l) { e.draw(); The method is also applied to the objects of the classes that do not implement the Shape interface! The method is applied to the objects of the Shape class and its descendents Public void drawlist(list<? extends Shape> l) { for (Shape e : l) { e.draw(); Public void drawlist(list<? super Shape> l) { for (Shape e : l) { e.draw(); The method is applied to the objects of the Shape class and its parents The Object class is an its parent, but, of course, it does not provide the Shape interface! 7

8 Wildcards (3/3) public class Integer extends Number implements Comparable<Integer> { //... fields and methods... public class BigInteger extends Number implements Comparable<BigInteger> { //... fields and methods... public class BiggerInteger extends BigInteger { //... fields and methods... This method cannot be applied to the objects of BiggerInteger class! public static <T extends Comparable<T>> T max(list<t> c) { //... method code... This method can be applied to the objects of BiggerInteger class public static <T> T max(list<? extends Comparable<? super T>> c) { //... method code... 8

9 Generic Implementation (1/2) List <String> l1 = new ArrayList()<String>(); List<Integer> l2 = new ArrayList<Integer>(); System.out.println(l1.getClass() p g == l2.getclass()); The printed value is true: they are the same! There eeis sonly yone ecompiled pedversion eso of age generic e ccasso class or function regardless of the number of parameterized types used Collection cs = new ArrayList<String>(); if (cs instanceof Collection<String>) { //... block code... Collection<String> cstr = (Collection<String>) cs; List<String>[] lsa = new List<String>[10]; Public <T> T create() { return new T(); The element of an array object cannot be defined by a parameterized type! There is not a specific constructor! Checking if an object is an instance of a particular invocation of a generic type has no sense!! Compiler gives an unchecked warning because the runtime system will have not information about the parameterized type 9

10 Generic Implementation (2/2) List final class Test { public static void main (String[ ] args) { List<String> ys = new List<String>(); ys.add("zero"); ys.add("one"); String y = ys.get(0); List final class Test { public static void main (String[ ] args) { List ys = new List(); ys.add("zero"); ys.add("one"); String y = (String) ys.get(0); Generics parameter is removed Appropriate casts are inserted 10