Generic method add query

[ahmed yehia]

    
import java.util.*;
 
public class Main {
public static void main(String[] args) {
List<Number> list = new ArrayList<Number>();
add(list);
}
 
public static <E extends Number> void add(List<E> list) {
       list.add(42); // error can't add
}
}

I m testing this generic method, it works well but when add something to the List it reports an error.. What is the reason?

Thanks

[al nik]

public static <E extends Number> void add(List<E> list) {

says "I accept a list of type E that 'extends Number'
so you can call this method with something like

List<Integer> listInt = new ArrayList<Integer>();
add(listInt);
or
List<Long> listLong = new ArrayList<Long>();
add(listLong);

<E extends Number> don't let you to put what you want in the List<E> simply because the compiler has no way to resolve what's the type at compile time and you can corrupt your List.
Think about if the add() method add a new Float() to your type safe 'listInt'... that's no good
you can add only an E obj..
something like this
E e = list.get(0);
list.add(e);
not so useful
correct me if I am wrong :roll:
[ October 22, 2007: Message edited by: al nik ]