Generics

[ujjawal rohra]

import java.util.*;
class exp{

public static void main(String ar[])
{
List <? extends Number>l1=null;
List<? super Number> l2=null;
l2=l1;
}
}

Why does this code not work?

[Devaka Cooray]

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[Devaka Cooray]

It is because the generics-scope of l1 and l2 do not match.
Please read GenericsSuperAndExtends to understand the scope of 'extends' and 'super'.

[Ireneusz Kordal]

Consider what would be if this assignment was legal:

List <? extends Number>l1=new List<Number>();
List<? super Number> l2=null;
l1.add(new Integer(2)); // at index 0 - Integer is ok, because it is subclass of Number
l2=l1;  // assume this is legal - now both l1 and l2 reference the same list
l2.put(0, new Object()); // replace Integer(2) on the list. Object is ok, because l2 contains "super Number"
Number i = l1.get(0); // <-- this must cast ClassCastException, because list at index 0 contains Object, not Number

In this code in line 6 we get Object from the list that is declared that can contain only Numbers.
This breaks program with ClassCastException.
To avoid situation like this, compiler prevents assignment in line 04.