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Generics

 
ujjawal rohra
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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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Welcome to JavaRanch.
Please UseCodeTags when you post a code at JavaRanch. Please edit your post and add code tags.
Not related to Servlets - Moving to BG...
 
Devaka Cooray
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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
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Consider what would be if this assignment was legal:

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.
 
It is sorta covered in the JavaRanch Style Guide.
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