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Generics  RSS feed

 
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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?
 
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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'.
 
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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.
 
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