Doubt with List assignment using Generics

[Satya Maheshwari]

Please see he code below:

class Gen1
{
    
}
 
class Gen2 extends Gen1
{
    
}
 
class Gen3 extends Gen2
{
    
}
 
public class GenericsDemo {
    public static void main(String[] args) {        
        List<? extends Gen2> list1 = null;
        List<? super Gen2> list2 = null;
 
        List<Gen2> list3 = (List<Gen2>  )  list1;     //Compiles OK 
        List<Gen1> list4 = (List<Gen1>  )  list1;     //Error. WHY??
        List<Object> list5 = (List<Object>  ) list2;  //Compile OK
        List<Gen1> list5 = (List<Gen1>  ) list2;      //Error OK
    }
 
}

Why is List<Gen1> list4 = (List<Gen1> ) list1; above, a compilation error?
[ November 21, 2008: Message edited by: Satya Maheshwari ]

[Ankit Garg]

see this example to see why compiler disallows this

class Gen1
{}
[B]class MyGen extends Gen1
{}[/B]
class Gen2 extends Gen1
{}
class Gen3 extends Gen2
{}
public class GenericsDemo
{
    public static void main(String[] args)
    {        
        List<? extends Gen2> list1 = new ArrayList<Gen2>();
        List<Gen1> list2 = (List<Gen1> )  list1;     //if this was allowed then there will be problems as shown below
        list2.add(new MyGen);   //the actual list is of type Gen2 but we added it's sibling into the list
        Gen2 obj = list1.get(0);  //ClassCastException as it will try to cast MyGen into Gen2 which are siblings
    }
 
}

[ November 21, 2008: Message edited by: Ankit Garg ]

[ramesh maredu]

List<Gen1> list4 = (List<Gen1> ; ) list1; //Error. WHY??

List<? extends Gen2> list1 = null;

You may be thinking that since Gen2 is a subclass of Gen1 it should compile, if so. See below code and think why it won't compile.

List<Object> list = new ArrayList<String>();

[Bert Bates]

Please tell us the author of this code!

Thanks!
[ November 21, 2008: Message edited by: Bert Bates ]

[meera kanekal]

In the above code why won't the following compile?
List<Gen1> list5 = (List<Gen1> ) list2; //Error
List<? super Gen2> list2
because list2 is bounded by Gen2 and its super classes. Gen1 is super for Gen2 and you should be able to assign list2 to list5.
Kindly explain.
Thanks,
Meera

[Satya Maheshwari]

Please tell us the author of this code!

I myself wrote this code, though I think using this anywhere would be a crime The sole purpose is to teach myself generics.

[Satya Maheshwari]

Originally posted by meera kanekal:
In the above code why won't the following compile?
List<Gen1> list5 = (List<Gen1> ) list2; //Error
List<? super Gen2> list2
because list2 is bounded by Gen2 and its super classes. Gen1 is super for Gen2 and you should be able to assign list2 to list5.
Kindly explain.
Thanks,
Meera

List2 contents are anything which is a superclass of Gen2. If Gen2 were 'Green Apple' and 'Gen1' were Apple, List2 contents would be any kind of 'Fruit', which does not mean that List2 contents are 'Apples' only. Hence its an error. I hope I am not too cryptic here

[Ankit Garg]

@meera Let's take an example to see why the compiler doesn't allow this

import java.util.*;
class Super
{}
class Sub1 extends Super
{}
class Sub2 extends Super
{}
class MainClass
{
   public static void main(String[] args)
   {
      List<? super Sub1> sub1List = new ArrayList<Sub1>();
      List<Super> superList = sub1List;   //if this was allowed then there will be problems 
      superList.add(new Sub2());          //oops we have added sibling of Sub1 into the list
      Sub1 subObj = sub1List.get(0);      //oops ClassCastException as we are trying to convert Sub2 into Sub1
   }
}

So as you can see if the assignment was allowed, then you will have a ClassCastException at your hand. This is the whole thing that we try to avoid by using Generics...