generics - why does this work?

[Lighted Jones]

Why does the code below generate the following output?
md=[cereal.D@126b249, cereal.D@182f0db]
class type of duane is class cereal.D
wierdGen=cereal.D@126b249

The List called duane is typed to A but since the reference variable is ? super D its allowing me to successfully add D type objects to the List thats typed to A?

Shouldn't I get a runtime error when I add the D object into the List?

  
package cereal;
import java.util.*;
 
class A { void ameth() { System.out.println("ameth");}}
class B extends A { void bmeth() { System.out.println("bmeth");}}
class C extends B { void cmeth() { System.out.println("cmeth");}}
class D extends C { void dmeth() { System.out.println("dmeth");} }
 
public class Generic 
{
   List<? super D> duane = new ArrayList < A >();
   
   public static void main( String args[] )
   {
      Generic manzer = new Generic();
      manzer.duane.add(new D());
      manzer.duane.add(new D());
      
      System.out.println("class type of duane is " + manzer.duane.get(0).getClass());
   
      wierdGen(manzer.duane);
            
   }
 
   
   public static <T> void wierdGen(List<? super D> typ) {
 
   D d = new D();
   
   typ.add(d);
   System.out.println("wierdGen=" + typ.get(0));
      
   
   }
}
  

Output is
md=[cereal.D@126b249, cereal.D@182f0db]
class type of duane is class cereal.D
wierdGen=cereal.D@126b249
[ July 08, 2007: Message edited by: Lighted Jones ]

[Keith Lynn]

But a D object is an A object.

[Lighted Jones]

yes but generic types must be adhered to strictly, unlike for arrays.

for example in an

A [] arrayA = new [] A ;

In the case above we can add objects of type A, B, C , D to arrayA.

But for
List<A> la = new ArrayList<A>();

we cannot add B,C, or D objects to la because generic types are strict.

Therefore in the example I posted shouldn't List duane reject any non A additions?

or am I wrong?

[Manfred Klug]

Originally posted by Lighted Jones:
But for
List<A> la = new ArrayList<A>();

we cannot add B,C, or D objects to la because generic types are strict.

You can. What you can't do is something like this:
List<A> la = new ArrayList<B>();
Which will work with arrays without problems.
A[] a = new B[10];

[Keith Lynn]

Also consider the case of a List<Object>.

What kind of elements can you add to the list? Anything that is an Object.

[Lighted Jones]

Right......

Yes I see what you mean..

Thanks guys.

[aslika bahini]

md=[cereal.D@126b249, cereal.D@182f0db]

may i know what is the above one in the output.




Thanks
samura

[aslika bahini]

originally posted by Lighted Jones:

List<? super D> duane = new ArrayList < A >();

we can add anything which is super of D. it is declared in the LHS of duane. what about the actual object side(RHS) ArrayList<A>() means what?
why we need to say this? when we can add all super of D and even retrieve it without run time exception.

Thanks
samura

[Manfred Klug]

Originally posted by samura babu:
we can add anything which is super of D.

That is not correct. The super keyword refers to the lists that can be used. Have a look at the following sample.

import java.util.*;
 
class A { }
class B extends A { }
class C extends B { }
class D extends C { }
 
public class Test {
    private static List<D> list = new ArrayList<D>();
 
    public static List<? super D> getList() {
        return list;
    }
 
    public static void main(String... args) {
         List<? super D> duane = getList();
         duane.add(new C());
    }
}

As you can see, it would be easy to add an element of the wrong type. The only type that is valid for all possible lists is D.

[Jesper de Jong]

Dear Lighted Jones,

I am one of the moderators of the JavaRanch forums. Here on JavaRanch, we have a naming policy. We want the users of JavaRanch to use a real name as their display name.

Unfortunately, your name "Lighted Jones" does not comply to the naming policy. Your name should consist of a first name, space, and second name and it should not be an obviously fake name.

Please read the naming policy carefully and change your display name. You can change your name by editing your profile.

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