<? super Integer>

[Balaji Bang]

From JavaBeat

	List <? super Integer> al = new ArrayList<Number>(); //line 1
		Integer i1=12;
		al.add(12); //line 2
		al.add(12+ 13); //line 3
		al.add(i1);
		for (Number no:al) //line 4
		{
		System.out.println(no);
		}

At Line 4 it is giving copiler error. Here List <? super Integer> means it takes Integer or its super type. But we can add only Integers to this List al. Then we know that elements from this list will have only Integers. Why it is allowing only for(Object : al) in Line 4

[Henry Wong]

At Line 4 it is giving copiler error. Here List <? super Integer> means it takes Integer or its super type. But we can add only Integers to this List al. Then we know that elements from this list will have only Integers. Why it is allowing only for(Object : al) in Line 4

Actually, no.... <? super Integer> means that it is an unknown type, but can be Integer or its super type. We can add only Integers to this List, because that is the only way to satisfy that it be all the possible unknown types.

You don't know what type of elements are actually in the list -- hence, you don't know if the elements are actually Number types. The only thing that you do know is that the elements are Object types.

Henry

[K. Tsang]

You know what <? super Integer> means? How about <? extends Number>?

[Henry Wong]

Another way to look at it....

List <Object> al0 = new ArrayList<Object>(); 

List <? super Integer> al = al0; 

Just because you can only add integer types to the list using the "al" reference, doesn't mean that there are only Integers in the list. In the example above, we can add other object types into the list, using the "al0" reference instead.

Henry

[Balaji Bang]

if List <? super Dog> dogs=new ArrayList<Animal>();
here also we should have only Object not Animal Or Dog???

[Punit Singh]

Banu Chowdary wrote:if List <? super Dog> dogs=new ArrayList<Animal>();
here also we should have only Object not Animal Or Dog???

Anything super to Dog:

  List <? super Dog> dogs=new ArrayList<Dog>();

 List <? super Dog> dogs=new ArrayList<Animal>();

 List <? super Dog> dogs=new ArrayList<Object>();

  

[Henry Wong]

Punit Singh wrote:

Banu Chowdary wrote:if List <? super Dog> dogs=new ArrayList<Animal>();
here also we should have only Object not Animal Or Dog???

Anything super to Dog:

  List <? super Dog> dogs=new ArrayList<Dog>();

 List <? super Dog> dogs=new ArrayList<Animal>();

 List <? super Dog> dogs=new ArrayList<Object>();

  

Yes, you can assign the list to any list type of Dog or anything super to Dog... but I believe the question was refering to the elements, which is.... yes, when you iterate through the Dogs list, the element type is assumed to be Object. (You will need to cast for anything else).

Henry