Wildcards and assignment

[ramesh maredu]

I was reading about generics in below link

Wild Cards

according to link above

  
 
List<? extends Integer> listExtendsInteger=new ArrayList<Integer>();
Collection<? extends Number> collectionExtendsNumber=new ArrayList<Number>();
collectionExtendsNumber=listExtendsInteger;
 

above code compiles fine, the answer to it from the link is as below

"List of Integer types can be treated as some Collection of Number types through the wildcard instantiation. If you think about it, you'll see that there is no conflict here. A List is certainly a Collection. And all we're doing is widening the type by which we can read the elements from Integer to Number".

if so why below code does not compile

 
List<Integer> listIntegers=new ArrayList<Integer>();
Collection<Number> collectionNumbers=new ArrayList<Number>();
collectionNumbers=listIntegers;

How wildcards can make difference here.

[Daniel Del Moral]

Generics types must be the same. Period. You can't do this:

List<Number> = new ArrayList<Integer>();

It only works with

List <Number> = new ArrayList<Number>();
or
List<Integer> = new ArrayList<Integer>();

It might be strange because you CAN do it with arrays:

Number[] num = new Integer[5];

Well, the explanation is a little long, but short: Generics verification only occurs at compiling time. At runtime, java use these classes as if they were no generic restriction. Array does use types at runtime.

So, why is it important?

What happens if I do this?

List <Number> = new ArrayList<Integer>;

List.add(new Float(2.1));

See what happened? It should be legal, but it is wrong. At runtime, java won't notice it when you add it, but it will explode when you retrieve it.

When you use wildcard using extends, you're saying that you won't add anything to the list, so you can assign it. By the way, you can add items if you're using wildcard with super, just think about why.

Test what happens using arrays:

Number [] num = new Integer[4];
num[0] = new Float (2.1);

It will explode at runtime!

[ May 24, 2008: Message edited by: Daniel Del Moral ]
[ May 24, 2008: Message edited by: Daniel Del Moral ]

[ramesh maredu]

Hi Daniel

thanks for your fast and clear reply.

I read K & B book before from that i understood that polymorphic assignment applies to collection type or base type not to generic type.

but i was surprised to see that it is not same with wildcards,now i understood from your explanation that just because i am using extends it is allowing me to compile.

i tried with super it is giving me compile time error

  
List<? super Integer> listSuperIntegers=new ArrayList<Integer>();
Collection<? super Number> collectionSuperNumbers=new ArrayList<Number>();
collectionSuperNumbers=listSuperIntegers;

[ May 24, 2008: Message edited by: ramesh maredu ]

[Daniel Del Moral]

That's because Integer is not a superclass of Number. Try this:

List<Number> listSuperIntegers=new ArrayList<Number>();
Collection<? super Integer> collectionSuperNumbers=new ArrayList<Number>();
collectionSuperNumbers=listSuperIntegers;

[Daniel Del Moral]

You can also try:

List<? super Number> listSuperIntegers=new ArrayList<Number>();
Collection<? super Integer> collectionSuperNumbers=new ArrayList<Number>();
collectionSuperNumbers=listSuperIntegers;

[ramesh maredu]

i think the example i gave with super is not compiling because


Collection<? super Number> can take Number or super types of Number but it can not take Integer.

[Santosh Arun Kumar Gudipati]

Hi Ramesh..i am preparing for SCJP 1.5.

[Santosh Arun Kumar Gudipati]

Hi Ramesh..i am also preparing SCJP 1.5
I want your advice on this regarding what are the inportant topics to be covered that appear mostly in the exam.
I will also appreciate if you can provide me with good URL's and e-books.
My gmail id is [email protected].
Thanks in advance.