asList() doubt

[sharan vasandani]

Integer i={3,5}
List l=Arrays.asList(i);
l.set(1,9);

asList method returns list,but list is a interface then how can we invoke a method set on it.

and why we cannot invoke add method on the same list,and if we use add method it compiles well but with some warnings and then it fails at runtime.

please explain.

[Chandra Bhatt]

Hi Saran,

Integer i={3,5}
List l=Arrays.asList(i);
l.set(1,9);

asList method returns list,but list is a interface then how can we invoke a method set on it.

Think about the line:
List<String> alist = new ArrayList<String>();
alist.add("Hello");
alist.add("howdy");
alist.add("hi");
alist.set(1,"rowdy"); // Can we do this? Absolutely you can


Now your question:
When you are using set(), it is not going to add a new object to the list,
it is just modification, but add() will add a new object so this is not allowed. What asList() returns is fixed List.

java.lang.UnSupportedOperationException is thrown at run time when you apply
add() method to it.

Note: What warning you are getting is not due to the add() method.
Actually your List reference "l" is raw type. It should be parameterized
as :
List<Integer> l1;

Regards,
cmbhatt
[ April 11, 2007: Message edited by: Chandra Bhatt ]

[sharan vasandani]

ok,

but list is a interface how does it have a method set then?
thnks

[Chandra Bhatt]

List is an Interface and has set() method but it is defined by concrete classes as ArrayList.

Your doubt will disappear when you see the output of the following code:

Integer[] i1  = {12,546,6};
List<Integer> L1 = Arrays.asList(i1);
L1.set(1, 555);

System.out.println(L1.getClass()); /*Output of this line should open your eyes   :wink:   */

Any Doubt???

Saran, I liked your question!!! Keep it up!

Regards,
cmbhatt
[ April 11, 2007: Message edited by: Chandra Bhatt ]