Generics Doubt

[Balaji Bang]

19)Which call(s) to method(addandDisp) are error free? From JavaBeat

public static <T> void addandDisp(Collection<T> cs, T t)
{
for (T o : cs)
{
s.add(o);
}
for (T o : cs)
{
System.out.println(o);
}}
//call 1
List<? super Object> ls1 = new LinkedList<Object>();
addandDisp(ls1,new String());
//call 2
List<? extends Object> ls2 = new LinkedList<Object>();
addandDisp(ls2,new Object());
//call 3
List<Object> ls3 = new LinkedList<Object>();
addandDisp(ls3,new String());
//call 4
List<? super Object> ls4 = new LinkedList<Object>();
addandDisp(ls4,new Object());

a)only 3
b)only 1 and 3
c)only 1 and 4
d)only 1,2 and 3
e)only 1,3 and 4

Given Answer : e
My Doubt : public static <T> void addandDisp(Collection<T> cs, T t)

Here Collection is of type T and second parameter also T. So I thought only 4 is Correct.

List<? super Object> ls4 = new LinkedList<Object>();
addandDisp(ls4,new Object());

How can we send this :

List<Object> ls3 = new LinkedList<Object>();
addandDisp(ls3,new String());

Her els3 of type Object means in the method addandDisp() first parameter is T holds Object , How come second Orameter T holds String???

[Henry Wong]

Her els3 of type Object means in the method addandDisp() first parameter is T holds Object , How come second Orameter T holds String???

A String IS-A Object.

Henry

[Balaji Bang]

Explanation given is : Method call 2 cannot work,because the compiler cannot gurantee the type to
substitute.But in all other calls it is guranteed.

I didn't understand this ..... Please..

[Henry Wong]

Banu Chowdary wrote:Explanation given is : Method call 2 cannot work,because the compiler cannot gurantee the type to
substitute.But in all other calls it is guranteed.

I didn't understand this ..... Please..

With the second option, T is an unknown type that may extends Object. T may not be an Object type, as required by the second parameter.

Henry