Generic methods doubt

[Andry Dub]

Hi! Please explain me why I can't invoke addandDisp with parameters ls1 and object of type Object? What second parameter is allowed with first parameter will be ls1?

public class TestSamp {
 
    public static void main(String[] args) {
        List<? super Object> ls1 = new LinkedList<Object>();
        addandDisp(ls1,new Object());
    }
 
    public static <T> void addandDisp(Collection<T> cs, T t) {
        
    }
}

[Sandip Sarkar]

hi,

Have a look at the below forum discussion---

web page

i hope it will answer you.

thanks.

[Nitesh Kant]

mmm, i am not sure what you wish to intend by List<? super Object>
This means either a superclass of Object(No class qualifies this rule) or the Object itself.
Since, there is no superclass of Object, its as good as saying List<Object>
If you do the above, everything should work fine.

[Kelvin Chenhao Lim]

Hi Andry,

I'm not sure what the issue here is. The code you provided compiles fine.

[prajal Mehta]

Hi kevin,

That code is from this site- http://www.javabeat.net/javabeat/scjp5/mocks/Generics-part2-7questions.php
question no. 3

  3)Which call(s) to method(addandDisp) are error free?
 
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
  

Answer is 3)e)
(call 2)Method call 2 cannot work,because the compiler cannot gurantee the type to substitute.But in all other calls it is guranteed.    

[prajal Mehta]

I am not sure if the answer given is correct.
Please somebody explain the correct answer to me ..

[Alexsandra Carvalho]

I really dont understand why this does not compile:

//call 2
List<? extends Object> ls2 = new LinkedList<Object>();
addandDisp(ls2,new Object());