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Regarding generics

 
Greenhorn
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One question from javabeat.net

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());

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

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 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.

My doubt is how we can add String in call1.
 
Ranch Hand
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Because in that case declaring the type in the List to be ? super Object, means Object or a superclass of Object.

Since there is no superclass of Object, the only type that can be in the List is Object.
 
Rajeswari Kumar
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Thanku. Sometimes iam really get confused
 
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