Generics

[Anil Pradhan]

I have declared a method as shown below.
public static <T> void addandDisp(Collection<T> cs, T t) {
Set s = new HashSet();
for (T o : cs){
s.add(o);
}
for (T o : cs){
System.out.println(o);
}
}

which of the following calls are compiler error free?
//call 1
List<? super String> 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());

Answers with explanations will be well appreciated!

[Deepak Jain]

Lets take one by one
// Call 1
List<? super String> ls1 = new LinkedList<Object>();
Here you are defining T as <? super String> which means generic type 'T' is of type String or any super class of String which is Object and hence T is of type String, Object.
addandDisp(ls1, new String(""));
Now ls1 can be assigned to cs of method addandDisp(), Since the generic type T can be String or Object, when you create new String(""), as you know it can be assigned to a reference of String and/or to a reference of type Object and hence this invocation is compiles fine.

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

Here you define the generic type T as <? extends Object> which means T can be Object or anything that extends class Object which as you know can be all the known java classes to man-kind. So now you can create a list of any type and assign it to ls2 , say you created a list of type String which can be assigned to ls2 but then the second argument new Object cannot be assigned to T since T is of type String and hence compilation fails here.
To compile the code you will need to change the code as follows
public static <T,T2> void addandDisp(Collection<T> cs, T2 t1) {

//call 3
List<Object> ls3 = new LinkedList<Object>();
addandDisp(ls3,new String());
Here T is of type Object and ls3 can be assinged to first argumnet of method and new String() can again be assigned to second Object reference.

//call 4
List<? super Object> ls4 = new LinkedList<Object>();
addandDisp(ls4,new Object());
Here ? super Object == Object and hence the above explanation is valid.