Collection addAll method Strange Behavior

[Anand Wadhwani]

Hi,
I am preparing for SCJP 1.4. Sunday is my exam!

I came across following question on collection addAll method:

What is the result of executing the following fragment of code:

import java.util.*;
public class ListTest
{
   public static void main(String args[])
   {
      List s1 = new ArrayList( );
      s1.add("a");
      s1.add("b");
      s1.add("c");
      List s2 = new ArrayList(  s1.subList(1, 1) ); // [b]
      s1.addAll(s2); // 3
      System.out.println(s1);
   }
}
 
a  The sequence a, b, c is printed.   
b  The sequence a, b, c, b is printed.   
c  The sequence a, c, b, c  is printed.   
d  The sequence a, c, b is printed.   
e  None of the above. 

The answer to above question is d. Which indicates that addAll does not adds a value if that is already present in collection.

Is this correct understanding? Shouldn't this be behavior of only Set collection? What all collection addAll method behaves in similar way?

Now here is bigger problem in my head:

if I replace line //3 addAdd above with following two lines:

s2.add("a");// 1
s1.addAll(2, s2);// 2

then the output of program is:

[a, b, a, c]

now here addAll is adding same element !!

I am confused, please help!
TIA!

[Animesh Shrivastava]

If u read the api for the subList method, it says:

Returns a view of the portion of this list between the specified fromIndex, inclusive, and toIndex, exclusive. (If fromIndex and toIndex are equal, the returned list is empty.)

If the fromindex and toIndex are same, the list returned is empty.
So, the statement:

  
List s2 = new ArrayList(  s1.subList(1, 1) ); 

will return u an empty list
Got it.