Thread: Question on Set

In a set if we have 5 objects {'a','v','d','w','r'}
And if i add one more same element say 'd', will the earlier 'd' object be retained in Set or it will be replaced by new 'd' object.

Hello tcstcs!
The javadoc for Set.add(E e) says;
"Adds the specified element to this set if it is not already present (optional operation). More formally, adds the specified element e to this set if the set contains no element e2 such that (e==null ? e2==null : e.equals(e2)). If this set already contains the element, the call leaves the set unchanged and returns false. In combination with the restriction on constructors, this ensures that sets never contain duplicate elements."

From what I get from the above, I would answer to your question that the new 'd' will never be added to the set, therefore the earlier 'd' will be retained.
You could also take a look at an interesting discussion about duplicates in sets that I found.
Hope it helps.

HashSet is implemented with HashMap. So in HashMap, if i add element with existing key the older value gets replaced with new one. Then in set as its based on HashMap, older value should be replaced with new one rite?

with the program, we will not be able to see the results. Reason being
If set has {a,s,d,f}
I will add 's' to set and after addition it will be {a,s,d,f}
So we will not know whether its prev 's' or new 's'.

I wrote this program but its giving o/p as false,false only without duplicates.
public static void main(String[] args)
{
HashSet hh = new HashSet();
String aa = new String("a");
String bb = new String("b");
hh.add(aa);
hh.add(bb);

Iterator it = hh.iterator();
while(it.hasNext())
{

System.out.println("---"+aa==it.next());
}

}

IF you make a set of with Set<object> declaration u need to make sure that Class(for object) overrides the hash codes and equals method correctly else duplicates will be allowed. when you are working with Set<String> it will not allow you to add "A" or "D" two times as the String class has hashcode and equals method implemented.