Collection: Set overwrite or reject duplicate value ?

[Rob Kiu]

Hi All,
What is the behaviour of Set when dealing with duplicate values, eg we use method add("One") then add("One") to say a HashSet, does it overwrite the earlier "One" with latest value of "One" or reject the latest "One"?
For comparison, below code shows for Map which does not allow duplicate key, the behavior is overwrite the earlier key, so output is Two for all Map classes.
I am not sure how to write code for Set to verify its behavior. Anyone can help with some code sample for Set similar to verify Map?
Thanks very much,
Rob
credits: original code from Marcus Green 1.4 Mock Question Bank ID 250,
modified with comments

import java.util.*;
public class Slove{
 public static void main(String argv[]){
Slove sl = new Slove();
sl.go();    }
    public void go(){
          Map hm = new HashMap();//Output: Two at 1
          /*
          Map hm = new Hashtable();//Output: Two at 1
  Map hm = new TreeMap();//Output: Two at 1
  Map hm = new LinkedHashMap();//Output: Two at 1
  */ 
Integer iwKey = new Integer(1);
hm.put(iwKey,"One");
hm.put(iwKey,"Two");
Iterator it = hm.keySet().iterator();
while(it.hasNext()){
    Integer i = (Integer) it.next();
    System.out.println(hm.get(i));//1. Output ?
}
      }
} 

[Narasimha Rao B.]

If you try to add duplicate values to the Set, false will be returned.
When ever you add any object to the Set, boolean value will be returned. True will be returned, if it adds successfully and False will be returned, if it could not add the element. So if you are adding the duplicate elements, it will not add and false will be returned.
You can practice my code, which is attached at the bottom.
Comment the line, //(1) and you can see false values, try to add here some distinguish values(Say your name, "Rob Kiu"), you will see 'true' in the output.
---------
See the code.......
----
import java.util.*;
public class SetExample {
public void method(){
// Here i am creating the HastSet object, you can create other Set objects too.
HashSet setColl;
setColl=new HashSet();
for (int i=0;i<4;i++){
// Here i am adding the elements to the set.
setColl.add("SetElement"+i);
// Here i am adding the duplicate entries. But those will not be stored and all the duplicate entries will
// be regected. And boolean will be retruned. To see the same uncomment the below second line, //(1).
setColl.add("SetElement"+i);
System.out.println(setColl.add("SetElement"+i)); // (1)
}
// I am creating the Iterator, so that i can loop through the iterator and can access the elements of the set class.
Iterator it = setColl.iterator();
while(it.hasNext()){
P.p(it.next());
}
}
public static void main(String[] args) {
SetExample obj = new SetExample();
obj.method();
}
}
-------
Let me know if you couldn't understand any thing in the code.
Thanks..

[Rob Kiu]

Narasimha,
You code is clear to me. Thanks very much ! Didn't realise result of method add(element) is a boolean and thus can be used to check.
So now I understand & confirm for myself: Set will neglect (or some mock exam use the term reject) duplicate elements while Map will overwrite duplicate keys with latest one.
Cheers,
Rob.
p.s. Narasimha, hope u don't mind, modify your codes below to form more easily understood by me...hope it'll be of help 2 others as well.

  
import java.util.*;
public class SetExample {
public void method(){
// Here i am creating the HastSet object, you can create other Set objects too.
Set setColl;
setColl=new HashSet();
// Here i am adding the elements to the set., OK: printout: true,true,true,true
for (int i=0;i<4;i++){
System.out.println(setColl.add("SetElement"+i));
}
// Here, adding same (duplicate) elements to set again, rejected (neglected), printout: false,false,false,false
for (int i=0;i<4;i++){
System.out.println(setColl.add("SetElement"+i));
}
// I am creating the Iterator, so that i can loop through the iterator and can access the elements of the set class.
Iterator it = setColl.iterator();
while(it.hasNext()){
System.out.println(it.next()); //Printout elements added, for verification
}
}
public static void main(String[] args) {
SetExample obj = new SetExample();
obj.method();
}
}
/* output:
true
true
true
true
false
false
false
false
SetElement3
SetElement0
SetElement2
SetElement1
*/