Adding duplicate values into HashSet

[Harshal Gurav]

Hi,
we know that,HashSet does not allows duplicate values .But when we try to insert duplicate values into hashSet,it does not give any compile or run time error ?
What is exactly means by

HashSet does not permits duplicate value

suppose we have this code

HashSet <Integer> arr1=new HashSet<Integer>();
arr1.add(1);
arr1.add(2);
arr1.add(2);
arr1.add(1);

When we complile and run this programme, it will give output as [1,2]

Thanks

[Campbell Ritchie]

Where did you get that quote from? It looks misleading.

HashMap does not permit duplicate keys. If you have two values with the same associated key, it retains which ever was "put" more recently, and loses the older value.

You cannot have a HashMap<Integer>; it needs two types. It has no add() method.You need to read the Java™ Tutorials section, particularly about Map.

[Harshal Gurav]

Sorry for previous post.
All this is related to HashSet.
Yes, In HashMap, we have value is associated with Key.
How it is work with HashSet.

[Martin Vanyavchich]

Harshal Gurav, are you were reffering to a HashSet? If so, note that add() method returns a boolean value of true, if insertion was successful.

[Steve Luke]

As Campbell said, you are confused somewhere with your code. A HashMap can have multiple duplicate values, just not duplicate keys. A HashMap has to be declared with two generic types, one for the key and one for the value. And there is no add(value) method in a HashMap. Assuming you meant no duplicate keys, the generics stuff was a mistake, and you meant put(key, value), then the API documentation tells you what will happen:
HashMap#put(key, value)

Or you could mean a HashSet. A HashSet allows no duplicate values, has a single generic attribute and does have an add(value) method. Here, too, the API explains what happens when you try to add duplicates:
HashSet#add(value)

[Campbell Ritchie]

No, it's me getting confused. Sorry. I read "Set" as "Map".

[Harshal Gurav]

Thanks for reply.
If HashSet does not give error while adding duplicate,then what is exactly use of Hashset?
Is add() method is used for any comparison purpose?

[Martin Vanyavchich]

Campbell Ritchie wrote:No, it's me getting confused. Sorry. I read "Set" as "Map".

I think you've read it just fine before it was edited

[Campbell Ritchie]

It was edited, was it?
Harshal Gurav, don't make that sort of change which makes subsequent posts look like nonsense

[Campbell Ritchie]

The java.util.Set interface is designed to operate like the mathematical abstraction of a set. You will have to learn a little basic set theory, then you will understand why add() doesn't insert the same value twice.

[satish varma]

we know that,HashSet does not allows duplicate values .But when we try to insert duplicate values into hashSet,it does not give any compile or run time error ?
What is exactly means by

"HashSet" duplicate values are not allowed "if we are trying to add duplicate objects into HashSet there is no compile time and run time error simply add() returns false. thats it"
check out below code

HashSet <Integer> arr1=new HashSet<Integer>();
arr1.add(1);
System.out.println(arr1.add(2));//returns true , it means object successfully added
System.out.println(arr1.add(2));//returns false, it means object not added successfully
arr1.add(1);