util package

How does an implementation of the Set interface know that an object is already there in its collection and its duplicate shall not be added?
Eg:
Set s=new HashSet();
s.add("S");
s.add("S");
Now how does s know that "S" is already there in the collection?

Take a look at the source code for the class HashSet (and HashMap) and you'll find the answer.
Do you know where to find the source code?

The javadoc for Set.add() says:

public boolean add(Object o)
Adds the specified element to this set if it is not already present (optional operation). More formally, adds the specified element, o, to this set if this set contains no element e such that (o==null ? e==null : o.equals(e)). If this set already contains the specified element, the call leaves this set unchanged and returns false. In combination with the restriction on constructors, this ensures that sets never contain duplicate elements.

So I think, an implementation of Set.add()
have to call equals() for each object in the
collection.
Hope this helps
[ Changed CODE tags to QUOTE tags to help with spacing ]
[ May 08, 2002: Message edited by: Jessica Sant ]

Karl, really... tsk, tsk, tsk... SCJP perhaps, but certainly not JavaRanch UBB Certified!

I dont know wether karl is right,coz when I defined my own equals method in my class and added it to hashset,than the equals method was not called.
And dirk,what caused u to think that I dont know where the source code is?

Sir Gautam,
I did not know whether you knew how to find the source files. You had not mentioned whether you had consulted and understood them.
So, did you find your answer in the source files?

Originally posted by Gautam Sewani:
I dont know wether karl is right,coz when I defined my own equals method in my class and added it to hashset,than the equals method was not called.

How did you define your equals method? And did you override the hashCode method as well?

I defined the equals method as:
public boolean equals(Object a)
{
System.out.println("in my equals");
return super.equals(a);
}

Gautam
Is your class extending Object?
Have you overriden hashCode to return the same int for those instances considered the same by equals?