how to compare two user defined objects in java?

You use either a Comparator or make the object implement Comparable

http://lkamal.blogspot.com/2008/07/java-sorting-comparator-vs-comparable.html

suppose we have Employee object, if the empId is same, then we should not allow that object to add in collection....

Yes. All you'd need for that is to use a Set. Return the empId as the value of hashCode and implements Employee.equals based on empId. Simple

> if the empId is same, then we should not allow that object to add in collection....

you need to first check if the object is in the collection.
The actual comparison is done with the equals() method which you need to overrride as a_b has shown above. In that method you define when two objects are considered to be equal

Thanks..