Hashcode() of value called during session.setAttribute() why?

Question

When I set a Vector value into session hashCode() is called on the elements of the Vector? Why?
In the code below, MyObject extends a base class that has the following implementation of hashCode() so as to enforce its implementation in all it's children if and only if it is required (i.e. not really a case for an abstract class as I want to try to avoid its implementation).
public int hashCode () {
    throw new RuntimeException("hashCode is not supported in base class");          
}What I find is the following code throws the hashcode exception on the 2nd set of Key1, but I dont understand why. Why is hashCode() called on the elements of the vector? is this some set optimization - i.e. it does not update session if the objects are the same?
Vector v1 = new Vector();
v1.add( new MyObject( someUniqueParam1 ) );
v1.add( new MyObject( someUniqueParam2 ) );
Vector v2 = new Vector();
v2.add( new MyObject( someUniqueParam1 ) );
v2.add( new MyObject( someUniqueParam2 ) );
session.setAttribute( "Key1", v1 ); // successful
session.setAttribute( "Key2", v2 ); // successful
session.setAttribute( "Key1", v2 ); // exception thrown from MyObject - why?Please help me understand? My workaround for the moment is to call session.removeValue() first.

Answer 1

The hashCode() method in Vector calls hashCode() in its superclass (AbstractList) which calls hashCode() on each of the objects stored in the List. It looks as if the servlet container calls hashCode() on the object being added as a session attribute when the key is already present. Why this happens is anyone's guess (checking if object is same as the one currently stored?) and is probably servlet engine specific.
Overriding hashCode() to throw an exception is probably not a good idea as many Java classes call hashCode() "behind the scenes" which is why the default implementation is part of Object. Can you use that default implementation?