Saving Vector in session problem

Question

Hi,
Im trying in my servlet to save vector into session and then retrive it:
// new vector
Vector VTree = new Vector();
// session
session = request.getSession(true);
//here I add some elements into vector VTree
VTree.size() // now using method size() I get number>0, so vector is not empty
// saving vector into session attribute SessionTree
session.setAttribute("SessionTree",VTree);
// then I clear vector VTree
VTree.clear();
// and Im trying to get vector from session, still in the same servlet, same method (doGet)
VTree = (Vector)session.getAttribute("SessionTree");
// and result, VTree.size() returns 0...
VTree.size()
Please help me, Im not possible to find error or bad expression...
Thanks, Jan

Answer 1

There are no errors. That is how Java works.
//here I add some elements into vector VTree
VTree.size() // now using method size() I get
number>0, so vector is not empty
// saving vector into session attribute SessionTree
session.setAttribute("SessionTree",VTree);You save a reference to that Vector in the session. So both your class and the session have a reference to the same Vector.
// then I clear vector VTree
VTree.clear();So you clear the Vector object. It has no elements now. Both your class and the session have a reference to this Vector, which is now empty.
// and Im trying to get vector from session, still in
the same servlet, same method (doGet)
VTree = (Vector)session.getAttribute("SessionTree");Now you get the reference to the (empty) Vector from the session and assign it to a variable in your class. (Your variable already had a reference to that same Vector.)
// and result, VTree.size() returns 0...
VTree.size()Yes, it does. Because you cleared the Vector earlier.
Remember, assigning a variable doesn't make a copy of an object. It just makes a copy of a reference (like a pointer) to the object.