Yes!!! In fact I went round and round this issue all morning and (finally!) came
to (most of) the conclusions you have arrived at. Though i could certainly not
have put it as clearly as you have. Thank you!
Geeta
Milt Epstein wrote:
> On Wed, 13 Sep 2000, Geeta Ramani wrote:
>
> > Properly chastised, (;-), I went ahead and tested several things in
> > a trivial extension of Kevin's Test class and after some reading
> > concluded that i was indeed wrong, and Kevin was indeed right.
> > Apparantly the fact that String is an "immutable" class is the
> > underlying reason for the fact that a putValue is neccessary for
> > String objects, but, not needed for say, StringBuffer objects or
> > Vector objects or Array objects (or probably most anything else?).
> > So I guess the fact that it is a String is sort of special.
> > Hopefully i am not too far off..?
>
> That is only part of it. It also has to do with the way Java passes
> objects around, as references (I'm avoiding that nasty "p" word :-).
>
> For example, suppose you do:
>
> String myString = (String) session.getValue("myString");
> Vector myVector = (Vector) session.getValue("myVector");
>
> myString and myVector now refer to the same objects as are stored in
> the session. If you then do:
>
> myString = "some new string";
> myVector = new Vector();
>
> *Neither* of these change anything in the session, you are simply
> changing what myString and myVector refer to, that is, making them
> refer to different objects. So String is not special in this regard.
>
> However, if instead you do something like:
>
> myVector.addElement(someObject);
>
> This *does* change something in the session. Here, you are
> manipulating the object that myVector refers to (which is the same
> thing that the session refers to), as opposed to making it refer to a
> different object. Here is where the immutability of Strings makes a
> difference, making Strings "sort of special" -- there is no way to
> manipulate what the String refers to.
>
> Hope this is clear.
>
> > Kevin Mukhar wrote:
> >
> > > Geeta Ramani wrote:
> > > >
> > > > Hmm, really? I thought that since String is not a primitive data
> > > > type, when you pass a String to a variable, you are passing a
> > > > reference to that String obect, (and it is not passed by
> > > > value)..? So my guess would have been that doing a "putValue"
> > > > wouldnt have been necessary.. Of course it should easy enough to
> > > > check..
> > >
> > > And if you had checked it before leaping to reply, you would have
> > > found...
> > >
> > > h.get("myInfo") is [this is string 1]
> > > myInfoB is [This is an entirely different string]
> > >
> > > ------------------ test code ------------------------------
> > > import java.util.*;
> > >
> > > public class Test {
> > > public static void main(String[] args) {
> > > Hashtable h = new Hashtable();
> > > String myInfo = "this is string 1";
> > > h.put("myInfo", myInfo);
> > >
> > > //simulate servlet b with a different myInfo variable
> > > String myInfoB = (String) h.get("myInfo");
> > > myInfoB = "This is an entirely different string";
> > >
> > > System.out.println("h.get(\"myInfo\") is [" + h.get("myInfo") + "]");
> > > System.out.println("myInfoB is [" + myInfoB + "]");
> > > }
> > > }
> > > ------------------ test code ------------------------------
> > >
> > > You are partly right. A reference to the string is passed to the
> > > session object. When servlet B gets the string from the session,
> > > it is really getting a reference to the string. That reference is
> > > assigned to some variable. If a new reference is assigned to that
> > > variable, the reference inside the session object is unchanged--
> > > it still points to the original string object. Assigning a new
> > > reference (of ANY type) to a new variable does not change the
> > > references held by other variables.
> > >
> >
>
> Milt Epstein
> Research Programmer
> Software/Systems Development Group
> Computing and Communications Services Office (CCSO)
> University of Illinois at Urbana-Champaign (UIUC)
> [EMAIL PROTECTED]
>
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