Milt,
what do you mean by "indirect" manipulation? Or, if objects are
manipulated indirectly by accessing their methods and variables, how
"direct manipulation" will be more direct than the indirect method you
mention?
All object variables are refferences, passing them in a stack as method
parameters doesnt makes them less or more "direct".
Cezar.
On Fri, 13 Aug 1999, Milt Epstein wrote:
> On Fri, 13 Aug 1999, Chris Pratt wrote:
>
> > I think you need to look back at those old computer texts, Pass by
>
> Sorry, but I think *you* should; I actually posted some text from one
> of "those old computer texts" in support of what I said.
>
> > Value means that the Value of the data is placed on the stack and
> > directly referenced, so that any changes to the data are not
> > reflected in the data that it was copied from. Pass by Reference
> > means that a reference (or pointer) to the original data is copied
> > to the stack so that the original data may be changed by
> > dereferencine the pointer. No where are you allowed to change the
> > copied data (either the pass by value copy of the data or the pass
> > by reference copy of the reference) and have that change propogated
> > outside the called procedure.
>
> Actually, that is the *key* part of pass-by-reference, as it is
> originally and strictly defined.
>
> There are a couple of points of confusion here. One has to do with
> the way Java handles objects -- variables that represent objects are
> really references to those objects; that reference is passed by value,
> and hence you can manipulate objects indirectly. Another is the fact
> that objects do have data and methods that allow indirect manipulation
> of the object (although pass-by-reference refers to direct
> manipulation). Also, as I said before, some of these terms were
> defined a long time ago (as the computer era goes, we could say they
> are "ancient"), and they may not be adequate to describe/cover all the
> variations in common use today. At least if we don't agree about the
> names, we seem to agree about the behavior, which perhaps is the most
> important thing.
>
>
> > ----- Original Message -----
> > From: Milt Epstein <[EMAIL PROTECTED]>
> > To: <[EMAIL PROTECTED]>
> > Sent: Friday, August 13, 1999 7:14 AM
> > Subject: Re: will this code work?
> >
> >
> > > On Thu, 12 Aug 1999, Chris Pratt wrote:
> > >
> > > > Wrong, Wrong, Wrong. Your example is correct, but that's because
> > > > you're changing the value of the reference, not the value of the
> > > > String (remember String's are immutable). Java uses Pass by
> > > > Reference for Objects and Pass by Value for intrinsic types (int,
> > > > float, char, etc). This has been a major source of confusion with
> > > > the Java detractors saying that the entire contents of each Object
> > > > must be copied to the stack since Java is "pointerless". If you
> > > > don't believe me, try this program. I bet you get "i = 10"
> > >
> > > First: I think you have a mistake in your code, at least in terms of
> > > what you are trying to illustrate. The line:
> > >
> > > obj.add(5);
> > >
> > > in your main was probably meant to be:
> > >
> > > process_me(obj);
> > >
> > > Second: Actually, *you* are wrong. Java does *not* use
> > > pass-by-reference, at least not what is traditionally known as
> > > pass-by-reference. If it did, and using your example, I could do the
> > > following:
> > >
> > > public class test {
> > >
> > > public static void process_me(Obj o) {
> > > o = new Obj(10);
> > > }
> > >
> > > public static void main(String[] args) {
> > > Obj obj = new Obj(5);
> > > System.out.println("obj = " + obj);
> > > process_me(obj);
> > > System.out.println("obj = " + obj);
> > > }
> > >
> > > }
> > >
> > > and the two println's would show something different. But they won't.
> > >
> > > It's true that Java uses pass-by-value for the primitive types, but
> > > what it uses for Objects is not quite pass-by-reference. It does pass
> > > a reference (i.e. pointer) to the object, but you can only use that to
> > > manipulate the object indirectly (i.e. through what methods the object
> > > allows), not directly (i.e. via assignment). Pass-by-reference allows
> > > the latter.
> > >
> > > You might think that because you can affect the value of an Object
> > > parameter, Java is using pass-by-reference, but there are other
> > > parameter passing techniques that allow this; pass-by-reference and
> > > pass-by-value aren't the only two possibilities.
> > >
> > > BTW, in an earlier post I said that C's '&' operator allowed
> > > pass-by-reference; this isn't technically correct. It's still the
> > > value that's being passed, it's just that the value is the address of
> > > (i.e. pointer to) a variable, and C's use of pointers allows you to
> > > manipulate the contents of a variable through it's pointer (which Java
> > > doesn't).
> > >
> > >
> > > > class Obj {
> > > > int i;
> > > >
> > > > public Obj (int i) {
> > > > this.i = i;
> > > > }
> > > >
> > > > public int add (int j) {
> > > > i += j;
> > > > return i;
> > > > }
> > > >
> > > > public String toString () {
> > > > return String.valueOf(i);
> > > > }
> > > >
> > > > }
> > > >
> > > > public class test {
> > > >
> > > > public static void process_me(Obj o) {
> > > > o.add(5);
> > > > }
> > > >
> > > > public static void main(String[] args) {
> > > > Obj obj = new Obj(5);
> > > > obj.add(5);
> > > > System.out.println("obj = " + obj);
> > > > }
> > > >
> > > > }
> > > >
> > > > ----- Original Message -----
> > > > From: Ted Neward <[EMAIL PROTECTED]>
> > > > To: <[EMAIL PROTECTED]>
> > > > Sent: Thursday, August 12, 1999 10:57 PM
> > > > Subject: Re: will this code work?
> > > >
> > > >
> > > > > Folks, the confusion, IMHO, is pretty simple: the difference between
> > > > > returning a parameter and the parameter passed in to the method.
> > > > >
> > > > > For example, had the code been written this way:
> > > > >
> > > > > public class Param
> > > > > {
> > > > > public static void main(String[] args)
> > > > > {
> > > > > String temp = null;
> > > > > manipParameter(temp);
> > > > > System.out.println("temp = " + temp);
> > > > > }
> > > > >
> > > > > private static void manipParameter(String param)
> > > > > {
> > > > > param = "This should be modified";
> > > > > }
> > > > > }
> > > > >
> > > > > the resulting output would be:
> > > > >
> > > > > temp = null
> > > > >
> > > > > because Java supports pass-by-value semantics, not pass-by-reference,
> > even
> > > > > for Object-derived types. (This is where Java's insistence that it has
> > no
> > > > > pointers really trips people up.) This means that the POINTER to
> > String,
> > > > > held by temp, is copied into the String reference named "param", and
> > > > > subsequent modification of "param" has no effect on the original
> > reference
> > > > > "temp".
> > > > >
> > > > > For all you C++-heads out there, the difference is one of
> > > > >
> > > > > void manipParameter(String* pString); // THIS is what Java does,
> > > > effectively
> > > > >
> > > > > vs.
> > > > >
> > > > > void manipParameter(String& pString); // THIS is NOT what Java does
> > > > >
> > > > > Hence, Shiraz is correct in that the *parameter*, temp, (not the
> > lvalue of
> > > > > the return value's assignment) will not be modified.
> > > > >
> > > > > However, because the original code was written to not only use the
> > > > (wrongly)
> > > > > assumed pass-by-reference semantics, but also to copy the return value
> > > > into
> > > > > temp:
> > > > >
> > > > > >> >> temp = process_me(temp);
> > > > >
> > > > > the code worked as expected.
> > > > >
> > > > > In short, everybody's right, bur for different reasons. :)
> > > [ ... ]
> > >
> > > 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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>
> Milt Epstein
> Research Programmer
> Software/Systems Development Group
> Computing and Communications Services Office (CCSO)
> University of Illinois at Urbana-Champaign (UIUC)
> [EMAIL PROTECTED]
>
> ___________________________________________________________________________
> To unsubscribe, send email to [EMAIL PROTECTED] and include in the body
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