java - Object by Reference vs. Reference by Value

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I read this comment here: href="https://stackoverflow.com/questions/1270760/passing-a-string-by-reference-in-java">Passing
a String by Reference in
Java?

Yes, it's a misconception. It's a huge, widespread misconception. It

leads to an interview question I hate: ("how does Java pass
arguments"). I
hate it because roughly half of the interviewers
actually seem to want the
wrong answer ("primitives by value, objects

by reference"). The
right answer takes longer to give, and seems to
confuse some of them. And
they won't be convinced: I swear I flunked a
tech screen because the
CSMajor-type screener had heard the
misconception in college and believed it
as gospel. Feh. – CPerkins
Aug 13 '09 at
14:34

Can someone
please explain, in terms that a new programmer can grasp, what is the difference between
saying:

"In Java primitives are passed by value
and objects are passed by
reference."

and:

"In
Java nothing is passed by reference and references are passed by
value."?

Are both of these
statements true in some sense? I don't want to invite a rant parade, but
this sounds like a really important concept, and one I still do not completely
understand.

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class="normal">Answer

I believe
the misconception lies in the fact that a variable can not contain an
object to begin with. If you grasp that, then obviously variables can
only contain references to objects (or primitive values). The step from there to
realizing that references are passed by value (just as primitive values) is quite
small.

There is a really easy test you can do to
figure out if a language supports pass by reference. Ask yourself if you can write a
swap function in the language, i.e. something that works
like

x == A, y ==
            B

swap(x, y);

x == B, y ==
            A

As a Java programmer
you realize quickly that you can't implement this in Java, thus you
(correctly) draw the conclusion that Java does not have pass by
reference.

Returning to your
sentences:

• In
  Java primitives are passed by value and objects are passed by
  reference.

This
is false. I would argue that you can only pass something
that is contained in a variable, and as I stated above, a variable can't contain an
object, thus you can't pass an object at all in
Java.

• In Java
  nothing is passed by reference and references are passed by
  value.

This
is true.