What is pass-by-value and pass-by-reference?

I have a hard time believing google couldn't come up with something you could understand. I wouldn't know how else to explain it. What didn't you understand?

Pass by value is when the parameter's value is copied and that copy is passed to the method. Changing the value of the variable inside the method does not affect the caller.

Pass by reference is when an "alias" to the variable is passed, so it's as if the method is operating directly on the caller's variable. Changing the variable's value inside the method changes the value of the caller's variable.

Java and C only pass by value.

C++ and C# support both PBV and PBR.

http://javadude.com/articles/passbyvalue.htm

http://java.sun.com/developer/JDCTechTips/2001/tt1009.html#tip1

http://www.javaranch.com/campfire/StoryPassBy.jsp

http://www.javaworld.com/javaworld/javaqa/2000-05/03-qa-0526-pass.html

http://www-106.ibm.com/developerworks/library/j-praxis/pr1.html

http://www.cs.toronto.edu/~dianeh/tutorials/params/

http://java.sun.com/docs/books/jls/second_edition/html/classes.doc.html#38698

http://radio.javaranch.com/channel/val/2004/05/21/1085125887000.html

jverda at 2007-7-12 20:03:35 >

in the next version of java, codenamed fo' jizzle, they're going to introduce a new feature called drive-by reference whereby objects on the heap and references in scope will be randomly matched up to each other by an undisclosed and non-deterministic algorithm, in a big car and baseball cap, known as Pimp Daddy Heap. this is so that any old skool developers who think java is getting a little safe and boring can now convince themselves they're "keepin' it real J-side". the downside, of course, being that for us confused uncles, we'll be forever at the mercy of ClassCastExceptions ( to be renamed "DatShitDontFitException" ) when our references keep pointing to the wrong things, and NullPointerExceptions ( or "FoolDoneGoneLikeElvisExceptions" ) when there aren't enough objects to go round

georgemca at 2007-7-12 20:03:35 >

> All of you are totally wrong!!!

>

> ...

No, YOU are wrong.

To quote James Gosling (he is the inventor of Java!):

"There is exactly one parameter passing mode in Java -- pass by value -- and that helps keep things simple."

James Gosling, "The Java Programming Language, Second Edition"

Now make way for the flame-throwers!

; )

prometheuzza at 2007-7-12 20:03:35 >

> All of you are totally wrong!!!

>

> Actually, Java has both pass-by-value and

> pass-by-reference

Nope.

Java is always pass-by-value. Always.

Always. Always. Always.

http://javadude.com/articles/passbyvalue.htm

http://java.sun.com/developer/JDCTechTips/2001/tt1009.html#tip1

http://www.javaranch.com/campfire/StoryPassBy.jsp

http://www.javaworld.com/javaworld/javaqa/2000-05/03-qa-0526-pass.html

http://www-106.ibm.com/developerworks/library/j-praxis/pr1.html

http://www.cs.toronto.edu/~dianeh/tutorials/params/

http://java.sun.com/docs/books/jls/second_edition/html/classes.doc.html#38698

http://radio.javaranch.com/channel/val/2004/05/21/1085125887000.html

There is exactly one parameter passing mode in Java -- pass by value -- and that helps keep things simple.

-- James Gosling, "The Java Programming Language, Second Edition"

(James Gosling being the father of Java)

jverda at 2007-7-21 22:36:18 >

Objects, are always passed by reference as someone has already mentioned in this thread. Changing the attributes of an object in a method called, would result in the parent object being modified.

Let's not keep on referring to what is given in some thread or book or tutorial. I just wrote a small example and the results confirmed the pass by reference concept. Please go ahead and write a sample program on your own and confirm.

SourabhGoeLa at 2007-7-21 22:36:18 >

> Objects, are always passed by reference as someone

> has already mentioned in this thread.

Some people will say incorrectly that objects are passed "by reference."

-- Arnold, K., Gosling J., Holmes D. (2006). The Java Programming Language Fourth Edition. Boston: Addison-Wesley.

Answer that.

> Let's not keep on referring to what is given in some

> thread or book or tutorial. I just wrote a small

> example and the results confirmed the pass by

> reference concept. Please go ahead and write a sample

> program on your own and confirm.

We have. There is no way to demonstrate pass-by-reference semantics in Java because it doesn't exist. You're confusing passing a reference with pass-by-reference. They're not the same thing. What you have demonstrated is pass-by-value semantics. The ability to modify an object via a pointer has nothing to do with how the parameter is passed.

~

yawmarka at 2007-7-21 22:36:18 >

> > Objects, are always passed by reference as

> someone

> > has already mentioned in this thread.

>

>

> Some people will say incorrectly that objects are

> passed "by reference."

> -- Arnold, K., Gosling J., Holmes D. (2006). The

> Java Programming Language Fourth Edition.

> Boston: Addison-Wesley.

Look man, when are you going to stop using your "references" and "facts" and "reality" as a crutch and just accept the truth?!

(It's best if you picture Chris Farley doing the over-exaggerated finger quotes while you read that)

hunter9000a at 2007-7-21 22:36:18 >

Here's an example of actual pass-by-reference in C#:public class Foo

{

public static void Main()

{

string s = "foo";

Console.WriteLine(s); // foo

passByRef(ref s);

Console.WriteLine(s); // bar

}

public static void passByRef(ref string s)

{

s = "bar";

}

}

The same thing is not possible in Java, because Java only has one type of parameter passing mechanism: pass-by-value.

~

yawmarka at 2007-7-21 22:36:18 >

Here is proof that pass by ref does not exist in Java. If it would, the result would be :

one

two

two

two

public class PassByRef {

public String name;

public PassByRef(String n) {

name = n;

}

public static void switchRefs(PassByRef a, PassByRef b) {

a = b;

}

public String toString() {

return name;

}

public static void main(String args[]) {

PassByRef a = new PassByRef("one");

PassByRef b = new PassByRef("two");

System.out.println(a);

System.out.println(b);

PassByRef.switchRefs(a, b);

System.out.println(a);

System.out.println(b);

}

}

but the switch never changes the reference in main.

Message was edited by:

Raphael.Blanchard

Raphael.Blancharda at 2007-7-21 22:36:18 >

> Objects, are always passed by reference

No. References are passed by value. Read the bloody liniks. Read the quote from the guy who invented the language.

> has already mentioned in this thread. Changing the

> attributes of an object in a method called, would

> result in the parent object being modified.

That's not PBR.

void foo(Bar bar) {

bar = somethingElse; // If the caller sees this, that's PBR.

bar.setSomething(baz); // The caller seeing this change does NOT mean PBR

bar.something = qux; // The caller seeing this change does NOT mean PBR

> Let's not keep on referring to what is given in some

> thread or book or tutorial.

What's given in those many references and by the inventor of the language and in the JLS is correct.

> I just wrote a small

> example and the results confirmed the pass by

> reference concept.

No, it confirms that you don't understand what PBR means.

jverda at 2007-7-21 22:36:18 >