Re: call by reference
- From: Dale King <DaleWKing@xxxxxxxxxxxxxxxxxxxx>
- Date: Sat, 02 Jul 2005 14:05:34 GMT
Chris Smith wrote:
rosty <dima.hristov@xxxxxxxxx> wrote:
I know there is no "call by reference" in Java. But please help me convince two of my colleagues. We started learning Java about one month ago, and they just don't believe me when i try to explain that there is a confusion when passing reference types as an argumetn to a function.
Please help. A link maybe to official documentation?
Most of the time, though, people who object to this fact *do* understand the behavior of Java, and just have a fuzzy definition of "pass by reference". When pressed, they will generally degrade to saying that the meaning of "pass by reference" is language-specific, thus creating a circular argument that you can't possibly disprove. Hopefully, they will later come to their senses.
Actually I find that more often it is not understanding the difference between the object living on the heap and the variable that refers to it. When people say objects are passed by reference they are showing this confusion.
Here is the canned response I put together on the subject:
Question: Does Java pass objects by reference or by value?
Answer: Since it makes no sense to begin any argument without agreed upon defintions let's formally define our terms. I will use abstract pseudocode to keep the issue from being clouded by the idiom of a particular language. The source of my information is the book "Advanced Programming Language Design" by Raphael A. Finkel.
For those unfamiliar with the term below an L-value is an expression that can appear on the left side of an assignment statement. It is basically a way to address where a variable is stored. Variables and other ways to refer to locations in memory are L-values. Most expressions are not L-values, e.g. ( x * 2 )
We assume the presence of a procedure named f that takes a formal parameter s. We call that function giving it an actual parameter g.
The calling code: f( g )
The function: procedure f( s ) begin -- body of the procedure end;
There are several parameter passing semantics that have been proposed or used:
value The value of the actual parameter is copied into the formal parameter when the procedure is invoked. Any modification of the formal parameter affects only the formal parameter and not the actual parameter. This is the most common form of parameter passing and is the only one provided in C and Java.
result The value of the formal parameter is copied into the actual parameter when the procedure returns. Modifications to the formal parameter do not affect the formal parameter until the function returns. The actual parameter must be an L-value. It is usually invalid to pass the same L-value to more than one result parameter, but the compiler cannot always detect this. The best example of this is out parameters in CORBA.
value result Combination of value and result semantics. The best example of this are inout parameters in CORBA.
reference The L-value of the formal parameter is set to the L-value of the actual parameter. In other words, the address of the formal parameter is the same as the address of the actual parameter. Any modifications to the formal parameter also immediately affect the actual parameter. FORTRAN only has reference mode (expressions are evaluated and stored in a temporary location in order to obtain an L-value). C++ has reference parameters by putting a & before the formal parameter name in the function header. Reference mode can be simulated in C using pointers and adding the & to the actual parameter and dereferencing the formal parameter within the function.
readonly Can use either value or reference mode, but modification of the formal parameter is forbidden by the compiler.
macro name These two have been used in the past, but are very much out of favor because they are confusing and difficult to implement. Therefore I won't bother trying to explain them.
Now that we have some definitions of terms we can return to the question. Does Java pass objects by reference or by value?
The answer is NO! The fact is that Java has no facility whatsoever to pass an object to any function! The reason is that Java has no variables that contain objects.
The reason there is so much confusion is people tend to blur the distinction between an object reference variable and an object instance. All object instances in Java are allocated on the heap and can only be accessed through object references. So if I have the following:
StringBuffer g = new StringBuffer( "Hello" );
The variable g does not contain the string "Hello", it contains a reference (or pointer) to an object instance that contains the string "hello".
So if I then call f( g ), f is free to modify its formal parameter s to make it point to another StringBuffer or to set it to null. The function f could also modify the StringBuffer by appending " World" for instance. While this changes the value of that StringBuffer, the value of that StringBuffer is NOT the value of the actual parameter.
Imagine for instance if I set g to null before passing it to f. There is no StringBuffer now to modify and f can in no way change the value of g to be non-null.
The bottom line is Java only has variables that hold primitives or object references. Both are passed by value.
-- Dale King .
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