when using Class type as method parameters, I have a question...

Thread view:

www - 07 Nov 2007 17:46 GMT

Hi,

I thought I have understood this issue without any problems already.
When using class type parameters for a method, the behavior is kind of
like pass-by-reference. The object can be modified by the code inside
the method, right? But today, I ran into an interesting problem: the
object cannot get the change it wants.

Suppose I have a class called Species. I have a method

//oldSpecies is an object of Species, its content has been set already.
//newSpecies is an object of Species, but its content has not been set
yet. i.e. its content all has been set by the default values. The
purpose of this method is to set newSpecies content based on different
condition.
public static void convertOldSpeciesToNewSpecies(Species oldSpecies,
Species newSpecies)
{
    if(..) //BLOCK A
    {
        newSpecies = oldOne; //just assign it. Here is the problem! The
caller, the second parameter, does not get the value
    }
    else //BLOCK B
    {
        //in this case, newSpecies get assigned values. No problem, the caller
gets the value.
        newSpecies.setXXX
        newSpecies.setYYY

    }

    //print out newSpecies content
    System.out.println(newSpecies.toString());
}

Helper.convertOldSpeciesToNewSpecies(speciesOne, speciesTwo);

Above calling, if the condition fits BLOCK B, result is correct
(speciesTwo gets the values it should get from inside the method).

But if the condition fits BLOCK A, speciesTwo does NOT get the content
of speciesOne. The printing inside the method shows that newSpecies has
got the correct content (same as speciesOne).

What is the problem? Thank you very much.

For my practical reason, I do not want to change the method to

public Species getNewSpecies(Species oldSpecies){..}

Reply to this Message

lyallex - 07 Nov 2007 19:32 GMT

> Hi,
>
[quoted text clipped - 46 lines]
>
> public Species getNewSpecies(Species oldSpecies){..}

I think I see what you are asking although your example is not very
clear, you are using multiple names (newSpecies, oldSpecies, speciesOne,
speciesTwo, oldOne and what have you) which confuses things a bit.

Arguments to methods are passed call by value in Java, it just so
happens that when you pass a reference in, the value is a memory
address. In the method body, copies of the values are used so you will
have a copy of (say) the memory address of newSpecies and a copy of the
memory address of oldSpecies.

Now to begin with, these copies still point to the original Objects so
you can modify those objects directly by calling their methods, however
if you assign one reference to another all you are doing is modifying
the copies not the values (memory addresses) of the arguments.

Write some more code to assign a value to a modified local copy and see
if you can figure out what's going on, stick with it, you will get there
in the end.

Reply to this Message

www - 07 Nov 2007 19:52 GMT

> Arguments to methods are passed call by value in Java, it just so
> happens that when you pass a reference in, the value is a memory
[quoted text clipped - 6 lines]
> if you assign one reference to another all you are doing is modifying
> the copies not the values (memory addresses) of the arguments.

Thank you for your help. Now I understand it:

public static void convertOldSpeciesToNewSpecies(Species oldSpecies,
Species newSpecies){..}

Helper.convertOldSpeciesToNewSpecies(speciesOne, speciesTwo);

Now, oldSpecies has the memory address of the object speciesOne,
newSpecies has the memory address of the object speciesTwo.

If condition is such so that goes to BLOCK A, newSpecies has the memory
address of the object speciesOne, due to the code BLOCK A. *BUT*, the
object speciesTwo remain un-modified. That is why speciesTwo didn't get
what it should get after the method calling.

Reply to this Message

lyallex - 07 Nov 2007 20:23 GMT

>> Arguments to methods are passed call by value in Java, it just so
>> happens that when you pass a reference in, the value is a memory
[quoted text clipped - 21 lines]
> object speciesTwo remain un-modified. That is why speciesTwo didn't get
> what it should get after the method calling.

It takes a while doesn't it, but I reckon you are getting there.
Keep at it, it's all worth it in the end :-)

Reply to this Message

Roedy Green - 07 Nov 2007 20:54 GMT

> newSpecies = oldOne; //just assign it. Here is the problem! The
>caller, the second parameter, does not get the value

That will not work. That just makes newSpecies point to the same
object that oldOne does. Any changes you make to the fields of
newSpecies will modify the fields in oldOne.

You need do something like this;

newSpecies = oldOne.clone();

See http://mindprod.com/jgloss/clone.html

or

newSpecies = new Species (oldOne);
// a copy constructor

or

newSpecies = new Species();
newSpecies.legCount = oldOne.legCount;
...

Also be aware that if you make the parameter local variable point to a
new object, it will have NO effect on the caller. You have either
modify the object passed or return a reference to a new object.

I think you would do well to invest some time reading a introductory
Java textbook that explains these mysteries. Even a badly out of date
second hand one (and hence very cheap) would do.

Signature

Roedy Green Canadian Mind Products
The Java Glossary
http://mindprod.com

Reply to this Message

Daniel Pitts - 07 Nov 2007 22:51 GMT

> Hi,
>
[quoted text clipped - 46 lines]
>
> public Species getNewSpecies(Species oldSpecies){..}

The reference itself is passed by value. newSpecies is a reference
holder, not an actual reference.

Signature

Daniel Pitts' Tech Blog: <http://virtualinfinity.net/wordpress/>

Reply to this Message

when using Class type as method parameters, I have a question...

Free Magazines