Java pass by reference issue

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Is Java pass by reference?

I have this class here:

public class Cat { 
    private String catNum;

    private static Cat cat1;
    private static Cat cat2;


    public Cat(String catNumber) {
        catNum = catNumber;
    }

    public static void change(Cat cat1, Cat cat2) {  
        Cat temp = cat1;
        cat1 = cat2;
        cat2 = temp;
    }

    public String toString() {
        return "cat number: " + catNum;
    }

    public static void main(String[] args) {
        cat1 = new Cat("1");   
        cat2 = new Cat("2");  
        System.out.println("cat1=" + cat1);  
        System.out.println("cat2= " + cat2);
        change(cat1, cat2);
        System.out.println("cat1=" + cat1);  
        System.out.println("cat2= " + cat2);
    }
}

I would like to get the change() function working. I know it's some issue about passing objects by reference but not sure how to fix it. Someone please help.

Welcome to Stack Overflow! As written, your question is too vague to be answered. What do you want the function to do? What does it do right now? What have you tried so far? The more details you provide and the more of an effort you've demonstrated, the more likely we are to help you out. — templatetypedef, Sep 15 '11 at 19:59
You can't. See: _[Java is Pass-by-Value, Dammit!](http://javadude.com/articles/passbyvalue.htm)_ — Bart Kiers, Sep 15 '11 at 20:00
http://stackoverflow.com/questions/1363186/is-it-possible-to-write-swap-method-in-java — , Sep 15 '11 at 20:10

score 3 · Answer 1 · answered Sep 15 '11 at 20:02

3

You problem is that java is pass by VALUE not reference. So you can't write a swap function the way you did.

you could do something like

class CatContainer {
    Cat cat1;
    Cat cat2;

    CatContainer(Cat cat1, Cat cat2) {
       this.cat1 = cat1;
       this.cat2 = cat2;
    }
    ...
}

and then have a method

public static void swapCatsInContainer(container) {
    Cat tmp = container.getCat1();
    container.setCat1(container.getCat2());
    container.setCat2(tmp);
}

something like that. Now in the scope that called swapCatsInContainer cat1 and cat2 are swapped.

answered Sep 15 '11 at 20:02

hvgotcodes

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1

to be strict you are passing the reference by value – ratchet freak Sep 15 '11 at 20:05
1

@ratchet right, which is to say you are passing by value. no need to complicate things with the word reference. – hvgotcodes Sep 15 '11 at 20:08
ratchet, while this is old post, your comment (passing reference by value) saved me. – Makketronix Aug 17 '15 at 18:51

score 2 · Answer 2 · answered Sep 15 '11 at 20:01

2

You have both change() arguments and class variables named cat1 and cat2. If you rename one set, I think you'll see what's going on. If not, drop a comment.

answered Sep 15 '11 at 20:01

Ed Staub

15,480
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Or atleast use this.catx when refferencing a global variable. – Shivan Dragon Sep 15 '11 at 20:03
or ClassName.catx when referencing static member variables. – Usman Saleem Sep 15 '11 at 23:01

Charles Goodwin · Answer 3 · 2011-09-15T23:03:50.020

You have a scope problem.

Each method defines its arguments as their own references. The values passed in are assigned to the references defined in the method signature. The method signature is essentially a declaration of local variables that are assigned values when the method is called. This what the other answers mean by 'pass by value'.

private static Cat cat1;  // < this cat1 and cat2 are
private static Cat cat2;  //   never referred to

public static void change(Cat cat1, Cat cat2) {  
    Cat temp = cat1; //         ^         ^
    cat1 = cat2;     // < cat1--'         |
    cat2 = temp;     // < cat2 means------'
}

If it helps, you could think about it like this:

// pseudo code
method change() {
    Cat cat1 = method.Argument[0];
    Cat cat2 = method.Argument[1];
    ...
}

By writing to cat1 and cat2, you are simply writing to the local variables defined as part of the method signature. You are not writing to the identically named static scoped variables.

To make the code work, you can explicitly refer to the static values.

public static void change(Cat cat1, Cat cat2) {
    Cat.cat1 = cat2;  // cat1 and cat2 are static
    Cat.cat2 = cat1;  // i.e. defined on the class
}

Or you could rename them.

public static void change(Cat c1, Cat c2) {  
    cat1 = c2;
    cat2 = c1;
}

Or, since they are static, you could just do away with the method arguments altogether.

public static void change() {
    Cat temp = cat1;
    cat1 = cat2;
    cat2 = temp;
}

Notice that only the final one still required the temp variable.

Java pass by reference issue

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