Java: pass-by-value references in methods

Question

I read a few articles and similar question in Stackoverflow; however, I did not quite get an answer to my question. Here is the code:

public class CoinFlipping {

    Random random = new Random();

    Boolean head = null;

    public void flip(Boolean b){
        b = random.nextBoolean();
    //      head = b; 
    }

    public static void main(String [] args){
        CoinFlipping cf = new CoinFlipping();
        cf.flip(cf.head);
        System.out.println("Head: "+cf.head);
    }

    }

I refer to this arcticle: Is Java "pass-by-reference" or "pass-by-value"? I can understand why that piece of code behaves as it does. When we call new dog() we basically create a new Dog object. I get it. However, the example that I provided seems to be confusing a bit. I am not creating an object in this case (aren't I?). I am changing a value. Then why in this case when I print results, I get null and not true or false?

b = random.nextBoolean(); creates a new reference, so modifies the value of b, not the value referenced by b. — AntonH
– AntonH, Commented Jul 17, 2014 at 17:18
@AntonH: b does not "reference a value." b has a value which is a reference to an object (or a null reference). The assignment statement gives b a new value (i.e., a reference to a new object). — Solomon Slow
– Solomon Slow, Commented Jul 17, 2014 at 17:23
@jameslarge I was trying to simplify, so that OP would see what was wrong. But too much, apparently. — AntonH
– AntonH, Commented Jul 17, 2014 at 17:28
there are answers on the linked question that directly address this. see stackoverflow.com/a/40507/217324 closing this as a duplicate. — Nathan Hughes
– Nathan Hughes, Commented Jul 17, 2014 at 17:29
@AntonH Hello, so am I getting this right? When I pass any objects to methods, following happens: It looks up the value of the object through the reference, copies it and this copy is assigned to the local var (argument)? In this case I have another question. Is there any ways to pass a reference to method, so that I could take this reference, access the object and change it internal state? Is there a way to actually have a reference to an original object but not to copy inside of the method? — George Revkov
– George Revkov, Commented Jul 17, 2014 at 17:45

ajb · Accepted Answer · 2014-07-17 17:19:03Z

2

The parameter b is passed by value, even though b is itself a reference. Thus, when you call flip, the flip method treats b as a local variable that is a copy of what you pass into it. Changing this copy doesn't change cf.head.

If b were a reference to a mutable object, you could change the object b refers to, and that change would be seen by the rest of the program after flip returns. But a change to b itself is a change to a copy, and any such change will not be seen by the rest of the program. (Boolean is not mutable.)

answered Jul 17, 2014 at 17:19

ajb

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Rika · Accepted Answer · 2017-09-06 10:59:19Z

To answer your question:

Then why in this case when I print results, I get null and not true or false?

I have created a step-by-step walktrough of your code to show you the pointers (references) of the specific variables and answer why it prints out null in the end:

1. Step: New CoinfFlipping object is created with default field values

CoinFlipping cf = new CoinFlipping();

Inside the CoinFlipping class:

Random random = new Random();
Boolean head = null;

cf ---- points to ----> CoinFlipping object
cf.random ---- points to ----> Random object
cf.head ---- points to ----> null

2. Step: Calling the flip method with cf.head as parameter on the cf object

cf.flip(cf.head);

When evoking this method, a variable b is created inside the scope of the flip method. The pointer (reference) from cf.head is copied into the variable b. So cf.head and b point then at the same thing.

public void flip(Boolean b){

cf ---- points to ----> CoinFlipping object
cf.random ---- points to ----> Random object
cf.head ---- points to ----> null
b ---- points to ----> null

3. Step: Inside the flip method b is being reassigned

b = random.nextBoolean();

cf ---- points to ----> CoinFlipping object
cf.random ---- points to ----> Random object
cf.head ---- points to ----> null
b ---- points to ----> a boolean value - either true or false

4. Step: Back in the main method to print out cf.head

System.out.println("Head: "+cf.head);

cf ---- points to ----> CoinFlipping object
cf.random ---- points to ----> Random object
cf.head ---- points to ----> null
b ---- points to ----> a boolean value - either true or false

As you can see, cf.head is still pointing to null, therefore null is being printed.

Solution:
To change this, you could let the method return a boolean value instead and assign that to cf.head in order to make cf.head point to a new boolean value.

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