java private members call by reference

Question

class insertionSort
{
    public void print(int arr[])
    {   
        arr[0]=999999999;
    }
}

public class Sorting 
{
    private  int[] arr=   {0,56,8,75,45,21,68,9,54,2,65,87,52,65,8,5,20,0,2,2};

    public static void main(String[] arg)
    {
        Sorting sort = new Sorting();
        insertionSort inSort = new insertionSort();
        inSort.print(sort.arr);
        System.out.println(sort.arr[0]);
    }
}

Although Sort.arr is a private variable , why inSort object is able to manipulate arr[0] ?

I know the parameter is passing by reference or address.

In a function how can I pass by value ?

Java is always pass by value, however `arr` is a reference, and this reference is passed by value. You don't get the array copied. — Peter Lawrey, Oct 03 '16 at 09:42
http://stackoverflow.com/questions/373419/whats-the-difference-between-passing-by-reference-vs-passing-by-value will answer your question — Vimal Bera, Oct 03 '16 at 09:48
You didn't create a copy of it and it won't automagically copy it for you. It will create a copy of the reference to the array, but there is only one array. — Peter Lawrey, Oct 03 '16 at 09:48
arrays are actually pointers --- `arr[]` is internally `*(arr+0)` --- `arr` is just the address of first element `&arr[0]` ---- when you pass `arr` you are actually passing address of the first element (pass by reference); So what ever Changes you make will be made on that memory directly — mohanen b, Oct 03 '16 at 09:49
The value is the int[] references. So you are accessing the same object, that why I prefer to write `int[] arr` to see that I pass an array and not a integer. — AxelH, Oct 03 '16 at 09:50
@mohanenb so, how can I pass this array "by values" ? or copied array — enamel, Oct 03 '16 at 09:55
create a copy of that array and send it to the functions ---cause in java all object and array variables are reference variables — mohanen b, Oct 03 '16 at 10:08
@markrotteveel: No, this is not a duplicate of http://stackoverflow.com/questions/40480/is-java-pass-by-reference-or-pass-by-value. Despite appearances, the question isn't about pass-by-value/pass-by-reference at all. — T.J. Crowder, Oct 03 '16 at 11:06
@MarkRotteveel: No. The question is asking how the array entry can be changed by code that isn't in the class where the `arr` member is declared, even though `arr` is `private`. The OP mentioned pass-by-value, etc., but is mistaken about that. It's about object references, nothing to do with passing conventions at all. — T.J. Crowder, Oct 03 '16 at 12:17

score 2 · Answer 1 · edited May 23 '17 at 11:48

In a function how can I pass by value?

Java is always pass-by-value, never pass-by-reference (see this question and its answers). The value being passed in your example is the object reference¹ that points to the array.

Your issue has nothing to do with pass-by-value vs. pass-by-reference, it has to do with that fact that you're passing an object reference (which is a value) into a method. That gives the method access to the object that reference refers to. So naturally it can modify the state of that object; in your case, it can modify what's stored at index 0 of the array.

Separately, it doesn't matter that the instance variable (field) you're getting that object reference from is private; the code inSort.print(sort.arr); is in the Sorting class, so it has access to sort.arr, reads its value (an object reference), and passes that value (the object reference) into the method.

¹ It's important to realize that the word "reference" in "object reference" and "pass-by-reference" is talking about two very different things. In "object reference" it's talking about a reference to an object (a pointer to it). In "pass-by-reference" it's talking about a reference (pointer) to a variable. Java doesn't have pass-by-reference.

java private members call by reference

1 Answers1