A simple ambiguity regarding the thing that in java everything is passed by copy??

[j.gohel]

Hi,

I have read everywhere that java passes-by-copy everything.But the behaviour of the following code i am not able to understand:

import java.util.*;

public class  Test1
{
	public static void main(String[] args) 
	{
		ArrayList list  = new ArrayList();
		list.add("1");
		list.add("2");
		list.add("3");
		func(list);
		System.out.println("Arr1 ---> " + list);

	}

	static void func(ArrayList arr)
	{
		arr.add("4");
		arr.add("5");
		arr.add("6");
		System.out.println("Arr2 --> " + arr);

	}
}

It o/p is:

C:\JIGNESH_GOHEL\JigneshG\JavaProgs>java Test1
Arr2 --> [1, 2, 3, 4, 5, 6]
Arr1 ---> [1, 2, 3, 4, 5, 6]

But it should be somewhat like this as per my understanding(correct me if am wrong)

Arr2 --> [4, 5, 6]
Arr1 ---> [1, 2, 3]

So can i get the explaination why this is happening??
It seems the arraylist object is passed by reference

[nspils]

I don't know what you've been reading. In Java, all parameters are passed by reference EXCEPT the primitive data types (int, char, byte, etc.). All non-primitive data type memory reference is indirect - the "data" for the object is stored "on the heap" while the variable points to a location "on the stack" which points to the object on the heap. What you observed is correct.

[Phaelax]

You're both sort of right. What java technically passes is a copy of the memory address (reference) of the object.

So any changes you make on an object inside a function are shown outside of it, however, assigning a new object to that reference wont be seen.

the variable "object" will not change after this method call because you're assigning a new object to the copied reference. (i'm not very good at explaining this am i?)

MyObject object = new MyObject("13");
public void stuff(MyObject obj)
{
    obj = new MyObject("42");
}

[nspils]

Yes - I have to correct what I said before. The Java method of passing variables is "pass by VALUE" - where the value of all but primitive data types is a memory location (a reference), so the passing of an object is "as if it were" "pass by reference".