Java pass by value clarified

Question

I have a list of Cell object that represent a board game inside the board class.

Cell boardGame[][] = new Cell[8][8];

I needed a temporary cell to try the player move on him and compare it to the other cells, so I though that I could use a java pass-by-value to do it.

test(board.boardGame);
board.printBoard(); 

private static void test(Cell[][] boardGame) {
Cell c = new Cell((new Soldier(ChessPiece.QUEEN, Color.WHITE)), 7, 4);
    boardGame[7][7] = c;

}

I read some post about java here, but apparently I still didn't catch it 100%.

I expected to see only one white queen on the board, but I saw two. I know that if you pass a reference you can change its values, but I though that if I would pass the array itself its members won't be modified unless I would execute a return.

Please help me to understand this subject better. Thanks

Edit:

I think I don't understand when it called attributes and where it doesn't. I though the different it if you are call "new" or not.

When its part of another object it called attribute right? but every object can be created as part of another object. I can create a new string in dog class and then create the dog class in the animal class and then create it in another class. So only the top class is in the stack?

For exemple:

public class Board  { //fake class

    int num= 0;
    public void test(int i){
        i = 1;
    }
}

and on another class:

    public class Main {
        static void outsideTest(Board board){
            board.num = 1;
        }
    public static void main(String[] args) {
        Board board = new Board();
        System.out.println(board.num);
        board.test(board.num);
        System.out.println(board.num);
        outsideTest(board);
        System.out.println(board.num);
}
}

Now I didn't understand why on test() method the num didn't change and on outsideTest() the num change, num as been created in the heap because its part of the board object, so its need to be changed on both cases no?

Answer 1

The best and least confusing way to remember it is as follows: Java passes everything by value, that includes the references. :)

When you have a variable:

Object a = new Object();

you don't actually have an object stored in a. What you have is a reference to an object somewhere in memory.

Likewise when you call a method on an object:

String b = a.toString();

you don't do that. What you call is a method that uses the data of the referenced object as its context.

So when you pass an object as an argument

System.out.println(b);

You don't pass the whole object. You pass the reference and that reference is passed by value.

edit:

If the reference were not passed by value, but by reference, you could do something like this, which fortunately you can't.

public void swap(Object a, Object b){
   Object swap = a; a = b ; b = swap;
} 

String a = "a";
String b = "b";
swap(a,b);

// would print "b a" if references were 
// passed by reference but instead prints 
// "a b" as they're passed by value.
System.out.println(a + " " b);

Answer 2

The reference to the object is passed by value, which means that

boardGame = new Cell[8][8];

does not do any harm, but changing anything you get from boardGame does.

Answer 3

Java is essentially always "pass-by-value".

Caveat: It passes the value stored in the memory for the variable.

For primitive data types, the memory is allocated in stack space, whereas for objects reference memory is allocated in stack space but the object itself is created in heap. Similar can be stated for arrays too though they are not exactly objects in a strong sense.

Diagrams below would make it clearer.

Your Cell object 2D array should seem something like anObjectArrayVar (not exactly as the ref in the diagram pointing to the objects should now be pointing to the rows and we would need another level of allocation in heap in between ref and objects for each row (a set of cells refering to the objects).

So, when you pass boardGame, the value stored in the stack is passed that stores the reference to the array of objects (just like the value stored in the anObjectArrayVar). If say the list of refs is stored in location numbered 50 then anObjectArrayVar would have stored that and it passes that value to the test method when we call it. In such a scenario the test method wont be able to goto memory location anObjectArrayVar and change its value (to say 100) as it has only a copy of the value but it could easily change what it refers to(directly or indirectly) like the values in ref or the next level (and add new objects as in your case adding a new cell with queen) or the objects pointed to by them and those changes would reflect through out the program!

I would also like to draw your attention to the fact that the code

boardGame[7][7] = c;

would replace the current cell (as well as the soldier currently in it) which would create major issues if there was originally a soldier in that place at that point in the game. The game state would actually change.

As a suggestion (given the limited knowledge about your design) I would say at least save the cell in some other value in test method before replacing it.

Cell old = boardGame[7][7];
//now do all your operations
boardGame[7][7] = old;//just before returning from the function