When does a 'void' method affect the parameter, and when does it affect the original object?

Question

I am brand new to programming, as well as to this website, so forgive me if I screw anything up. I'm having a heck of a time figuring out how to properly post my code in here.

package tester;
import java.util.*;
public class Mainclass2 {
    public static void main(String[] args) {

        int y = 3; 
        int[] x = {1, 2, 3, 4};

        editnumbersArray(x);
        editnumbersNotArray(y);

        System.out.println(x[2]); **//now this was changed from 3 to 9...**
        System.out.println(y);    //but this one went unchanged.

    }

    //this accepts 'x[]' into the 'a[]' parameter.
    public static void editnumbersArray(int[] a){
        a[2] = 9;  **//<---why does this one CHANGE the actual x[2] instead of just a[2]?**
    }

    //this accepts 'y' into the 'a' parameter.
    public static void editnumbersNotArray(int a){
        a = 9;  **//<--while this one only changes 'a' instead of 'y'?**
    }

}

So my question is basically typed in there as comments. Why does the array that is passed into the method change the values of the original array (x[]) when the int that is passed into the other method doesnt change? I'm sure it's a simple answer, but when I did my research I couldn't figure out what to search. I don't know what this is called so everything I searched led me the wrong way. Thanks for any help!!

EDIT: Thanks for that analogy with the address! That is by far the best way you could have explained it to me. So basically when you pass an array into a parameter, its passing a reference, not the actual value? So when I make adjustments within my method, its changing whatever the array is referencing? I noticed that this also happens with a list. So the list isnt actually passed by value? It seems as if the array/list itself is basically passed in for editing, no matter what I name it within my method (a[] in this case.)

EDIT http://javadude.com/articles/passbyvalue.htm this page really cleared it up. And sorry for posting a duplicate question. The problem was that I didn't know what I was trying to ask. I had never even heard these terms "pass-by-value/reference", so now I know

Answer 1

Changing the value of the parameter itself never affects the argument in Java, because all arguments are passed by value. However, look at this method:

public static void editnumbersArray(int[] a){
    a[2] = 9;
}

That assignment doesn't change the value of the parameter. The value of a is still the same reference, to the same array - it just changes the contents of the array.

Imagine if I wrote my home address on a piece of paper for you. It wouldn't matter what you did to that piece of paper - that wouldn't change where I lived. However, if you visited the address and painted the front door green, without ever changing the piece of paper at all, I would see that change.

It's very important to differentiate between different concepts:

• A variable is a named storage location; it holds a value, which is always either a primitive value (e.g. an int) or a reference. In my example above, the piece of paper was like the variable.
• A reference is just a value which allows you to navigate to an object. It's not the object itself. It's like the address on the piece of paper.
• An object contains other variables. There may be several variables which all have values which are references to the same object. It's like the house in my example: I can write my address on several pieces of paper, but there's only one house.

An array is an object which acts as a container for other variables. So the value of a is just a reference to the array.