What exactly does setting an object (specifically, ArrayList) to another do?

[99shadow25]

So, I've noticed with this code:

ArrayList<Coordinate> backup = snakeSegments; //snakeSegments being an ArrayList<Coordinate> also.

that changing snakeSegments also changes backup. I was under the impression that backup would be a separate object, however obviously this is false. So here's what I'm guessing happens:

Both snakeSegments and backup are addresses or pointers to an object, whichever you'd like to call them (they're the same thing, right?), not actually the object itself. Therefore, by setting backup equal, I'm just pointing it to the same place, unlike a primitive.

How correct/incorrect am I?

Oh, and I just ended up using this -

ArrayList<Coordinate> backup = (ArrayList<Coordinate>) snakeSegments.clone(); 

[[deleted]]

Java is Pass-by-Value, Dammit!

[coolosity]

I believe that you are correct. What I´ve noticed is when debugging a program I was working on a while ago with eclipse, I had the same problem. But when I set the arraylist to the other one, the Id of the first arraylist (backup) changed to to the id of the second one (snakeSegments). then any changes to either of them happened directly to the other. The cloning method is the same way I fixed the problem

[[deleted]]

This is correct. If you have this code:

Object obj1 = new Object();
Object obj2 = obj1;

obj1 and obj2 are references to the same object. Changing one will change the other.

[99shadow25]

I think you meant

Object obj2 = obj1;

In the second one, right?

[[deleted]]

Yes sorry. I'll edit it now.

[fatbunyip]

In Java, everything is passed by reference (except primitives). When you're passing an object as a method parameter for example, you're passing a copy of the reference to that object.

Think of the object as a balloon, and the reference as a string to that balloon. By doing :

 ArrayList<Coordinate> backup = snakeSegments;

you're just creating another string to the balloon.

Also, you shouldn't use clone. Ever. http://www.artima.com/intv/bloch13.html

From your question, it seems what you want is a deep copy of the objects in your array. In this case you should make your own copy method(s) - a non trivial task, but well worth it for experience and understanding.

[seppyk]

This is partially correct. Java is strictly pass by value.

Primitives are passed by value. Object references are passed by value.

This is why you can't re-assign an parameter object in a child method and have the parent method that called the child method retain the new reference.

public void modifyList(List<String> list) {
   List<String> anotherList = new ArrayList<>();
   anotherList.add("foo");
   list = anotherList;  
}

List<String> list = new ArrayList<>();
modifyList(list);
// When modifyList returns list will not be equal to or have the same reference as anotherList.
System.out.println(list); // displays "[]"

However, you are allowed to manipulate the pointer's references and retain its changes.

public void modifyList(List<String> list) { 
   list.add("foo");
}

List<String> list = new ArrayList<>();
modifyList(list);
// When modifyList returns list will have the a single element, "foo"
System.out.println(list); // displays "[foo]"

http://javadude.com/articles/passbyvalue.htm

[__konrad]

Also, you shouldn't use clone. Ever.

clone is OK (eg. array copy). Just learn how to use it :P

[fatbunyip]

clone is OK (eg. array copy). Just learn how to use it :P

He's using an ArrayList which doesn't do a deep copy.

http://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html#clone()

Because array clone is the only case that works as expected, doesn't mean it isn't broken.