mysterious adressing problem

General Discussion Other

rafallo

in General Discussion • Other • 8 months ago

Hi

I don't have so much experience in OOP, but I'm trying to program a moderately complex game in Processing. It is not my first in Processing, but it is my first serious project.

I am having a recurring problem that I can't explain. I'm not copy-pasting all the code cause that's too long, so let me explain.

One of my arrayList, let's call it LIST, is modified dynamically by adding elements at some moments in the game. Actually, each time it is augmented by the current configuration of the game (a special class containing the current configuration, a variable that i call CONFIG - I write in caps to distinguish from my text here -).

Problem: When I try to come back to a previous configuration, the program actually calls the current value of CONFIG, and not the old one when it was supposed to be stored in LIST at that moment. As if what I was putting in LIST was not the actual value of CONFIG, but the address of the variable itself...I clearly checked in my program that the only moments where LIST is modified is by a .add method (no .set methods), so I don't think it is a mistake from me.

Sorry for the abstract explanations, I hope I made myself clear. I see really no way out of this.

Replies(6)

quarks

Re: mysterious adressing problem

8 months ago

As you say the description is a bit abstract so I am going to make some assumptions here.

I am assuming that

there is a class which I will call GameConfig (your special class containing the current configuration)
an ArrayList which I will call configList which is used to store game configurations at different stages e.g. ArrayList<GameConfig> configList
you have created a single confiuration object (GameConfig object) which I will call config
config is regularly updated
every now and again config is added to configList

If this is the case then it explains your problem.

Consider the following code

ArrayList<GameConfig> configList = new ArrayList<GameConfig>();
GameConfig config = new GameConfig();
configList.add(config);
// game continues and changes made to config
configList.add(config);
// game continues and changes made to config
//...

In this program it might appear that we have three GameConfig objects

two stored on the array list
one in config

but this is not true.

In Java config is an 'object reference' and it references a part of memory where the objects attributes are stored. In line 4 and 7 you are adding an object reference not the actual object.

So by line 10 what you have is one GameConfig object and three object references -

two object references in the arraylist
and config

Since all the references refer to the same object changes made using one, will be accessable using another reference.

The solution is to store a copy of config on the arraylist. There are many ways to create an exact copy of an object and you might do some searching on "Java cloning".

This code shows how you might implement a copy for my GameConfig class

public class GameConfig {
int x, y;
// other attributes
public GameConfig get(){
GameConfig clone = new GameConfig();
clone.x = x;
clone.y = y;
// copy other attributes
return clone;
}
}

Then when you want to store the current configuration use

configList.add(config.get());

If any of the attributes are also object references then you will need to make copies of them as well, add a get method to that class, and copy the attribute in get()

HTH

rafallo

Re: mysterious adressing problem

8 months ago

Thumbs up, you perfectly understood this. I will try to implement your answer tmr.

So the only variables you actually copy by an array affectation are only the built-in classes int, float, etc? That's good to know.

Thanks!

rafallo

Re: mysterious adressing problem

8 months ago

I just read on wikipedia that there exists a .clone() method that is not adequate in most of the problems.

Basically, duplicating a complicated class seems complicated. I don't understand why. It's actually quite annoying. My class actually contains arrays of objects of some other classes, etc...If I have to all copy by hand I'll get crazy.

Do prgrammers actually do all this hand copying in their class definitions? Or am I doing it the wrong way?

PhiLho

Re: mysterious adressing problem

8 months ago

" duplicating a complicated class seems complicated. I don't understand why"
Because there are several ways to do that, and to make an object.
An object can contain other objects, some of them can be collections of yet other objects, etc. A complex class can lead to a complex chain of references to other objects, and you can even have loops in the relations!
When you say "I copy an object", it can be a shallow copy, where all references just point to the same objects than the previous one. Or it can be a deep copy, where you also copy each referenced objects. But, in this case, you can have several ways to create these copies, if they have several different constructors, etc.
As you can see, the problem isn't trivial.
That's why it is recommended to make copy constructors, for example, (constructors of a class taking an object of same class as parameter) and to define manually the rules of the copy.

rafallo

Re: mysterious adressing problem

8 months ago

As you can see, the problem isn't trivial.

Indeed. Thanks for the explanations. I'll be less ambitious in my programming next time. Is there a "good way" of OOP programming? Or java OOP? Is there a good reference book?

PhiLho

Re: mysterious adressing problem

8 months ago

There are lot of free resources on Internet about OOP and Java, but with abundance comes the task to sort out and read the good stuff... Alas, it is hard to give on precise reference.
The Thinking in Java book is available for free and even if a bit old, it is generally seen as a good book.
On the physical side (and pay one), I loved Head First Java which is the first of the series and is excellent in introducing OOP and Java at the same time in a pleasant and concrete way.