Copying value of objects, not reference?

Question

This has been bothering me for a while. I learned that when you set one object equal to another you are really just assigning the same reference to both, so that if one is altered they are both altered. Well, how can you set one object equal to the value of another but not the same reference so that altering one will not alter the other. My concern stems from returning arrays in methods. Since the reference itself is passed, any changes I make to the array are permanent, correct, and not just existing inside the method? So I was thinking maybe the best solution would be to use a tmp{] variable that is equal to the VALUE of the array I pass in, and then return that, so the original array is not altered in any way. Is this the customary practice for dealing with such situations? If not, what is typically done? Oh and just for future reference, because it might be of value to know, how DO I copy a value and not a reference? (and yes, that WAS on purpose ;) )

Answer 1

violagirl23 wrote:
My concern stems from passing arrays to [KRC] methods. Since the reference itself is passed, any changes I make to the array are permanent, correct, and not just existing inside the method? So I was thinking maybe the best solution would be to use a tmp{] variable that is equal to the VALUE of the array.Both georgemc and tschodt have already given you some good pointers, I'll try not to reiterate the same advise.
This is an old "problem", discussed at length by folks much smarter than myself, and (IMHO) there still is no definitive question, let alone universal answer. What I mean is that sometimes this ability to modify the contents of a collection or array within a method is highly desirable; and sometimes of course it's just a source of bugs, when a noob calls your method not realising that it modifies the contents of the passed array (which shouldn't make it past unit testing anyway)... So, I think probably the best thing I can do for you is to give you the terms to google.
What you're talking about is called a defensive copy... which, especially with arrays and collections, typically involves a deep copy (make a copy of every attribute of every object, all the way down the reference tree)... as apposed to Arrays.copyOf, which is a shallow copy, i.e. it just copies the references to the "top" objects.
If you're writing a system from scratch you could decide to implement clone methods for everything you ever need to defensive copy. Cloning is ugly, no matter how you cook it. It requires every part of the class heirarchy being cloned to support the clone method (properly).
In the past, I've tried to write a "generic" copy method, in the abstract-base-type of a class heirarchy using reflections. Everything I've tried has "pretty fatal" flaws. The only "proper" solution (AFAIK) is to just implement clone in every single frickin class... and that's a lot more "boiler plate" code than I personally think should be required to implement such a common, mundane task.
Alternately, you could make all your data transfer objects immutable, so a shallow copy (Arrays.copyOf) of the array is enough to protect the calling code from unexpected changes to the array contents because you are passing "a copy" of the references to things-that-cannot-change... So the callee cannot mutate a pointed-to object, and if the callee changes which object an array element points-to, you won't "see" the change... and of course the callee cannot change which array your reference points to, because Java passes references by value.
Google all the italic stuff... Sun's stuff is the best, then lookout for IBM's stuff, and most Wikipedia articles are really good (a few not so good).
HTH. Cheers. Keith.