ArrayList

kobystud · May 25th, 2004, 08:04 AM

hello
i have two classes

class1
{
private ArrayList arr;

   public class1()
   {
    this.arr = new ArrayList();
    arr.add("bye");
    arr.add(.....);
    . . . .
    }

   void func1()
  {
   new class2(this.arr);
   }

   public ArrayList arr1
   {
   set{this.arr=value}
   get{return this.arr}
   }
}

class2
{
  class1 c1;
  private ArrayList arrCopy;

  public class2(ArrayList arr)
{
  this.arrCopy = arr;
}
........
void func()
{
c1.arr1.insert(0 , "hello");
}

my question is: suppose i change element 0 from "bye" to "hello" in class2 , why arrCopy is change element 0 too????
how can i avoid it??
thank you very much
koby

planoie · May 25th, 2004, 08:11 AM

An ArrayList is an object. This line:

this.arrCopy = arr;

does not create a copy of the array. It is assigning the object reference of "arr" to "this.arrCopy" so, as you have found, changes to one affect the other.

What you might be looking for is the Clone() method. This creates a shallow copy of the arraylist to another arraylist.

Peter
-------------------------
Work smarter, not harder

kobystud · May 25th, 2004, 10:10 AM

hi
i made a clone and its still the same problem
koby

planoie · May 25th, 2004, 12:56 PM

Hmmm.. Well that's no good then is it.

I looked in the docs and I didn't see any other method to copy an array. You might need to just copy the array manually element by element. Just keep in mind that any elements of the arraylist that are objects will still be the same instance.

Imar · May 25th, 2004, 02:05 PM

Fortunately, you can let the Framework do the copying for you. One overloaded constructor for an ArrayList accepts another ICollection, and all elements it contains are *copied* to the new ArrayList. So, instead of assigning one list to the other like this:

Code:

this.arrCopy = arr;

simply create a new ArrayList and pass in the old version:

Code:

this.arrCopy = new ArrayList(arr);

The following code is a working example that shows you what I mean:

Code:

// Create two List Instances
ArrayList List1 = new ArrayList();
ArrayList List2 = new ArrayList();

// Declare 3rd list for Constructor demo
ArrayList List3;

// Add items to List1
List1.Add("Hi");
List1.Add("Bye");

// Assign List1 to List2 
List2 = List1;

// Change second item in second list
// This should also affect List1
List2[1] = "New Bye";

// Both Message boxes should return New Bye
MessageBox.Show(String.Format("List1[1] is now {0}", List1[1]));
MessageBox.Show(String.Format("List2[1] is now {0}", List2[1]));

// Now use the overloaded contructor to
// *copy* the elements in List1 to List3
List3 = new ArrayList(List1);

// Change second item in third list. Was New Bye
// Is now Old Bye. Will *not* affect List1
List3[1] = "Old Bye";

// Now the MessageBox for List1 will return New Bye (the old value)
// while the listbox for List3 will return Old Bye (the new value)
MessageBox.Show(String.Format("List1[1] is now {0}", List1[1]));
MessageBox.Show(String.Format("List3[1] is now {0}", List3[1]));

Hope this helps,

Imar
---------------------------------------
Imar Spaanjaars
Everyone is unique, except for me.