Copying ArrayList content to another ArrayList

Question

I have Customer class which holds 5 fields. And I have 10 unique instance of Customer class in ArrayList<Customer> firstlist. I copied firstlist content to new ArrayList<Customer> secondlist list with method:

public void copy( ArrayList<Customer> a,  ArrayList<Customer> b){
 for (int i=0;i<a.size();i++){ b.add(a.get(0)); } }

Problem is if I change secondlist instances, it also changes firslist instances because of reference..

How can I copy firstlist instances to secondlist so that when I change secondlist instance it doesn't effect to firtlist instance(copy of itself). Is there any method which can do that?

What about `Copy Constructor` of Customer class? Create a new instance of Customer. some thing like that `public Customer(Customer c){...}` — Braj, Apr 29 '14 at 17:48
You have to make a [deep copy of your list](http://stackoverflow.com/questions/64036/how-do-you-make-a-deep-copy-of-an-object-in-java) — Juan Sánchez, Apr 29 '14 at 17:50
@Braj you're assuming `Customer` class has such constructor, but this isn't stated anywhere in the question. — Luiggi Mendoza, Apr 29 '14 at 17:50
@Braj me too, by voting this question to close as duplicate. — Luiggi Mendoza, Apr 29 '14 at 17:51
@LuiggiMendoza Your solution is perfect but the question is about object references in `ArrayList`. Somewhat similar to it. — Braj, Apr 29 '14 at 17:53
@Braj create a deep clone of each `Customer` in the `List` and pass each clone to the new `List`. I don't see any complication on that. — Luiggi Mendoza, Apr 29 '14 at 17:54
@LuiggiMendoza I think now the OP has enough solutions to solve this issue. — Braj, Apr 29 '14 at 17:55

score 0 · Accepted Answer · answered Apr 29 '14 at 17:50

0

Seems you need to clone the Customer instances.

Check the Cloneable interface:

http://docs.oracle.com/javase/7/docs/api/java/lang/Cloneable.html

and implement it in Customer, then in your loop add clones to
the second list, not the originals: b.add(a.get(i).clone());.

answered Apr 29 '14 at 17:50

peter.petrov

38,363
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Check the possible duplicate Q/A to see why this is not a good option. – Luiggi Mendoza Apr 29 '14 at 17:57

score 0 · Answer 2 · answered Apr 29 '14 at 17:51

0

I suggest you add a clone to your Customer class by implementing Cloneable.

public void copy( ArrayList<Customer> a,  ArrayList<Customer> b){
  for (int i=0;i<a.size();i++){ 
    b.add(a.get(i).clone()); // get(i) I think. 
  } 
}

answered Apr 29 '14 at 17:51

Elliott Frisch

198,278
20
158
249

score 0 · Answer 3 · answered Apr 29 '14 at 17:52

You'll need some method of performing a deep copy of a Customer instance. There are several ways of doing this: implementing clone(), creating a copy constructor, or writing your own deepCopy() method or something like that that returns a deep copy of the object it's called on. Which one you use is up to you.

Copy constructors are constructors which take an instance of the same object as the argument. Generally copy constructors would just deep copy all fields from the passed object to the newly constructed one, and so are pretty easy to implement.

deepCopy() or other such methods would be implemented in a similar way.

I've heard varying things about Java's clone() method and I usually lean towards other methods of performing a deep copy, but that you can implement by implementing the Cloneable interface and properly implementing the clone() method

Copying ArrayList content to another ArrayList

3 Answers3