Cloneable and Collection

Question

I am trying to code my own version of ArrayList (to learn how it works) and I was looking at the Cloneable interface which is implemented by java.lang.ArrayList (I would like to keep the same contract than the original version).

I am a bit confused by the definition of the clone() method and hope someone can clarify it for my case.

The part I am not really sure of, is that if someone use the clone() method on my collection, it should return a new ArrayList (meaning not a reference to the existing one). However:

Does it means that each object contained needs to be cloned as well?
Do I have to explicitly create a new instance using 'new' for each of them (which should be quite slow?)
Should I be trying to use the clone() method on my collection's objects, and how to be sure they override the clone() method as the definition in Cloneable states that it is not explicitly required (the method is not in the Interface)?

Thanks in advance for any help.

The link you gave said it must do a field-for-field copy. To me, that sounds like a shallow copy, not a deep (Recursive) copy. — Patashu, Jun 05 '13 at 01:30
See also http://stackoverflow.com/questions/4081858/about-java-cloneable — Edward Falk, Jun 05 '13 at 01:46

score 3 · Accepted Answer · answered Jun 05 '13 at 01:33

3

In the context of a Collection the contract is to get a new Collection with the same objects, not copies or clones of the objects. Anything else would break the contract.

So what you want is a "shallow" copy of the elements, just their references, in the new collection. What you are suggesting with the cloning/new is a "deep copy" which is very different, and you shouldn't do in this case.

So

You definitely should not clone each of the objects in the Collection.
Same as 1. You should not use new, this would be doing a "deep" copy of the collection.
No, same as 1. 2.

answered Jun 05 '13 at 01:33

greedybuddha

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In that case would the following statement be enough? return new ArrayList().addAll(this); – clement Jun 05 '13 at 01:36
Well it would be fine as long as the method `addAll` returns the `new ArrayList`. But, usually `addAll` returns a boolean value. – greedybuddha Jun 05 '13 at 01:39
Yeah, got the point, just write it like that, but got the principle, thanks! – clement Jun 05 '13 at 01:41

Cloneable and Collection

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