Question

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Java: how to clone ArrayList but also clone its items?

I have a sample program like the following:

ArrayList<Invoice> orginalInvoice = new ArrayList<Invoice>();

//add some items into it here

ArrayList<Invoice> copiedInvoice = new ArrayList<Invoice>();

copiedInvoice.addAll(orginalInvoice);

I thought I can modify items inside the copiedInvoice and it will not affect these items inside originalInoice. But I was wrong.

How can I make a separated copy / clone of an ArrayList?

Thanks

Answer 1

Yes that’s correct – You need to implement clone() (or another suitable mechanism for copying your object, as clone() is considered “broken” by many programmers). Your clone() method should perform a deep copy of all mutable fields within your object. That way, modifications to the cloned object will not affect the original.

In your example code you’re creating a second ArrayList and populating it with references to the same objects, which is why changes to the object are visible from both Lists. With the clone approach your code would look like:

List<Foo> originalList = ...;

// Create new List with same capacity as original (for efficiency).
List<Foo> copy = new ArrayList<Foo>(originalList.size());

for (Foo foo: originalList) {
  copy.add((Foo)foo.clone());
}

EDIT: To clarify, the above code is performing a deep copy of the original List whereby the new List contains references to copies of the original objects. This contrasts to calling ArrayList.clone(), which performs a shallow copy of the List. In this context a shallow copy creates a new List instance but containing references to the original objects.