Deep copy when Using ArrayList in java

Question

I know that it is a trivial matter but I want to ask this question. Suppose that I have a get method that returns an ArrayList of objects. First do we have to returns a copy of it? If so, does it have to be a deep copy of the ArrayList? Do we still need to do a deep copy when the objects inside it are immutable? If we use this type of constructor

ArrayList<T> arr = new ArrayList<>(collection);

then are the elements of the array copied or they still point to the old values? Thanks

aioobe · Accepted Answer · 2015-02-21 12:57:09Z

5

It depends on your use case. Sometimes it's desirable to expose the underlying collection, some times it's not.

If you for instance want to leverage the collection methods for your class you could return the underlying collection (or a modifiable view of the underlying collection). This enables clients of your class to do, for instance

phoneBook.getPeople().removeIf(Person::isAddressInvalid);

If you don't want to expose the underlying collection I would go with

return new ArrayList<>(yourCollection);

I think it's very rare to return a deep copy in a get method.

answered Feb 21, 2015 at 12:57

aioobe

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2 Comments

randomDisplayName Over a year ago

Ok what if the type is String then can I return it this way: return new ArrayList<>(yourCollection);

aioobe Over a year ago

Absolutely. Strings are immutable, so returning a deep copy of strings is particularly pointless.

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