CODEWITHSUNDEEP
javamemoryreference
well actually
well actually

Reputation: 12370

Simple Java question about references to ArrayList element

Say I have the following code,

LinkedList partials = new LinkedList();
partials.add(new ArrayList());
ArrayList head = partials.element();
head.add("Test");

I want "head" to simply be a copy of the Arraylist that is a result of partials.element(). However, now when I make changes to "head", it is reflected in the Arraylist partials. How do I make a copy of the Arraylist that is the first element of partials such that making changes to the Arraylist won't be reflected in partials?

Upvotes: 1

Views: 1064

Answers (2)

Tanguy
Tanguy

Reputation: 191

actually the code can't compile at line 3 : partials.element() returns an Object (the first element of your LinkedList) and you have to cast it as an ArrayList if you want to use it this way.

I agree you don't need generics to make the code easier to add. You could also define partials and head as List instead of LinkedList & ArrayList. Use interface instead of implementations is a good practice and avoid you to use too much specific method like element() one when you don't need to.

Upvotes: 1

Jon Skeet
Jon Skeet

Reputation: 1500585

Two options come to mind:

  • Call the ArrayList constructor which takes another collection - that'll make a copy
  • Call clone() on the ArrayList

Note that both of these will create shallow copies - each list will contain references to the same objects. That's fine if they're immutable (like strings) but if you want to create a deeper copy you'll need to do more work.

Is there any reason you're not using generics, by the way? For example, I would probably use:

LinkedList<ArrayList<String>> partials = new LinkedList<ArrayList<String>>();
ArrayList<String> list = new ArrayList<String>();
list.add("Test");

// Create a shallow copy and add a reference to that into the linked list
partials.add(new ArrayList<String>(list));

list.add("Another element");

// Prints 1, because the lists are distinct
System.out.println(partials.element().size());

If nothing should ever change the contents of the lists in the linked list, you may want to look at the immutable collections available in Guava, or wrap your clone using Collections.unmodifiableList. Note that the latter only creates a view on the original list, so you'd still need to perform the clone step first.

Upvotes: 7

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