Warning cloning ArrayList in Java

Question

public void addProductList(ArrayList<ViewOrderProduct> globalOrderProductList) {

    this.productOrderList =  (ArrayList<ViewOrderProduct>)globalOrderProductList.clone();
}

This gives me the warning:

Type safety: Unchecked cast from Object to ArrayList

I know that I can simply solve the problem adding @SuppressWarnings("unchecked").

But I want understand the problem. I can be sure that all goes good if I add the suppressWarnings? Is there another solution for this warning?

It is just a warning for you, because he can't verify at runtime that only `ViewOrderProduct` objects are in your cloned list. — Thomas Jungblut, Nov 22 '13 at 14:37
@ThomasJungblut Nope, `clone()` returns an `Object`, so the warning is correct and has nothing to do with type erasure or the like. — Neet, Nov 22 '13 at 14:40
@Neet you better reread the definition of an `Unchecked cast` ;) — Thomas Jungblut, Nov 22 '13 at 14:42
Related about `clone` method and cloning/copying: http://stackoverflow.com/q/2326758/1065197, http://stackoverflow.com/q/2156120/1065197. Now, your problem is about the warning, related: http://stackoverflow.com/q/9499908/1065197 — Luiggi Mendoza, Nov 22 '13 at 14:43
@ThomasJungblut ... *cough* ... yes, you're right. My fault^^ — Neet, Nov 22 '13 at 15:42

Neet · Accepted Answer · 2013-11-22T14:59:33.407

9

clone() returns Object by default, the correct way to clone an ArrayList is to use the appropriate constructor:

this.productOrderList =  new ArrayList<ViewOrderProduct>(globalOrderProductList);

Edit: The preferred way is to use the appropriate constructor, and both methods only return a shallow copy anyways.

Edit: And there's no other way I'm aware of, to remove the warning using clone() without a SuppressWarning.

edited Nov 22 '13 at 14:59

answered Nov 22 '13 at 14:37

Neet

3,937
15
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That will give you a shallow copy, not a in-depth copy. This, of course, depends on OP's needs. – Luiggi Mendoza Nov 22 '13 at 14:39
1

@LuiggiMendoza `clone()` on `ArrayList` also returns a shallow copy. – Neet Nov 22 '13 at 14:41
Refer to links I posted in OP's question about usage of `clone` method. – Luiggi Mendoza Nov 22 '13 at 14:43
@LuiggiMendoza well, the OP should skip the whole clone thing. Using the ctor is the preferred way anyways. – Neet Nov 22 '13 at 14:47
In short, yes. But the question itself is about using `@SuppressWarnings("unchecked")`. – Luiggi Mendoza Nov 22 '13 at 14:47
The only operation made over the globalOrderProductList is a .clear(), so I suppose I don't need a deep copy – GVillani82 Nov 22 '13 at 14:59

score 0 · Answer 2 · answered Nov 22 '13 at 14:55

arraylist is wrapper over Object[], anything inside stored as Object, it does not matter what constructor is used result will be same.

as stated above, to evoid warning you shoud use constructor public ArrayList(Collection c) {}

eg new ArrayList(globalOrderProductList);

this will return copy of content without warnings.

Warning cloning ArrayList in Java

2 Answers2