What does cloning an ArrayList get me?

When I clone an arrayli开发者_开发技巧st am I actually getting the data copied? The API says a shallow copy but I'm not sure what this means.

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Shallow copy means only the references are copied into a new ArrayList. That is, the objects referenced in the new cloned ArrayList are the same objects in the original ArrayList.

It's the equivalent of doing something like:

// shallow copy 
Object[] original = {new SomeObject(), new SomeObject(), new SomeObject()}; 
Object[] copy = new SomeObject[original.length]; 

for(int i = 0; i < original.length; ++i) 
    copy[i] = original[i];

Now, assume SomeObject has a variable "int x":

SomeObject obj1 = original[0];
SomeObject obj2 = copy[0];

obj1.setX(123456);

System.out.println("obj2.x " + obj2.getX());

You'll see:

obj2.x 123456

However, if you were to add a new SomeObject to original, it would not be in copy.

original.add(new SomeObject());
original.size(); //4
copy.size(); //3

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That means that you get a new list but with the same data in it

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It means that the a duplicate of the List is created, but the references the duplicated List stores point to the same objects on the heap that the original does.

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That means it clones the list but the elements are not cloned.

To copy the data you need to iterate over the list and clone each item.

List<SomeObject> newList = new ArrayList<SomeObject>();
for(SomeObject o : list) {
    newList.add(o.clone());
}

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A new ArrayList is created but the objects in the list still point to the old objects