ArrayList not initializes with predefined size

Question

I have an ArrayList of type my model, which conatins 3 items, that is the ArrayList size is 3.

ArrayList<Model> mModels; // mModels.size() = 3

I have to copy this ArrayList in to another ArrayList, so for that I have created another ArrayList of same type as follows.

ArrayList<Model> localModels = new ArrayList<>(mModels.size());

next step is to copy data from member variable to local variable, since I dont want to copy the reference of member variable I have used Collections.copy()

Collections.copy(localModels,mModels);

but I am getting ArrayOutOfBoundException by telling destination size should be greater than source size. so I have logged both variable size. then for the member I got the size as 3 and for the localVariable logged the size as 0.

UPDATE

and I have tried copiying member ArrayList to local one. but it copies only reference. is there any way to copy the data instead of reference ?

I have tried these methods

 //1        
       for(Model model: mModels){    
             localModels.add(model);
        }

 //2    
       for(Model model: mModels){    
          localModels.add((Model)model.clone());
       }

 //3
       Collections.copy(localModels, mModels);

 //4
       localModels = (ArrayList<Model>)mModels.clone();

 //5 
       localModels = new ArrayList<>(mModels);

so my questions are

1- how can I copy (value change should not reflect) value from one ArrayList to another ?

2- why java/android always copiying the reference

3- how to intialize ArrayList with predefined size (already answered)

My bad, if you want to copy an arraylist from an arraylist , you can just do `List newList = new ArrayList(oldList);` — BrockLee, Sep 28 '15 at 13:55
@Sotirios How this qn marked as duplicate. The question you pointed and this are entirely different. — droidev, Sep 28 '15 at 16:48
You seem to have confused the initial capacity as the size. Those are two different things. You're trying copy three elements into a list that does't have three elements. — Sotirios Delimanolis, Sep 28 '15 at 20:09
@SotiriosDelimanolis actually I read that question before posting this question. I posted this question only because it is not of my case. — droidev, Sep 29 '15 at 04:21
If you think the posts in the link don't provide an answer to your question, please edit your question to explain why you think so. — Sotirios Delimanolis, Sep 29 '15 at 04:22

score 5 · Answer 1 · answered Sep 28 '15 at 13:49

5

ArrayList<Model> localModels = new ArrayList<>(mModels.size());

creates an ArrayList whose initial capacity is equal to mModels.size() (3 in your example), but it contains 0 elements.

You have to add elements to the ArrayList in order for it to have a positive number of elements.

If you want your localModels ArrayList to contain the same elements as mModels, you can initialize it with :

ArrayList<Model> localModels = new ArrayList<>(mModels);

This will give you a shallow copy of the original ArrayList though.

answered Sep 28 '15 at 13:49

Eran

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if i changed the values in localModels will it effect the mModels ? – droidev Sep 28 '15 at 13:53
1

@robodev If you add or remove elements from `localModels`, it won't affect `mModels`. However, if you change the state of one of the elements in `localModels`, the change would be reflected in `mModels`, since both lists contain references to the same objects. – Eran Sep 28 '15 at 13:55
is there any way to get separate memory location for the local variable ? – droidev Sep 28 '15 at 13:57
Look at this question: http://stackoverflow.com/questions/10427109/how-to-copy-values-not-references-of-listinteger-into-another-list – Fausto Carvalho Marques Silva Sep 28 '15 at 14:02
Or this http://stackoverflow.com/questions/715650/how-to-clone-arraylist-and-also-clone-its-contents – Fausto Carvalho Marques Silva Sep 28 '15 at 14:22

score 1 · Answer 2 · answered Sep 28 '15 at 13:49

1

You can copy the ArrayList directly using its constructor:

ArrayList<Model> localModels = new ArrayList<>(mModels);

Then there is no need to use Collections.copy.

answered Sep 28 '15 at 13:49

Andy Turner

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it copies the reference only, if I change the any fields in localModels, it effects the member also – droidev Sep 28 '15 at 13:50
`Collections.copy` does too. It doesn't know how to deeply copy the contents of your list. – Andy Turner Sep 28 '15 at 13:51

score -1 · Answer 3 · answered Sep 28 '15 at 13:50

Array Lists have dynamic size, so you don't need to initialize it with a pre-defined size. You can just use ArrayList.addall() to add the contents of one List object to another.

See the Docs here - http://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html#addAll(java.util.Collection)

score -1 · Accepted Answer · answered Sep 29 '15 at 09:30

finally I found the answer:

I have to copy each fields in the Models separately to second ArrayList

like

ArrayList<Model> localModels = new ArrayList<>(mModels.size());

for(Model model: mModels){
 Models tempModel = new Model();
 tempModel.setId(model.getId());
 tempModel.setName(model.getName());
 ....
 ....
 localModels.add(tempModel);
}

return localModels();

this is the way to copy contents inside one ArrayList to another ArrayList. any other method posted in question will copy only reference

ArrayList not initializes with predefined size

4 Answers4