Clearing a global ArrayList clears an other too

Question

I have a really weird bug.

I am programming a generic class and in one of its method, there is a strange thing. Here the class code (not completely):

public abstract class GenericFragment<M extends BaseModel, A extends BaseAdapter> extends HtmlFragment {

    private ArrayList<M> datas = new ArrayList<>();    
    private A adapter;

    protected abstract String getUrl();

    protected abstract A setAdapter();

    protected A getAdapter() {

        if(adapter == null) {
             adapter = setAdapter();
        }

        return adapter;
    }

    protected ArrayList<M> getDatas() {
        return datas;
    }

    protected void refreshData(ArrayList<M> datas) {

        Log.d("before glob", String.valueOf(this.datas.size()));
        Log.d("before loc", String.valueOf(datas.size()));

        this.datas.clear();

        Log.d("clear glob", String.valueOf(this.datas.size()));
        Log.d("clear loc", String.valueOf(datas.size()));

        this.datas.addAll(datas);

        Log.d("after glob", String.valueOf(this.datas.size()));
        Log.d("after loc", String.valueOf(datas.size()));

        getAdapter().notifyDataSetChanged();
    }
}

And when I call the function refreshData() the global ArrayList is cleared but also the local ArrayList. Here are the logs:

D/niviel: before glob: 5
D/niviel: before loc: 5
D/niviel: clear glob: 0
D/niviel: clear loc: 0
D/niviel: after glob: 0
D/niviel: after loc: 0

I just want to clone the local array to the global array.

EDIT:

The function is called in a child class.

requestData(activity, new requestDataCallback() {
    @Override
    public ArrayList<? extends BaseModel> parseDatas(Document document) {
        return RecordProvider.getRecord(activity, document);
    }

    @Override
    public void runOnUIThread(ArrayList<? extends BaseModel> datas) {

        refreshData((ArrayList<Record>) datas);
    }

});

What parameter are you passing to `refreshData`? If it is `this.datas`, why wouldn't you expect it to clear "both" `this.datas` and `datas`? ("both", because it's the same instance). — Andy Turner, Jul 01 '16 at 16:21
Was the class field set in the setter and used in refreshData? — KJaeg, Jul 01 '16 at 16:25
how do you pass the arraylist or how do you copy from global arraylist — intellignt_idiot, Jul 01 '16 at 16:25
The arraylist comes from a callback and the function is called in a child class in a callback. — Herel Adrastel, Jul 01 '16 at 16:46

score 3 · Answer 1 · answered Jul 01 '16 at 16:28

In your refreshData(), both the lists u are using are same.

protected void refreshData(ArrayList<M> datas) {

Log.d("before glob", String.valueOf(this.datas.size()));
Log.d("before loc", String.valueOf(datas.size()));

//here you are clearing datas
this.datas.clear();

//so now your datas size will be 0

Log.d("clear glob", String.valueOf(this.datas.size()));
Log.d("clear loc", String.valueOf(datas.size()));

//here you are adding it to datas again whose size is already 0
this.datas.addAll(datas);

Log.d("after glob", String.valueOf(this.datas.size()));
Log.d("after loc", String.valueOf(datas.size()));

getAdapter().notifyDataSetChanged();
}

I don't understand how they can be the same: even if I change the local array's name, there is still the problem — Herel Adrastel, Jul 01 '16 at 16:32

score 1 · Accepted Answer · answered Jul 01 '16 at 18:59

1

Ok I finally debugged my code. I noticed that the function refreshData() was passing in parameters the global ArrayList.

answered Jul 01 '16 at 18:59

Herel Adrastel

151
11

score 0 · Answer 3 · answered Jul 01 '16 at 16:22

0

Learn about Java memory model. List is reference type of data, so you work with the same list.

answered Jul 01 '16 at 16:22

Stanislav Parkhomenko

499
5
18

Wow, thank you for your fast answer. So according to you, what is the best way the solve the problem ? – Herel Adrastel Jul 01 '16 at 16:24
For example you can use [this.](http://stackoverflow.com/questions/15778645/passing-arraylist-as-value-only-and-not-reference) Also would be very nice if you provide part of code where you use refreshData function. – Stanislav Parkhomenko Jul 01 '16 at 16:32
the problem is if I make a new array, the adapter won't work, will it ? – Herel Adrastel Jul 01 '16 at 16:44
You have 1 ArrayList which added to adapter, if you change different ArrayList or its copy, then ofc no changes in your adapter would apear. – Stanislav Parkhomenko Jul 01 '16 at 17:39
Yeah, that's why I use clear and addAll. All I want is to replace the global array with the local one in order to call notifydatasetchanged – Herel Adrastel Jul 01 '16 at 17:42
You can call notifyDataSetChanged any time you want. – Stanislav Parkhomenko Jul 01 '16 at 17:48

Clearing a global ArrayList clears an other too

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