Java ArrayList of ArrayList non-dynamic

Question

I am using an ArrayList to track the path of a line drawn on the screen (Android).

I would like to save these values (in another ArrayList), then analyse them once I hit the "Submit" button.

However, it seems to be adding a pointer and not saving what is there at that particular time.

public static ArrayList<Float> drawnData = new ArrayList<>();
public static ArrayList<ArrayList<Float>> saveData = new ArrayList<>();

    //This is called from another class upon MotionEvent.ACTION_UP
    //drawnData is updated from another class during the drawing
    public void updateData(){
        saveData.add(drawnData);
    }

How do I set this up so it saves what is in drawnData every time the method is called, and not just a pointer to what is currently in drawnData?

For example, if this method is called 5 times, when I go to look at the data, it contains 5 ArrayLists, all containing data for the path that was last drawn, instead of each of the individual paths.

Bonus question: Should I just be using List instead of ArrayList?

Eran · Accepted Answer · 2015-06-27 06:26:07Z

2

Add a copy of drawnData:

saveData.add(new ArrayList<Float>(drawnData));

As for List vs ArrayList, it's always better to declare the variables using the interface types instead of the implementation types.

public static List<Float> drawnData = new ArrayList<>();
public static List<List<Float>> saveData = new ArrayList<>();

answered Jun 27, 2015 at 6:26

Eran

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