Add an arraylist to 2d arraylist in java [duplicate]

Question

I have a 2D arraylist which I want to fill it with several 1D arraylists. My code is the following:

ArrayList<ArrayList<String>> array = new ArrayList<ArrayList<String>>(); 
ArrayList<String> list = new ArrayList<String>();

while (ts2.next()) {
    list.add( ts2.getString("userName");
    list.add(ts2.getString("userId"));  
    array.add(list);
    list.clear();
}

I have noticed that list.clear() deletes the elements from the list however also deletes the element from the array. In the end, both array and list are empty. How can I add list in array and clear the list after array.add(list)

A.Sharma · Accepted Answer · 2019-04-02 15:25:42Z

1

You can clone the list:

array.add(list.clone());

Or you can instantiate the list object within the loop itself:

    ArrayList<ArrayList<String>> array = new ArrayList<ArrayList<String>>(); 

    while (ts2.next()) {
        ArrayList<String> list = new ArrayList<String>();
        list.add( ts2.getString("userName");
        list.add(ts2.getString("userId"));  
        array.add(list);
    }

Then you wouldn't even need to clear it.

edited Apr 2, 2019 at 15:25

answered Oct 28, 2016 at 12:31

A.Sharma

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4 Comments

konstantin Over a year ago

I got this message: The method add(ArrayList<String>) in the type ArrayList<ArrayList<String>> is not applicable for the arguments (Object)

A.Sharma Over a year ago

Try to cast the object as an arraylist: array.add((ArrayList<String>)list.clone());

adeel iqbal Over a year ago

array.add(list.clone()); don't work

A.Sharma Over a year ago

@adeeliqbal Not sure why it wouldn't work. You're adding an arraylist into an arraylist of arraylists.

Paolo Mastrangelo · Accepted Answer · 2016-10-28 12:35:36Z

1

You can do this:

    ArrayList<String[]> arr = new ArrayList<String[]>();
    String[] str = new String[2];
    while (ts2.next()) {
        str[0] = ts2.getString("userName");
        str[1] = ts2.getString("userId");  
        arr.add(str);
    }

if you would an arraylist also:

    ArrayList<ArrayList<String>> arr = new ArrayList<ArrayList<String>>();
    while (ts2.next()) {
        ArrayList<String> arrString = new ArrayList<String>();
        arrString.add(ts2.getString("userName"));
        arrString.add(ts2.getString("userId"));  
        arr.add(arrString);
    }

answered Oct 28, 2016 at 12:35

Paolo Mastrangelo

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Comments

Nikolas · Accepted Answer · 2016-10-28 12:39:12Z

1

You can create a new instance of ArrayList<> to be added in the existing one.

while (ts2.next()) {
    List<String> list = new ArrayList<>();
    list.add(ts2.getString("userName"));
    list.add(ts2.getString("userId"));  
    array.add(list);
}

However the better approach is to map these values into a new instance of a class.

List<MyClass> = new ArrayList<>();

while (ts2.next()) {
    MyClass i = new MyClass(ts2.getString("userName"), ts2.getString("userId"));
    array.add(i);
}

answered Oct 28, 2016 at 12:39

Nikolas

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