ListView problems, using a List instead of an Array

Question

I'm trying to set a ListView and I was able to do it using a simple String[] Array as you can see in the code.

Later on, I commented out the Array and used a List

My problem starts when I want to use the List (since I need to manipulate the contents) and a simple Array can't be dynamically changed. When I use the List I get a NullPointerException when I create the ArrayAdapter and I don't know why.

I read in the documentation of the ArrayAdapter that it is overloaded with 6 different constructors and one of them is

ArrayAdapter(Context context, int resource, List objects)

Why am I getting the NullPointerException?

What should I do to make it work with a List instead of an Array?

Thanks in advance!

public class MainActivity extends Activity implements OnItemClickListener {

ListView l;

// String[] days = {"Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"};

List<String> days;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        days.add(0, "Sunday");
        days.add(1, "Monday");
        days.add(2, "Tuesday");
        days.add(3, "Wednesday");
        days.add(4, "Thursday");
        days.add(5, "Friday");
        days.add(6, "Saturday");


        l = (ListView) findViewById(R.id.listView1);

        ArrayAdapter<String> adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, days);
        l.setAdapter(adapter);
        l.setOnItemClickListener(this);

    }

    @Override
    public void onItemClick(
            AdapterView<?> adapterView, // ListView that was clicked
            View view, // Reference to the row that was clicked (each row is a TextView)
            int i, // position
            long l) { // id of the TextView that was clicked

        Toast.makeText(this, days.get(i) + " " + i, Toast.LENGTH_LONG).show();
    }

}

C. K. Young · Accepted Answer · 2013-08-18 23:24:42Z

2

You forgot to initialise the days field with anything, so it remains null when you call the add method. Before the days.add(0, "Sunday"); line, add this:

days = new ArrayList<String>();

Also, take out the index numbers. Just use days.add("Sunday"); days.add("Monday");, and so on.

answered Aug 18, 2013 at 23:24

C. K. Young

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2 Comments

user2566468 Over a year ago

Chris, about not using index numbers, I'm actually using 2 different collections in my app and they reference each othe using the index number. If I use ArrayList is it guaranteed (if I'm not using index number) that they are going to be in the same position? In other words: is it guaranteed that the index will be the same as the order in they were introduced to the list?

C. K. Young Over a year ago

Yes, add with no index will always add to the end of the list.

andy256 · Accepted Answer · 2013-08-18 23:26:16Z

1

You need to create days:

List<String> days = new ArrayList<String>();

for example.

answered Aug 18, 2013 at 23:26

andy256

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