How can I check specific checkboxes on startup?

Question

I am using Android Studio to create an app using java but I am fairly new to it, most of my experience is in Visual Studio with C# and Winforms.

Currently I have a ListView with CheckBoxes inside of it and I'm using an ArrayAdapter to sync the list.

    ArrayList<String> items = new ArrayList<>();
    items.add("Alpha");
    items.add("Bravo");
    items.add("Charlie");
    items.add("Delta");

    ArrayAdapter<String> adapter = new ArrayAdapter<>(this, R.layout.row_layout, R.id.chkText, items);
    lv.setAdapter(adapter);

    // Test
    Toast.makeText(getApplicationContext(), "Count " + lv.getAdapter().getCount(), Toast.LENGTH_LONG).show();
    for (int j = 1; j < lv.getAdapter().getCount(); j++)
    {
        CheckBox cb = (CheckBox) lv.getAdapter().getItem(j);
        cb.setChecked(true);
    }

The getCount() does return the proper value but the app crashes when it tries to check the boxes. All the answers I found online seem way overcomplicated for such a simple task. Is there nice and easy way I can check boxes "Bravo" and "Delta" when the app loads for example?

`lv.getAdapter()` returns, well, an adapter, not a checkbox. — OneCricketeer, Aug 24 '17 at 20:30
Was missing a part, my fault, it now reads: lv.getAdapter().getItem(j); — Walter Bishop, Aug 24 '17 at 20:32

OneCricketeer · Answer 1 · 2017-08-24T22:24:47.090

The true way to get at that one Checkbox requires you to dig into the ListView children, not the adapter itself.

android - listview get item view by position

Basically, the adapter only stores the data and "binds" the data to the view. The Adapter doesn't actually hold the view information itself.

lv.getAdapter() returns you a generic, untyped Adapter<?>
When you do lv.getAdapter().getItem(j), that returns you an Object, which can only be cast as a String because you have used an ArrayAdapter<String>.

The better way to approach this problem is if you make a custom class that extends ArrayAdapter<String>, and then you would be able to write your own fields to store a boolean list called mChecked here, for example, and a method to update that list.

In more detail, here's the idea

private List<Boolean> mChecked = new ArrayList<>();

public void setChecked(int position, boolean checked) {
  mChecked.set(position, checked); // Might throw out of bounds exception! 
  notifyDataSetChanged(); // Need to refresh the adapter
}

public boolean isChecked(int position) {
  return mChecked.get(position);
}

@Override
public void getView( ... ) {
    ...

    View rowView = ... ;

    TextView tv = (TextView) rowView.findViewById(R.id.chkText);
    tv.setText(getItem(position));
    Checkbox cb = (Checkbox) rowView.findViewById(R.id.checkbox);
    cb.setChecked(isChecked(position));
}

The getView() method of that adapter class would control when you've checked a box or not via checkBox.setChecked(isChecked(position));

In the end, you won't be setting the boxes when the app loads, but rather when you initialize the adapter.

I think I'm getting confused on what the View is for. In your first approach, that's awesome if I can get a View but what do I do with it? I can't seem to spot a getChild() or getItem() method. With the second approach, say I did create a custom class, how could I get setChecked() to work if I cannot do so with the current adapter? Sorry, I am rather new to a lot of this. Everything seems backwards. — Walter Bishop, Aug 24 '17 at 21:34
`lv.getChild()`. You would get some `View` object which you then must `findViewById` for whatever checkbox ID you set within `row_layout.xml`. For the second option, see this for an example https://guides.codepath.com/android/Using-an-ArrayAdapter-with-ListView — OneCricketeer, Aug 24 '17 at 22:06

How can I check specific checkboxes on startup?

1 Answers1