When are constructors one and two used in the ArrayAdapter?

Question

ArrayAapter Class have some constructors:

When are constructors one and two used? Because we don't have data structure like input array or list at all in these constructors. If possible, explain with an example. Thank you

Answer 1

Those two constructors:

public ArrayAdapter (Context context, 
                int resource)


public ArrayAdapter (Context context, 
                int resource, 
                int textViewResourceId)

are used when you want to create an ArrayAdapter but you don't have the data at the moment of its creation. You can add data later on using methods like add(), insert(), etc. They actually internally call other constructors with empty collection as parameter:

public ArrayAdapter(@NonNull Context context, @LayoutRes int resource) {
    this(context, resource, 0, new ArrayList<>());
}

public ArrayAdapter(@NonNull Context context, @LayoutRes int resource,
        @IdRes int textViewResourceId) {
    this(context, resource, textViewResourceId, new ArrayList<>());
}

As for example, you might want to create an ArrayAdapter for a ListView in your activity's onCreate() method, but the data to populate the ListView is fetched later from a network request. In this case, you can use the first or second constructor to create the ArrayAdapter and then add data to it once your network request is complete.

Some useful links:

https://developer.android.com/develop/ui/views/layout/declaring-layout#AdapterViews

https://android.googlesource.com/platform/frameworks/base/+/refs/heads/master/core/java/android/widget/ArrayAdapter.javae

[edit] If the first constructor is used, then text view resource will be 0, from the source code (link above) you can learn that in this case the whole resource is considered a TextView:

    if (mFieldId == 0) {
            //  If no custom field is assigned, assume the whole resource is a TextView
            text = (TextView) view;
        } else {

Answer 2

These two constructors create empty ArrayAdapter instances.

ArrayAdapter has methods add() and addAll() that allow you to add elements to such an empty ArrayAdapter.