The spinner is empty when no new object is being generated for an ArrayAdapter

Question

To assign data of an ArrayList<String> alKategorien to a spinner auswahl_kategorie, I am using this snippet:

ad = new ArrayAdapter(
        this,
        android.R.layout.simple_spinner_item,
        alKategorien);
ad.setDropDownViewResource(
        android.R.layout
                .simple_spinner_dropdown_item);
auswahl_kategorie.setAdapter(ad);

I tried to reuse the ArrayAdapter by phrasing it like

if (ad == null) {
    ad = new ArrayAdapter(
            this,
            android.R.layout.simple_spinner_item,
            alKategorien);
    ad.setDropDownViewResource(
            android.R.layout
                    .simple_spinner_dropdown_item);
    auswahl_kategorie.setAdapter(ad);
}
else
{
    ad.clear();
    ad.addAll(alKategorien);
    ad.notifyDataSetChanged();
}

Yet unless ad is null, this yields an empty spinner.

How do I reuse the ArrayAdapter properly?

Answer 1

I have just made a very simple sample.

activity_main.xml:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
android:padding="10dp"
tools:context=".MainActivity">

<Button
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:text="Button 1"
    android:id="@+id/bt1" />

<Button
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:text="Button 2"
    android:id="@+id/bt2" />

<Button
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:text="Button 3"
    android:id="@+id/bt3" />

<Button
    android:id="@+id/btGetSpinnersValues"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:text="Get Spinners Values" />

</LinearLayout>

item_schreiben.xml:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:padding="10dp">

<TextView
    android:id="@+id/tvSample"
    android:layout_width="match_parent"
    android:layout_height="wrap_content" />

<Spinner
    android:layout_marginBottom="20dp"
    android:layout_width="match_parent"
    android:layout_height="70dp"
    android:id="@+id/auswahl_kategorie" />

</LinearLayout>`

MainActivity.java:

public class MainActivity extends AppCompatActivity {
private final int NO_OF_ITEMS = 3;
Button bt1, bt2, bt3, btGetSpinnersValues;
//Spinner auswahl_kategorie;
ArrayList<String> alKategorien = new ArrayList();
ArrayList<Integer> alDynamicChildren = new ArrayList();
ArrayAdapter ad;

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

    LinearLayout rootLayout = findViewById(R.id.rootLayout);
    bt1 = findViewById(R.id.bt1);
    bt2 = findViewById(R.id.bt2);
    bt3 = findViewById(R.id.bt3);
    btGetSpinnersValues = findViewById(R.id.btGetSpinnersValues);
    alKategorien.add("a");
    alKategorien.add("b");
    alKategorien.add("c");
    alKategorien.add("d");
    alKategorien.add("e");
    setSpinnerAdapter();
    for (int i = 0; i < NO_OF_ITEMS; i++) {
        View dl_item_schreiben = getLayoutInflater().inflate(R.layout.item_schreiben, null);
        rootLayout.addView(dl_item_schreiben);
        alDynamicChildren.add(rootLayout.indexOfChild(dl_item_schreiben));
        TextView textView = dl_item_schreiben.findViewById(R.id.tvSample);
        textView.setText("Spinner " + (i + 1));
        Spinner auswahl_kategorie = dl_item_schreiben.findViewById(R.id.auswahl_kategorie);
        auswahl_kategorie.setAdapter(ad);
        auswahl_kategorie.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
            @Override
            public void onItemSelected(AdapterView<?> adapterView, View view, int i, long l) {
            }

            @Override
            public void onNothingSelected(AdapterView<?> adapterView) {
            }
        });
    }

    bt1.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            alKategorien.clear();
            alKategorien.add("A");
            alKategorien.add("B");
            alKategorien.add("C");
            alKategorien.add("D");
            alKategorien.add("E");
            ad.notifyDataSetChanged();// or setSpinnerAdapter();
        }
    });
    bt2.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            alKategorien.clear();
            alKategorien.add("1");
            alKategorien.add("2");
            alKategorien.add("3");
            alKategorien.add("4");
            alKategorien.add("5");
            ad.notifyDataSetChanged();// or setSpinnerAdapter();
        }
    });
    bt3.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            alKategorien = new ArrayList<>();
            alKategorien.add("A");
            alKategorien.add("B");
            alKategorien.add("C");
            alKategorien.add("D");
            alKategorien.add("E");
            setSpinnerAdapter();
        }
    });
    btGetSpinnersValues.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            String msg = "";
            LinearLayout parent = (LinearLayout) view.getParent();
            for (int i = 0; i < NO_OF_ITEMS; i++) {
                Spinner auswahl_kategorie = parent.getChildAt(alDynamicChildren.get(i)).findViewById(R.id.auswahl_kategorie);
                msg += "Spinner " + (i +1) + ": " + auswahl_kategorie.getSelectedItem().toString() + "\n";
            }
            Toast.makeText(getApplicationContext(), msg, Toast.LENGTH_LONG).show();
        }
    });

}

private void setSpinnerAdapter() {
    if (ad == null) {
        ad = new ArrayAdapter(this, android.R.layout.simple_spinner_item, alKategorien);
        ad.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
        //auswahl_kategorie.setAdapter(ad);
    } else {
        //ad.clear();
        //ad.addAll(alKategorien);
        ad.notifyDataSetChanged();
    }
}
}

I tested the code and spinner changed content for bt1/bt2 clicked. But after bt3 clicked, no more change.

First, when the adapter is created, it is set with alKategorien as the data list, so call ad.clear() just clear alKategorien and ad.addAll(alKategorien); add back an empty list. Therefore those two lines should be removed.

Second, data list cannot be created again (i.e. = new ArrayList<>() and so no more change after bt3 clicked) but can be modified (clear, add, remove) because it acts like a linkage between activity and adapter. If data list is created again by new, then the adapter needs to be created again as linkage broken. There may be other conditions that can break the linkage. Since the code that I mentioned in first point is removed, so issue should be caused by the way of modifying data list which is not shown in the snippet.