Referencing Listview Null Adapter

Question

Im beginner android developer and trying on implementing ExpandableListView into my app but kept getting the error message. already done double checking and maybe my logic is weak that i could not identify the issue. Much appreciate that you could help me out or guide me the right way. Thanks <3

i found a reference link: link

Error:

     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ExpandableListView.setAdapter(android.widget.ExpandableListAdapter)' on a null object reference
    at com.example.n3333.myapplication.ExpendableList.onCreate(ExpendableList.java:31)

it stated that the list Adapter is null:

MyCodes:

List_Item xml:

List_Group xml:

Main Activity Class:

public class ExpendableList extends AppCompatActivity {
ExpandableListAdapter listAdapter;
ExpandableListView expListView;
List<String> listDataHeader;
HashMap<String, List<String>> listDataChild;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    // get the listview
    expListView = findViewById(R.id.lvExp);

    // preparing list data
  prepareListData();

    listAdapter = new ExpandableListAdapter(this, listDataHeader, listDataChild);

    // setting list adapter
    expListView.setAdapter(listAdapter);
}

/*
 * Preparing the list data
 */
private void prepareListData() {
    listDataHeader = new ArrayList<String>();
    listDataChild = new HashMap<String, List<String>>();

    // Adding child data
    listDataHeader.add("Top 250");
    listDataHeader.add("Now Showing");
    listDataHeader.add("Coming Soon..");

    // Adding child data
    List<String> top250 = new ArrayList<String>();
    top250.add("The Shawshank Redemption");
    top250.add("The Godfather");
    top250.add("The Godfather: Part II");
    top250.add("Pulp Fiction");
    top250.add("The Good, the Bad and the Ugly");
    top250.add("The Dark Knight");
    top250.add("12 Angry Men");

    List<String> nowShowing = new ArrayList<String>();
    nowShowing.add("The Conjuring");
    nowShowing.add("Despicable Me 2");
    nowShowing.add("Turbo");
    nowShowing.add("Grown Ups 2");
    nowShowing.add("Red 2");
    nowShowing.add("The Wolverine");

    List<String> comingSoon = new ArrayList<String>();
    comingSoon.add("2 Guns");
    comingSoon.add("The Smurfs 2");
    comingSoon.add("The Spectacular Now");
    comingSoon.add("The Canyons");
    comingSoon.add("Europa Report");

    listDataChild.put(listDataHeader.get(0), top250); // Header, Child data
    listDataChild.put(listDataHeader.get(1), nowShowing);
    listDataChild.put(listDataHeader.get(2), comingSoon);
  }
}

Adapter Class:

public class ExpandableListAdapter extends BaseExpandableListAdapter {
private Context _context;
private List<String> _listDataHeader; // header titles
// child data in format of header title, child title
private HashMap<String, List<String>> _listDataChild;

public ExpandableListAdapter(Context context, List<String> listDataHeader,
                             HashMap<String, List<String>> listChildData) {
    this._context = context;
    this._listDataHeader = listDataHeader;
    this._listDataChild = listChildData;
}

@Override
public Object getGroup(int groupPosition) {
    return this._listDataHeader.get(groupPosition);
}

@Override
public int getGroupCount() {
    return this._listDataHeader.size();
}

@Override
public long getGroupId(int groupPosition) {
    return groupPosition;
}

@Override
public View getGroupView(int groupPosition, boolean isExpanded,
                         View convertView, ViewGroup parent) {
    String headerTitle = (String) getGroup(groupPosition);
    if (convertView == null) {
        LayoutInflater infalInflater = (LayoutInflater) this._context
                .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        convertView = infalInflater.inflate(R.layout.list_group, null);
    }

    TextView lblListHeader = (TextView) convertView
            .findViewById(R.id.header);
    lblListHeader.setTypeface(null, Typeface.BOLD);
    lblListHeader.setText(headerTitle);

    return convertView;
}


@Override
public Object getChild(int groupPosition, int childPosititon) {
    return this._listDataChild.get(this._listDataHeader.get(groupPosition))
            .get(childPosititon);
}

@Override
public long getChildId(int groupPosition, int childPosition) {
    return childPosition;
}

@Override
public View getChildView(int groupPosition, final int childPosition,
                         boolean isLastChild, View convertView, ViewGroup parent) {



    final String childText = (String) getChild(groupPosition, childPosition);

    if (convertView == null) {
        LayoutInflater infalInflater = (LayoutInflater) this._context
                .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        convertView = infalInflater.inflate(R.layout.list_item, null);
    }
    TextView txtListChild = (TextView) convertView
            .findViewById(R.id.child);

    txtListChild.setText(childText);
    return convertView;
}

@Override
public int getChildrenCount(int groupPosition) {
    return this._listDataChild.get(this._listDataHeader.get(groupPosition))
            .size();
}

@Override
public boolean hasStableIds() {
    return false;
}

@Override
public boolean isChildSelectable(int groupPosition, int childPosition) {
    return true;
}

Answer 1

Please add this code to your main activity xml.

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical"
    android:background="#f4f4f4" >

            <ExpandableListView
                android:id="@+id/lvExp"
                android:layout_height="match_parent"
                android:layout_width="match_parent"/>   

</LinearLayout>