ExpandableListView: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference

Question

It shows that the problem is here:

@Override
public int getChildrenCount(int groupPosition) {
    return hashMap.get(groupPosition).size();
}

I tried to print the map size and it does. It has only two keys: "Upcoming Events" and "Past Events". I checked using the containsKey() and the result was true. But somehow I still can't get the size of the List object contained as a value.

It may help to know that I did not create another class file for this and all the code is bundled up in the same class file.

Here's my code:

class CustomEventListAdapter extends BaseExpandableListAdapter{
    HashMap<String, List<Event>> hashMap;
    Context con;
    CustomEventListAdapter(Context context, HashMap<String, List<Event>> map){
        hashMap = map;
        con = context;
    }

    List<String> groupTitle = Arrays.asList("Upcoming Events","Past Events");

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

    @Override
    public int getChildrenCount(int groupPosition) {
        return hashMap.get(groupPosition).size();
    }

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

    @Override
    public Object getChild(int groupPosition, int childPosition) {
        return hashMap.get(groupTitle.get(groupPosition)).get(childPosition);
    }

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

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

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

    @Override
    public View getGroupView(int groupPosition, boolean isExpanded, View convertView, ViewGroup parent) {
        if (convertView == null) {
            LayoutInflater layoutInflater = (LayoutInflater) getApplicationContext()
                    .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            convertView = layoutInflater.inflate(R.layout.expandable_list_group, null);
        }
        TextView listTitleTextView = (TextView) convertView
                .findViewById(R.id.groupHeader);
        listTitleTextView.setTypeface(null, Typeface.BOLD);
        listTitleTextView.setText(groupTitle.get(groupPosition));
        return convertView;        }

    @Override
    public View getChildView(int groupPosition, int childPosition, boolean isLastChild, View convertView, ViewGroup parent) {
        if (convertView == null) {
            LayoutInflater layoutInflater = (LayoutInflater) getApplicationContext()
                    .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            convertView = layoutInflater.inflate(R.layout.expandable_list_item, null);
        }
        TextView eventName = (TextView) convertView.findViewById(R.id.eventName);
        TextView eventDate = (TextView) convertView.findViewById(R.id.eventDate);
        TextView eventDesc = (TextView) convertView.findViewById(R.id.eventDesc);
        ImageView imageView = (ImageView) convertView.findViewById(R.id.eventPic);

        eventName.setText(hashMap.get(groupPosition).get(childPosition).getName());
        eventDate.setText(hashMap.get(groupPosition).get(childPosition).getDate());
        eventDesc.setText(hashMap.get(groupPosition).get(childPosition).getDetails());

        return convertView;        }

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

This is the onCreate() :

   expandableListView = (ExpandableListView) findViewById(R.id.eventList);
    hashMap  = new HashMap<>();
    FirebaseDatabase database = FirebaseDatabase.getInstance();
    DatabaseReference reference = database.getReference().child("Events");
    reference.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            List<Event> eventList = new ArrayList<>();
            //List<Event> upcoming = new ArrayList<>();
            //List<Event> past = new ArrayList<>();
            for (DataSnapshot postSnapshot : dataSnapshot.getChildren()) {

                Event event = new Event();
                event.name = String.valueOf(postSnapshot.child("Name").getValue());
                event.details = String.valueOf(postSnapshot.child("Details").getValue());
                event.add = String.valueOf(postSnapshot.child("PhotoAdd").getValue());
                event.date = String.valueOf(postSnapshot.child("Date").getValue());

                eventList.add(event);
                Toast.makeText(EventsActivity.this, "longDate = " + event.getLongDate() , Toast.LENGTH_SHORT).show();


            }
            long current = Calendar.getInstance().getTimeInMillis();
            Collections.sort(eventList, new Sortbyroll());
            int i=0;
            for( i=0; i< eventList.size(); i++){
                if(eventList.get(i).getLongDate() < current){
                    break;
                }
            }
            hashMap.put("Upcoming Events", eventList.subList(0,i));
            hashMap.put("Past Events", eventList.subList(i,eventList.size()));

            ExpandableListAdapter listAdapter = new CustomEventListAdapter(getBaseContext(),hashMap);
            expandableListView.setAdapter(listAdapter);
        }

        @Override
        public void onCancelled(@NonNull DatabaseError databaseError) {

        }
    });

Answer 1

Maybe the code you put here is not complete because I don't found something like "hashMap.put(groupPosition, ....)". I mean that you trying to GET the content of that Group but there isn't any piece of code where you SET it. The only two "put()" methods are called for special groups called "upcoming events" and "past events".....so the "hashMap" can contain only those two values.