how to implement Navigation drawer with submenu

Question

I am trying to create a submenu on one of the items in my navdrawer. This is my code so far:

navDrawerItems.add(new NavDrawerItem(navMenuTitles[0], navMenuIcons.getResourceId(0, -1)));

    navDrawerItems.add(new NavDrawerItem(navMenuTitles[1], navMenuIcons.getResourceId(1, -1)));

    navDrawerItems.add(new NavDrawerItem(navMenuTitles[2], navMenuIcons.getResourceId(2, -1)));

    navDrawerItems.add(new NavDrawerItem(navMenuTitles[3], navMenuIcons.getResourceId(3, -1)));

This list the items, however now under naMenuTitles[1] (for example) i want to create 4 sub titles and when tapping on them to start the activity on that particular item.

now i have these codes but not sure where to implement them:

public void setChildGroupData() {

        child = new ArrayList<String>();
        child.add("test1");
        child.add("test2");
        child.add("test3");
        child.add("test4");
        childItem.add(child);

So what i have tried is this:

navDrawerItems = new ArrayList<NavDrawerItem>();
    childItem = new ArrayList<Object>(); 


    navDrawerItems.add(new NavDrawerItem(navMenuTitles[0], navMenuIcons.getResourceId(0, -1)));

    navDrawerItems.add(new NavDrawerItem(navMenuTitles[1], navMenuIcons.getResourceId(1, -1)));
    child = new ArrayList<String>();
    child.add("test1");
    child.add("test2");
    child.add("test3");
    child.add("test4");
    childItem.add(child);

    navDrawerItems.add(new NavDrawerItem(navMenuTitles[2], navMenuIcons.getResourceId(2, -1)));

There are no errors in the app when launching, but it doesn't seem to work. What do i need to do to fix this problem?
Any help will be appreciated!

EDIT

This is my NavDrawerListAdapter Class:

public class NavDrawerListAdapter extends BaseAdapter {

private Context context;
private ArrayList<NavDrawerItem> navDrawerItems;
public ArrayList<Object> Childtem = new ArrayList<Object>();
private ArrayList<Object> childItem;

public NavDrawerListAdapter(Context context, ArrayList<NavDrawerItem> navDrawerItems){
    this.context = context;
    this.navDrawerItems = navDrawerItems;
    this.Childtem = childItem;

}

@Override
public int getCount() {
    return navDrawerItems.size();
}


@Override
public Object getItem(int position) {       
    return navDrawerItems.get(position);
}

@Override
public long getItemId(int position) {
    return position;
}

@Override
public View getView(int position, View convertView, ViewGroup parent) {
    if (convertView == null) {
        LayoutInflater mInflater = (LayoutInflater)
                context.getSystemService(Activity.LAYOUT_INFLATER_SERVICE);
        convertView = mInflater.inflate(R.layout.drawer_list_item, null);
    }

    ImageView imgIcon = (ImageView) convertView.findViewById(R.id.icon);
    TextView txtTitle = (TextView) convertView.findViewById(R.id.title);
    TextView txtCount = (TextView) convertView.findViewById(R.id.counter);

    imgIcon.setImageResource(navDrawerItems.get(position).getIcon());        
    txtTitle.setText(navDrawerItems.get(position).getTitle());

    // displaying count
    // check whether it set visible or not
    if(navDrawerItems.get(position).getCounterVisibility()){
        txtCount.setText(navDrawerItems.get(position).getCount());
    }else{
        // hide the counter view
        txtCount.setVisibility(View.GONE);
    }

    return convertView;
}

}