如何在ListView中显示HashMap中的项和子项

HashMap<String, List<String>> data = new HashMap<>();
List<String> subItems1 = new ArrayList<>();
subItems1.add("子项1");
subItems1.add("子项2");
data.put("项1", subItems1);

List<String> subItems2 = new ArrayList<>();
subItems2.add("子项3");
subItems2.add("子项4");
data.put("项2", subItems2);

public class MyAdapter extends BaseAdapter {
    private HashMap<String, List<String>> mData;

    public MyAdapter(HashMap<String, List<String>> data) {
        mData = data;
    }

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

    @Override
    public Object getItem(int position) {
        // 获取指定位置的项
        return mData.keySet().toArray()[position];
    }

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

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        ViewHolder holder;
        if (convertView == null) {
            convertView = LayoutInflater.from(parent.getContext()).inflate(android.R.layout.simple_list_item_1, parent, false);
            holder = new ViewHolder();
            holder.textView = convertView.findViewById(android.R.id.text1);
            convertView.setTag(holder);
        } else {
            holder = (ViewHolder) convertView.getTag();
        }

        // 获取当前位置的项
        String item = (String) getItem(position);
        holder.textView.setText(item);

        return convertView;
    }

    private static class ViewHolder {
        TextView textView;
    }
}

ListView listView = findViewById(R.id.listView);
MyAdapter adapter = new MyAdapter(data);
listView.setAdapter(adapter);