为什么SQLite里的数据用ListView在Fragment里显示不出来

<------CourseFragment.java---->
public class CourseFragment extends Fragment {    private TextView course_user_name;
    ListView r_listview;
    private SQLiteDatabase dbRead;
    MyDatabaseHelper dbHelper;
    String id,time,location,req_number,inrtoduction,now_number ;
    private Cursor result;
    private ArrayList<String>plan=new ArrayList<String>();
//    public CourseFragment() {
//        // Required empty public constructor
//    }
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
//        System.out.println("hello1");        View view = inflater.inflate(R.layout.fragment_course, container, false);
        r_listview=(ListView)view.findViewById(R.id.list_view);   //注意：ListView的实体化
        dbHelper=new MyDatabaseHelper(view.getContext(), "Activity.db", null, 1);  //注意：dbHelper的实体化
        //查询数据库
        dbRead=dbHelper.getReadableDatabase();
        String sql="select id,time,location,req_number from NewActivity order by id";//可能有错        result=dbRead.rawQuery(sql,null);        int resultCounts=result.getCount();
        if(resultCounts==0||!result.moveToFirst()){
            Toast.makeText(getActivity()
                    , "无数据", Toast.LENGTH_SHORT).show();
        }else{
            while(!result.isAfterLast()){
// idStr=String.valueOf(result.getInt(result.getColumnIndex("id")))+"\t";
                id=result.getString(result.getColumnIndex("id"))+":\t";
                time=result.getString(result.getColumnIndex("time"))+"\t";
                location=result.getString(result.getColumnIndex("location"))+"\t";
                req_number=result.getString(result.getColumnIndex("req_number"));
                plan.add(id+time+location+req_number);
                result.moveToNext();
            }
        }
        ArrayAdapter<String> arrayAdapter=new ArrayAdapter<String>(getActivity(), android.R.layout.simple_list_item_1,plan);
        r_listview.setAdapter(arrayAdapter);
        //r_listview.setAdapter(new ArrayAdapter<String>(getActivity(), android.R.layout.simple_list_item_1,plan));  //将ListView加到适配器里
        dbRead.close();
        //return inflater.inflate(R.layout.fragment_course, container, false);
        return view;
    }