数据库值存储在变量中，listview不能使用？

Question

我正在创建一个Android应用程序。我有一个TextView和一个ListView。我尝试检索数据并在TextView上显示，它工作。然后我进一步改进想要在listview上显示，但它只显示＆＃34; null＆＃34;。我不知道为什么......

public class History extends AppCompatActivity {
    ArrayList<String> listItems = new ArrayList<String>();
    ArrayAdapter<String> adapter;
    ListView HistoryList;


    String HotelName;
    String DoorCode;
    String DateIn;
    String DateOut;
    String RoomNum;
    String customerID;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_history);


        getData();

        addData();

        HistoryList.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                switch(position){
                    case 0:
                        startActivity(new Intent(History.this, UserApplication.class));
                        break;
                    case 1:
                        startActivity(new Intent(History.this, UserApplication.class));
                        break;
                    case 2:
                        startActivity(new Intent(History.this, UserApplication.class));
                        break;
                }
            }
        });
    }
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }
    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            startActivity(new Intent(History.this, Setting.class));
            return true;
        }
        else if(id == R.id.History){
            startActivity(new Intent(History.this, History.class));
            return true;
        }
        return super.onOptionsItemSelected(item);
    }
    public void addData(){
        adapter = new ArrayAdapter<String>(this,
                android.R.layout.simple_list_item_1,
                listItems);
        setListAdapter(adapter);
  //  listItems.add(HotelName[x]+"\n"+(CounterTime[x])+"\n Check In Date: "+(CheckInDate[x])+"\n Check Out Date: "+(CheckOutDate[x])
        //        +"\n Room Type: "+(RoomTypeName[x])+"\n Electrical Usage: "+(ElecticityTypeName[x]));
        listItems.add(HotelName+"\n"+DateIn+"\n"+DateOut+"\n"+RoomNum);
        listItems.add(1,"Deal");
        listItems.add(2,"Dea2");
        listItems.add(3,"Dea3");
        listItems.add(4,"Dea4");
        listItems.add(5,"Dea5");
        adapter.notifyDataSetChanged();


    }
    protected ListView getListView() {
        if (HistoryList == null) {
            HistoryList = (ListView) findViewById(R.id.HistoryList);
        }
        return HistoryList;
    }
    protected void setListAdapter(ListAdapter adapter) {
        getListView().setAdapter(adapter);
    }

    //---Start getData from Database ---//

    private ProgressDialog loading;

    private void getData() {

        loading = ProgressDialog.show(this,"Please wait...","Fetching...",false,false);
        customerID = MainActivity.getID();

        TextView txtviewbla;
        txtviewbla = (TextView) findViewById(R.id.textView3);
        txtviewbla.setText(customerID);

       // String abc = LoginActivity.getEmail();  // check back from login activity
        String url = "http://vlc2017-001-site1.1tempurl.com/get-purchasehistory.php?CustomerID="+customerID;

        StringRequest stringRequest = new StringRequest(url, new Response.Listener<String>() {
            @Override
            public void onResponse(String response) {
                loading.dismiss();
                showJSON(response);
            }
        },
                new Response.ErrorListener() {
                    @Override
                    public void onErrorResponse(VolleyError error) {
                        Toast.makeText(History.this,error.getMessage().toString(),Toast.LENGTH_LONG).show();
                    }
                });

        RequestQueue requestQueue = Volley.newRequestQueue(this);
        requestQueue.add(stringRequest);
    }

    public void showJSON(String response){


        try {
            JSONObject jsonObject = new JSONObject(response);
            JSONArray result = jsonObject.getJSONArray("result");
            JSONObject collegeData = result.getJSONObject(0);

            HotelName = collegeData.getString("HotelName");
            DoorCode = collegeData.getString("DoorCode");
            DateIn = collegeData.getString("DateIn");
            DateOut = collegeData.getString("DateOut");
            RoomNum = collegeData.getString("RoomNum");

            //    name = collegeData.getString("name");
            //   address = collegeData.getString("address");
            //  vc = collegeData.getString("vc");
        } catch (JSONException e) {
            e.printStackTrace();
        }
        TextView txtviewbla;
        txtviewbla = (TextView) findViewById(R.id.textView3);
        txtviewbla.setText(customerID+HotelName+DoorCode+DateIn+DateOut+RoomNum);

    }

}

Value可以在textview中显示，是不是我不能直接使用Listview中的变量？ ListView仅显示为null