当SQL查询没有返回结果时该怎么办？

Question

My SQL database is queried from an android app using PHP, however sometimes no results are returned from the search because the search criteria give no matches.

Here is my ProgressTask:

public class ProgressTask extends AsyncTask<String, Void, Boolean> {
    private ProgressDialog dialog;
    protected void onPreExecute() {
        dialog = new ProgressDialog(GetData.this);
        dialog.setMessage("Searching Database");
        dialog.show();
    }
    protected Boolean doInBackground(final String... args) {

        JSONParser jParser = new JSONParser();
        // get JSON data from URL
        JSONArray json = jParser.getJSONFromUrl(url);

        for (int i = 0; i < json.length(); i++) {

            try {
                JSONObject c = json.getJSONObject(i);
                String Listing_ID = c.getString(lblListing_ID);
                String Event_ID = c.getString(lblEvent_ID);
                String Venue_ID = c.getString(lblVenue_ID);
                String Start_Date = c.getString(lblStart_Date);
                String End_Date = c.getString(lblEnd_Date);
                String Frequency = c.getString(lblFrequency);

                HashMap<String, String> map = new HashMap<String, String>();

                // Add child node to HashMap key & value
                map.put(lblListing_ID, Listing_ID);
                map.put(lblEvent_ID, Event_ID);
                map.put(lblVenue_ID, Venue_ID);
                map.put(lblStart_Date, Start_Date);
                map.put(lblEnd_Date, End_Date);
                map.put(lblFrequency, Frequency);
                jsonlist.add(map);
             }
            catch (JSONException e) {
                e.printStackTrace();
            }
        }
        return null;
    }

    protected void onPostExecute(final Boolean success) {
        if (dialog.isShowing()) {
            dialog.dismiss();
         }

        ListAdapter adapter = new SimpleAdapter(GetData.this, jsonlist,
                R.layout.list_item, new String[] { lblListing_ID , lblEvent_ID,
                lblVenue_ID, lblStart_Date, lblEnd_Date, lblFrequency }, new int[] {
                R.id.Listing_ID, R.id.Event_ID, R.id.Venue_ID,
                R.id.Start_Date, R.id.End_Date, R.id.Frequency });
            setListAdapter(adapter);
        // select single ListView item
        lv = getListView();
        }
    }
}

And here is my JSONParser class:

public class JSONParser {

static InputStream iStream = null;
static JSONArray jarray = null;
static String json = "";

public JSONParser() {
}

public JSONArray getJSONFromUrl(String url) {

    StringBuilder builder = new StringBuilder();
    HttpClient client = new DefaultHttpClient();
    HttpGet httpGet = new HttpGet(url);
    try {
        HttpResponse response = client.execute(httpGet);
        StatusLine statusLine = response.getStatusLine();
        int statusCode = statusLine.getStatusCode();
        if (statusCode == 200) {
            HttpEntity entity = response.getEntity();
            InputStream content = entity.getContent();
            BufferedReader reader = new BufferedReader(new InputStreamReader(content));
            String line;
            while ((line = reader.readLine()) != null) {
                builder.append(line);
            }
        } else {
            Log.e("==>", "Failed to download file");
        }
    } catch (ClientProtocolException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }

    // Parse String to JSON object
    try {
        JSONObject object = new JSONObject( builder.toString());
        jarray = object.getJSONArray("listings");

    } catch (JSONException e) {
        Log.e("JSON Parser", "Error parsing data " + e.toString());
        }



        // return JSON Object
    return jarray;

}

}

PHP returns this JSON data when the serach returns no results:

{
"listings": [
    ""
]
}

When the JSON is empty i wish to output some kind of message saying "No results found". I have searched around a bit and tried out methods such as JSON.length() = 0 or setEmptyView but with no success. I was just wondering if anyone had any ideas on what is the best way to respond when no results are returned and if so how to implement it.

Thanks in advance for any responses and if you need anymore information please comment below. Thanks

Answer 1

check the size of JSONArray if its not more than 0 , then show a toast.

JSONObject object = new JSONObject(builder.toString());
jarray = object.getJSONArray("listings");

if (jarray == null || jarray.length() == 0) {
    Toast.makeText(context, "No results found", duration).show():
}
else{
  // existing code to process entries
}

EDIT

you can use following layout, if your json array is null or its size == 0 then set the visiblity of your listview to gone and visibility of textview to visible.

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@color/screen_background"
    tools:context=".FAQFragment" >

    <ListView
        android:id="@android:id/list"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" >
    </ListView>


        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_centerInParent="true"
            android:text="No Values to Show"
            android:visibility="gone"
            android:id="@+id/tvMsg" />

</RelativeLayout>

now in your AsyncTask:-

public class ProgressTask extends AsyncTask<String, Void, Boolean> {
    private ProgressDialog dialog;
    private  JSONArray json=null;
    protected void onPreExecute() {
        dialog = new ProgressDialog(GetData.this);
        dialog.setMessage("Searching Database");
        dialog.show();
    }
    protected Boolean doInBackground(final String... args) {

        JSONParser jParser = new JSONParser();
        // get JSON data from URL
        JSON = jParser.getJSONFromUrl(url);

        if(json!=null && json.length()>0)// returning from doInBackground() if json is null
            return null;
        for (int i = 0; i < json.length(); i++) {

            try {
                JSONObject c = json.getJSONObject(i);
                String Listing_ID = c.getString(lblListing_ID);
                String Event_ID = c.getString(lblEvent_ID);
                String Venue_ID = c.getString(lblVenue_ID);
                String Start_Date = c.getString(lblStart_Date);
                String End_Date = c.getString(lblEnd_Date);
                String Frequency = c.getString(lblFrequency);

                HashMap<String, String> map = new HashMap<String, String>();

                // Add child node to HashMap key & value
                map.put(lblListing_ID, Listing_ID);
                map.put(lblEvent_ID, Event_ID);
                map.put(lblVenue_ID, Venue_ID);
                map.put(lblStart_Date, Start_Date);
                map.put(lblEnd_Date, End_Date);
                map.put(lblFrequency, Frequency);
                jsonlist.add(map);
             }

            catch (JSONException e) {
                e.printStackTrace();
            }
        }
        return null;
    }

    protected void onPostExecute(final Boolean success) {
        if (dialog.isShowing()) {
            dialog.dismiss();
         }

       if(json!=null && json.length()>0){
        ListAdapter adapter = new SimpleAdapter(GetData.this, jsonlist,
                R.layout.list_item, new String[] { lblListing_ID , lblEvent_ID,
                lblVenue_ID, lblStart_Date, lblEnd_Date, lblFrequency }, new int[] {
                R.id.Listing_ID, R.id.Event_ID, R.id.Venue_ID,
                R.id.Start_Date, R.id.End_Date, R.id.Frequency });
            setListAdapter(adapter);
        // select single ListView item
        lv = getListView();
        }else{
           tvMsg.setVisibility(View.VISIBLE);  // displaying overlay text when no data is present
         yourListView.setVisibility(View.GONE);
        }
        }
    }
}

Answer 2

Usually, ListView are displayed in a ListActivity . Such an activity contains embedded a mechanism to handle exactly this situation:

Optionally, your custom view can contain another view object of any type to display when the list view is empty. This "empty list" notifier must have an id "android:id/empty". Note that when an empty view is present, the list view will be hidden when there is no data to display.

(in http://developer.android.com/reference/android/app/ListActivity.html )

Basically, you put a @android:id/list ListView for your list, and a @android:id/empty TextView for the No result found message. The ListActivity handles the switch between the 2.

Edit

From the methods you use, such as setListAdapter , I assume you are indeed using a ListActivity . You can therefore easily integrate the @android:id/empty mechanism.

Answer 3

You can check the first element
Ex:

JSONArray json = jParser.getJSONFromUrl(url);
if(json == null){
     //Oops!
     showToast();
     return null;
}
...
jarray = object.getJSONArray("listings");
if(jarray.get(0).toString().length() == 0)
//Oops!
return null;
...

Edit
Ex2:

...
JSONArray json = jParser.getJSONFromUrl(url);
if(json.get(0).toString().length() == 0){
     //Oops!
     showToast();
     return null;
}
...

Good luck