Catch statements not working in android

Question

I am working with an android database oriented program. This activity is from one of my tab. When i press that particular tab it will get the value from the db and if any null values from the db it will catch it and show an alertbox and changing to another tab. This is my logic. But it is not executing the catch block. It is going straight to the webview and showing there a null value. Please anyone help me to fix this error.Here is my class...

LocationActivity

import java.util.List;
import android.annotation.SuppressLint;
import android.app.Activity;
import android.app.AlertDialog;
import android.content.DialogInterface;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.view.Window;
import android.webkit.WebChromeClient;
import android.webkit.WebSettings;
import android.webkit.WebView;
import android.widget.ProgressBar;


@SuppressLint("SetJavaScriptEnabled")
public class LocationActivity extends Activity {

    MainTabActivity mainTab;

    WebView mWebView;
    GPSTracker g;
    double my_lat, my_lon;
    String lat, lon, to_lat, to_lon, addressText, temp_lat, temp_lon;
    DatabaseHandler db;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        db = new DatabaseHandler(this);
        g = new GPSTracker(this);



            Log.d("Reading: ", "Location Activity Reading all Values");

        List<Datas> datas = db.getAllDatas();       

        for (Datas dat : datas) {
            try{
            to_lat = dat.getlat();
            to_lon = dat.getlon();
            }catch(NullPointerException e){

                AlertDialog.Builder alert = new AlertDialog.Builder(this);

                alert.setTitle("Oops!");

                // set dialog message
                alert
                    .setMessage("Please select a destination before finding your route!")
                    .setCancelable(false)
                    .setPositiveButton("ok",new DialogInterface.OnClickListener() {
                        @SuppressWarnings("deprecation")
                        public void onClick(DialogInterface dialog,int id) {

                          ((MainTabActivity)getParent()).getTabHost().setCurrentTab(0);
                          }
                      });

                    // create alert dialog
                    AlertDialog alertDialog = alert.create();

                    // show it
                    alertDialog.show();

            Log.d(to_lat, to_lon);
            System.out.println("Frim thap thee "+ to_lat + " " + to_lon);
            }
        }
        setContentView(R.layout.webview);        
        getWindow().setFeatureInt(Window.FEATURE_PROGRESS,Window.PROGRESS_VISIBILITY_ON);
            final ProgressBar pb = (ProgressBar) findViewById(R.id.progressBar1);
            pb.setVisibility(View.VISIBLE);

            my_lat = g.getLatitude();
            my_lon = g.getLongitude();
            lat = String.valueOf(my_lat);
            lon = String.valueOf(my_lon);



            mWebView = (WebView)findViewById(R.id.webview1);
            mWebView.setWebChromeClient(new WebChromeClient() {
             public void onProgressChanged(WebView view, int progress) {
              pb.setProgress(progress);
              if (progress == 100) {
               pb.setVisibility(View.GONE);
              }
             }
            });

            WebSettings webSettings = mWebView.getSettings();
            webSettings.setJavaScriptEnabled(true);
            mWebView.setWebViewClient(new HelloWebViewClient());
            addressText = lat + "," + lon + "+to:" + to_lat + ","+ to_lon;
            addressText = addressText.replaceAll(" ", "%20");
            addressText = addressText.replaceAll("#", "%23");
            mWebView.loadUrl("http://maps.google.com/maps?q=from:"+ addressText);


  }

}

Answer 1

getlat and getlon may be null, and therefore not throw an exception. You should replace your try/catch with a if statement which checks for null.

Furthermore the real problem of you NullPointerException is:

Log.d(to_lat, to_lon);

Log.d should be used with the TAG as first param, and the String as second:

Log.d("LOCATION", "From " + to_lat + ", to " + to_lon);

If to_lat and to_lon are null:

log.d(null, null); 

will definately cause a crash.

Answer 2

Setting strings to null is by itself not an issue and will not throw a NullPointerException. However if you would try to call a method on String that is null you would get an Exception.

String s = null; //no problem
s.isEmpty(); //Would throw a nullpointer exception

Remove the try catch clause and do a simple check with if statements

to_lat = dat.getlat();
to_lon = dat.getlon();
if(to_lat == null || to_long == null()
{
    //Show the alert
}

Answer 3

first check if you are getting anything in the datas using if(datas.size()>0) and then use the for loop.

Answer 4

to_lat, to_lon are Strings and they can be null. So why do you expect to have NPE in the catch block?