issue with adding button programmatically

Question

actually i add some buttons programmatically with no problems.after that i need to recreate buttons and delete old buttons.also i had set OnClickListener for each button.my problem is that when i recall funcrion refresh() and recreate buttons,buttons added succefully,but when i click on buttons,program crashs! :|

      Main Class:

package com.example.mysqlex;

import com.example.mysqlex.DbHandler;
import com.example.mysqlex.DbHelper;

import android.annotation.SuppressLint;
import android.app.Activity;
import android.content.Context;
import android.content.Intent;
import android.database.sqlite.SQLiteDatabase;
import android.graphics.Typeface;
import android.graphics.drawable.Drawable;
import android.net.ConnectivityManager;
import android.net.NetworkInfo;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.ViewGroup.LayoutParams;
import android.widget.Button;
import android.widget.LinearLayout;
import android.widget.ScrollView;
import android.widget.TextView;
import android.widget.Toast;
import android.content.Context;

@SuppressLint("ResourceAsColor")
public class Main extends Activity {
static String Totalresult;
static LinearLayout Llayout,mainRel;
static TextView titleTest;
private static Context context;
private Typeface font;


private DbHandler dbHandler=new DbHandler(this);
public DbHelper dbHelper=new DbHelper(this);
private SQLiteDatabase database;

public static boolean haveNetworkConnection() {
    boolean haveConnectedWifi = false;
    boolean haveConnectedMobile = false;

    ConnectivityManager cm = (ConnectivityManager) context.getSystemService(Context.CONNECTIVITY_SERVICE);
    NetworkInfo[] netInfo = cm.getAllNetworkInfo();
    for (NetworkInfo ni : netInfo) {
        if (ni.getTypeName().equalsIgnoreCase("WIFI"))
            if (ni.isConnected())
                haveConnectedWifi = true;
        if (ni.getTypeName().equalsIgnoreCase("MOBILE"))
            if (ni.isConnected())
                haveConnectedMobile = true;
    }
    return haveConnectedWifi || haveConnectedMobile;
}
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main); 
    mainRel=(LinearLayout) findViewById(R.id.mainRel);
    Typeface font = Typeface.createFromAsset(getAssets(),"fonts/yekan2.TTF");
    TextView title=(TextView) findViewById(R.id.title);
    title.setTypeface(font);
    context=this;
    if(dbHandler.getSettings("catlist", "value")=="null"){
        Toast.makeText(getApplicationContext(), "اطلاعاتی ذخیره نشده است.لطفا نوسازی کنید...", 5000).show();
    }else{
        refresh("home");
    }
}
public void refresh(String refreshType){
    dbHandler=new DbHandler(context);
    dbHelper=new DbHelper(context);
    if(refreshType=="button"){
        if(dbHandler.getSettings("catlist", "value")=="null"){
            dbHandler.insertSettings("catlist", Totalresult, "");
        }else{
            dbHandler.updateSettings("catlist", Totalresult, "");
        }
    }else if(refreshType=="home"){
        Totalresult=dbHandler.getSettings("catlist", "value");
    }
    Typeface font = Typeface.createFromAsset(context.getAssets(),"fonts/yekan2.TTF");
    String[] childs=Totalresult.split("/.../<br />");
    mainRel.removeAllViews();

    for(int i=0; i<childs.length; i++){
        String[] parts=childs[i].split("<br />");
        final String[] values=parts[1].split("->");

        final String[] id=parts[0].split("->");

        Button btn = new Button(context);
        LinearLayout Llayout = new LinearLayout(context);
        btn.setBackgroundResource(R.drawable.cat_drawable);
        btn.setHeight(110); btn.setTextSize(24); btn.setTypeface(font); btn.setTextColor(R.color.white);
        btn.setId(i);
        btn.setOnClickListener(new OnClickListener() {
        public void onClick(View arg0) {
            Intent goToContent = new Intent(Main.this, Content.class); 
            goToContent.putExtra("catid", id[1]);
            goToContent.putExtra("pageTitle", values[1]);
            startActivity(goToContent);
        }
    });

        LayoutParams btnLp = new LayoutParams(LayoutParams.FILL_PARENT, LayoutParams.WRAP_CONTENT);
        LinearLayout.LayoutParams LLp = new LinearLayout.LayoutParams(LayoutParams.FILL_PARENT, LayoutParams.WRAP_CONTENT);

        LLp.setMargins(0, 5, 0, 5);
        btn.setLayoutParams(btnLp);
        Llayout.setLayoutParams(LLp);

        btn.setText(values[1]);
        Llayout.addView(btn);
        mainRel.addView(Llayout);
    }
}
public void refresh_btn(View v) {
    if(haveNetworkConnection()){
        SigninActivity.classReferrer="Main";
        new SigninActivity(this,0).execute("site_id","1","catlist","1","data_type","echo(/.../)");
    }else{
        Toast.makeText(getApplicationContext(), "لطفا جهت نوسازی اطلاعات،به اینترنت متصل شوید", Toast.LENGTH_LONG).show();
    }
  }
}


   SigninActivity Class :
package com.example.mysqlex;

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.lang.ref.WeakReference;
import java.lang.reflect.Array;
import java.net.URI;
import java.net.URL;
import java.net.URLConnection;
import java.net.URLEncoder;

import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;


import android.app.ProgressDialog;
import android.content.Context;
import android.os.AsyncTask;
import android.widget.TextView;
import android.widget.Toast;

public class SigninActivity  extends AsyncTask<String,Void,String>{
private ProgressDialog pdia;
String url="http://www.pbroilgas.com/index2.php";
private TextView roleField;
private Context context;
private int byGetOrPost = 0; 
static public String results,classReferrer;
static public String[] resultArray;
//flag 0 means get and 1 means post.(By default it is get.)
public SigninActivity(Context context,int flag) {
  this.context = context;
  byGetOrPost = flag;
}

protected void onPreExecute(){

   pdia = new ProgressDialog(context);
   pdia.setMessage("در حال نوسازی اطلاعات...");
   pdia.setProgressStyle(pdia.STYLE_SPINNER);
   pdia.show();
   super.onPreExecute();
}
@Override
protected String doInBackground(String... arg0) {
  if(byGetOrPost == 0){ //means by Get Method
     try{
         String command1 = (String)arg0[0];
         String value1 = (String)arg0[1];
         StringBuilder link = new StringBuilder();
         link.append(url+"?"+command1+"="+value1);
         for(int i=2; i<arg0.length; i+=2){
             link.append("&"+arg0[i]);
             link.append("="+arg0[i+1]);
         }
        URL url = new URL(link.toString());
        HttpClient client = new DefaultHttpClient();
        HttpGet request = new HttpGet();
        request.setURI(new URI(link.toString()));
        HttpResponse response = client.execute(request);
        BufferedReader in = new BufferedReader
       (new InputStreamReader(response.getEntity().getContent()));

       StringBuffer sb = new StringBuffer("");
       String line;
       while ((line = in.readLine()) != null) {
           sb.append(line);
        }
        in.close();
        return sb.toString();
  }catch(Exception e){
     return new String("Exception: " + e.getMessage());
  }
  }
  else{
     try{
        String command = (String)arg0[0];
        String value = (String)arg0[1];
        String link=url;
        String data  = URLEncoder.encode(command, "UTF-8") 
        + "=" + URLEncoder.encode(value, "UTF-8");
        URL url = new URL(link);
        URLConnection conn = url.openConnection(); 
        conn.setDoOutput(true); 
        OutputStreamWriter wr = new OutputStreamWriter
        (conn.getOutputStream()); 
        wr.write( data ); 
        wr.flush(); 
        BufferedReader reader = new BufferedReader
        (new InputStreamReader(conn.getInputStream()));
        StringBuilder sb = new StringBuilder();
        String line = null;
        // Read Server Response
        while((line = reader.readLine()) != null)
        {
           sb.append(line);
           break;
        }
        return sb.toString();
     }catch(Exception e){
        return new String("Exception: " + e.getMessage());
     }
  }
}

@Override
protected void onPostExecute(String result){
   pdia.dismiss();
   results=result;
   if(classReferrer=="Main"){
       Main  main= new Main();
       Main.Totalresult=result.toString();
       main.refresh("button");
   }else if(classReferrer=="Content"){
       Content  content= new Content();
       Content.Totalresult=result.toString();
       content.refresh("button");
   }
  }
}

my LogCat : 08-11 02:12:55.839: E/AndroidRuntime(8853): FATAL EXCEPTION: main 08-11 02:12:55.839: E/AndroidRuntime(8853): java.lang.NullPointerException 08-11 02:12:55.839: E/AndroidRuntime(8853): at android.content.ContextWrapper.getPackageName(ContextWrapper.java:127) 08-11 02:12:55.839: E/AndroidRuntime(8853): at android.content.ComponentName.<init>(ComponentName.java:75) 08-11 02:12:55.839: E/AndroidRuntime(8853): at android.content.Intent.<init>(Intent.java:3350) 08-11 02:12:55.839: E/AndroidRuntime(8853): at com.example.mysqlex.Main$1.onClick(Main.java:99) 08-11 02:12:55.839: E/AndroidRuntime(8853): at android.view.View.performClick(View.java:4147) 08-11 02:12:55.839: E/AndroidRuntime(8853): at android.view.View$PerformClick.run(View.java:17161) 08-11 02:12:55.839: E/AndroidRuntime(8853): at android.os.Handler.handleCallback(Handler.java:615) 08-11 02:12:55.839: E/AndroidRuntime(8853): at android.os.Handler.dispatchMessage(Handler.java:92) 08-11 02:12:55.839: E/AndroidRuntime(8853): at android.os.Looper.loop(Looper.java:213) 08-11 02:12:55.839: E/AndroidRuntime(8853): at android.app.ActivityThread.main(ActivityThread.java:4786) 08-11 02:12:55.839: E/AndroidRuntime(8853): at java.lang.reflect.Method.invokeNative(Native Method) 08-11 02:12:55.839: E/AndroidRuntime(8853): at java.lang.reflect.Method.invoke(Method.java:511) 08-11 02:12:55.839: E/AndroidRuntime(8853): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:789) 08-11 02:12:55.839: E/AndroidRuntime(8853): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:556) 08-11 02:12:55.839: E/AndroidRuntime(8853): at dalvik.system.NativeStart.main(Native Method) i appreciate your helps... thanks in advance...

Answer 1

Looking at the source for the Intent(Context, Class<?>)...

public Intent(Context packageContext, Class<?> cls) {
    mComponent = new ComponentName(packageContext, cls);
}

...then looking at the source for the ComponentName(Context, Class<?>) constructor...

public ComponentName(Context pkg, Class<?> cls) {
    mPackage = pkg.getPackageName();
    mClass = cls.getName();
}

...the only way using Intent(Context, Class<?>) can throw an NPE is at mPackage = pkg.getPackageName(); - in other words Main.this is null.

Looking at your updated code I can see why. In onPostExecute(...) of the AsyncTask in your SigninActivity you are doing the following...

Main main= new Main();

It is not possible to create an instance of an Activity using new - it simply doesn't work and that's why Main.this is null.

You're going to have to rethink things as you should never attempt to access any variables or methods of an Activity from any other app component.

If you need to access methods in an Activity from an AsyncTask, the easiest thing is to make the AsyncTask an inner class of the Activity itself.

Answer 2

The error says that it can't get the package name. So your Main.this is invalid at that point for some reason. Change it to this:

    public void onClick(View arg0) {
        Intent goToContent = new Intent(arg0.getContext(), Content.class);
        goToContent.putExtra("catid", id[1]);
        goToContent.putExtra("pageTitle", values[1]);
        startActivity(goToContent);
    }

You have an invalid context reference, but the view that calls the method should never be invalid.