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copied!<p>I have a problem with create a separate class in jave I have this part of code</p> <pre><code>private class Read_from_db extends AsyncTask <String, Long, Void> { private final ProgressDialog dialog = new ProgressDialog(Read.this); // can use UI thread here protected void onPreExecute() { this.dialog.setMessage("Wait\nSome SLOW job is being done..."); this.dialog.show(); } @Override protected Void doInBackground(String... arg0) { // TODO Auto-generated method stub try { //txtMsg.append("\n"); // obtain a list of from DB String TABLE_NAME = "classicpoems__poet_header"; String COLUMN_ID = "_id"; // String _ID = "_id"; String COLUMN_NAME = "poet_name"; String COLUMN_CENTURY = "century_start"; String [] columns ={COLUMN_ID,COLUMN_NAME,COLUMN_CENTURY}; Cursor c = db.query(TABLE_NAME,columns,null, null, null, null, COLUMN_ID); SimpleCursorAdapter adapter = new SimpleCursorAdapter(Read.this, R.layout.list_item, c, new String[] {COLUMN_ID,COLUMN_NAME,COLUMN_CENTURY}, new int[] {R.id.list_item_text_id,R.id.list_item_text_main,R.id.list_item_text_sub}, 0); ListView list = (ListView) findViewById(R.id.list_poet_name); list.setAdapter(adapter); } catch (Exception e) { //Toast.makeText(Read.this, e.getMessage(), 1).show(); Log.i(TAG, e.getMessage()); } db.close(); return null; } // can use UI thread here protected void onPostExecute(final Void unused) { if (this.dialog.isShowing()) { this.dialog.dismiss(); } // cleaning-up, all done this.dialog.setMessage("Done"); } } </code></pre> <p>That must repeat each time an activity loads (but with some changes for example <code>TABLE_NAME</code> and <code>R.id.list_poet_name</code> <code>columns</code> and ..) I think repeating this code is not a professional way so I want to convert this to a separate class and in each activity I use it But I don't know how to do it..(I tried but I always get error for example I don't know how to define context for <code>SimpleCursorAdapter</code> or make Toast work here can you help me how to convert this code to a separate class</p> <p>Here Is my class code</p> <pre><code> package co.tosca.persianpoem; import android.app.Activity; import android.app.Dialog; import android.app.ProgressDialog; import android.content.Context; import android.database.Cursor; import android.database.sqlite.SQLiteDatabase; import android.os.AsyncTask; import android.os.Environment; import android.util.Log; import android.view.View; import android.widget.ListView; import android.widget.SimpleCursorAdapter; import android.widget.Toast; public class Get_data extends AsyncTask<String, Long, Void> { //Context context; public String TABLE_NAME; public String COLUMN_ID; public String COLUMN_NAME; public String COLUMN_CENTURY; public String[] columns; public int target; private String DATABASE_NAME; private static final String SDcardPath = Environment.getExternalStorageDirectory().getPath(); private String DbPath = SDcardPath + "/Tosca/" + DATABASE_NAME; private static final String TAG ="DatabaseHelper"; private SQLiteDatabase db; private ProgressDialog dialog; private Activity callingActivity; public int adapter_list; //public Get_data(Context context){ // this.context=context; // DATABASE_NAME="persian_poem.db"; //} public Get_data(Activity activity) { callingActivity = activity; } public Get_data() { TABLE_NAME="classicpoems__poet_header"; COLUMN_ID="_id"; COLUMN_NAME = "poet_name"; } @Override protected void onPreExecute() { this.dialog.setMessage("Wait\nSome SLOW job is being done..."); this.dialog.show(); } @Override protected Void doInBackground(String... arg0) { // TODO Auto-generated method stub try { // obtain a list of from DB // String TABLE_NAME = "classicpoems__poet_header"; // String COLUMN_ID = "_id"; // String _ID = "_id"; // String COLUMN_NAME = "poet_name"; // String COLUMN_CENTURY = "century_start"; // String [] columns ={COLUMN_ID,COLUMN_NAME,COLUMN_CENTURY}; Cursor c = db.query(TABLE_NAME,columns,null, null, null, null, COLUMN_ID); SimpleCursorAdapter adapter = new SimpleCursorAdapter(callingActivity, adapter_list, c, new String[] {COLUMN_ID,COLUMN_NAME,COLUMN_CENTURY}, new int[] {R.id.list_item_text_id,R.id.list_item_text_main,R.id.list_item_text_sub}, 0); ListView list = (ListView)callingActivity.findViewById(target); list.setAdapter(adapter); } catch (Exception e) { Toast.makeText(callingActivity, e.getMessage(), 1).show(); Log.i(TAG, e.getMessage()); } db.close(); return null; } protected void onPostExecute(final Void unused) { dialog.dismiss(); } } </code></pre> <p>and when I want to use it I use this codes</p> <pre><code>enter code here Get_data poet_name=new Get_data(); poet_name.TABLE_NAME="classicpoems__poet_header"; poet_name.COLUMN_ID = "_id"; poet_name.COLUMN_NAME = "poet_name"; poet_name.COLUMN_CENTURY = "century_start"; poet_name.columns =new String[]{"_id","poet_name","century_start"}; poet_name.adapter_list=R.layout.list_item; poet_name.target=R.id.list_poet_name; poet_name.execute(); </code></pre> <p>But I get error ..Can you help me to find my mistakes?</p> <p>I changed initial code to <code>Get_data poet_name=new Get_data(this);</code> but I am getting error yet here is part of logcat </p> <pre><code>01-06 04:25:32.262: E/AndroidRuntime(3244): FATAL EXCEPTION: main 01-06 04:25:32.262: E/AndroidRuntime(3244): java.lang.RuntimeException: Unable to start activity ComponentInfo{co.tosca.persianpoem/co.tosca.persianpoem.Read}: java.lang.NullPointerException 01-06 04:25:32.262: E/AndroidRuntime(3244): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1967) 01-06 04:25:32.262: E/AndroidRuntime(3244): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1992) 01-06 04:25:32.262: E/AndroidRuntime(3244): at android.app.ActivityThread.access$600(ActivityThread.java:127) 01-06 04:25:32.262: E/AndroidRuntime(3244): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1158) 01-06 04:25:32.262: E/AndroidRuntime(3244): at android.os.Handler.dispatchMessage(Handler.java:99) 01-06 04:25:32.262: E/AndroidRuntime(3244): at android.os.Looper.loop(Looper.java:137) 01-06 04:25:32.262: E/AndroidRuntime(3244): at android.app.ActivityThread.main(ActivityThread.java:4441) 01-06 04:25:32.262: E/AndroidRuntime(3244): at java.lang.reflect.Method.invokeNative(Native Method) 01-06 04:25:32.262: E/AndroidRuntime(3244): at java.lang.reflect.Method.invoke(Method.java:511) 01-06 04:25:32.262: E/AndroidRuntime(3244): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784) 01-06 04:25:32.262: E/AndroidRuntime(3244): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551) 01-06 04:25:32.262: E/AndroidRuntime(3244): at dalvik.system.NativeStart.main(Native Method) 01-06 04:25:32.262: E/AndroidRuntime(3244): Caused by: java.lang.NullPointerException 01-06 04:25:32.262: E/AndroidRuntime(3244): at co.tosca.persianpoem.Get_data.onPreExecute(Get_data.java:54) 01-06 04:25:32.262: E/AndroidRuntime(3244): at android.os.AsyncTask.executeOnExecutor(AsyncTask.java:561) 01-06 04:25:32.262: E/AndroidRuntime(3244): at android.os.AsyncTask.execute(AsyncTask.java:511) 01-06 04:25:32.262: E/AndroidRuntime(3244): at co.tosca.persianpoem.Read.onCreate(Read.java:65) 01-06 04:25:32.262: E/AndroidRuntime(3244): at android.app.Activity.performCreate(Activity.java:4465) 01-06 04:25:32.262: E/AndroidRuntime(3244): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049) 01-06 04:25:32.262: E/AndroidRuntime(3244): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1931) 01-06 04:25:32.262: E/AndroidRuntime(3244): ... 11 more </code></pre>

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