Null-Zeiger-Ausnahme in Cursor-Adapter

Question

Ich versuche, den Cursor-Adapter zu verwenden, aber ich habe eine Null-Zeiger-Ausnahme beim Aufruf onPostExecute, ich kann nicht herausfinden, was das Problem ist. Hauptidee der Aktivität ist die Liste der Kontakte anzeigen. Code:

Haupttätigkeit:

public class AddressBook extends ListActivity {  
    public static final String ROW_ID = "row_id"; // additional Intent key 
    private ListView contactListView; // ListView component from ListActivity 
    private CursorAdapter contactAdapter; //CursorAdapter for ListView  
    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 

     contactListView = getListView(); // access to ListView 
     contactListView.setOnItemClickListener(viewContactListener); 

     // display contact name on TextView in ListView 
     String[] from = new String[]{"name"}; 
     int[] to = new int[]{R.id.contactTextView}; 
     CursorAdapter contactAdapter = new SimpleCursorAdapter(AddressBook.this, R.layout.contact_list_item, null, from, to); 
     setListAdapter(contactAdapter); // setting adaptercontactView 
    } 

    // creating Activity menu 
    @Override 
    public boolean onCreateOptionsMenu(Menu menu) { 
     super.onCreateOptionsMenu(menu); 
     MenuInflater inflater = getMenuInflater(); 

     inflater.inflate(R.menu.addressbook_menu, menu); 
     return true; 
    } 

    // Selectng option 
    @Override 
    public boolean onOptionsItemSelected(MenuItem item) {  
     //create new Intent object for AddEditContact method 
     Intent addNewContact = 
       new Intent(AddressBook.this, AddEditContact.class); 
     startActivity(addNewContact); // start the AddEditContact Activity 
     return super.onOptionsItemSelected(item); // call superclass method 
    } 

    @Override 
    protected void onResume() { 
     super.onResume(); 
     // Create new GetContactsTask object and call it 
     new GetContactsTask().execute((Object[]) null); 
    } 

    @Override 
    protected void onStop() { 
     Cursor cursor = contactAdapter.getCursor(); // access to current Cursor  
     if (cursor != null) 
      cursor.deactivate();  // deactivating  
     contactAdapter.changeCursor(null); // change Cursor to null 
     super.onStop(); 
    } 

    private class GetContactsTask extends AsyncTask<Object, Object, Cursor> {  
     DatabaseConnector databaseConnector = 
       new DatabaseConnector(AddressBook.this); // performing access to database  
     @Override 
     protected Cursor doInBackground(Object... params) { 
      databaseConnector.open(); // access to Cursor 
      return databaseConnector.getAllContacts(); 
     } 

     @Override 
     protected void onPostExecute(Cursor result) { // using Cursor, returned by doInBackground method 
      contactAdapter.changeCursor(result); // change Cursor adapter //NULL POINTER EXCEPTION(ContactAdapter is null) 
      databaseConnector.close(); 
     } 
    }  
} 

LogCat:

E/AndroidRuntime: FATAL EXCEPTION: main 
java.lang.NullPointerException 
at com.va.testappadr.AddressBook$GetContactsTask.onPostExecute(AddressBook.java:92) 
at com.va.testappadr.AddressBook$GetContactsTask.onPostExecute(AddressBook.java:77) 
at android.os.AsyncTask.finish(AsyncTask.java:631) 
at android.os.AsyncTask.access$600(AsyncTask.java:177) 
at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:644) 
at android.os.Handler.dispatchMessage(Handler.java:99) 
at android.os.Looper.loop(Looper.java:153)... 

Answer 1

Das Problem ist, dass Sie dies, indem Sie Shadowing den Cursor-Adapter sind:

CursorAdapter contactAdapter = new ... 

in der onCreate-Methode ... daher manipuliert die Async-Aufgabe ein Objekt, das w wie nie initialisiert ....

Warum?

, weil Sie ein neues Objekt in der onCreate erklärt ... die globale anstatt und es initialisieren ...

im onCreate den Code ändern ...

// display contact name on TextView in ListView 
     String[] from = new String[]{"name"}; 
     int[] to = new int[]{R.id.contactTextView}; 
     //HERE!! 
     contactAdapter = new SimpleCursorAdapter(AddressBook.this, R.layout.contact_list_item, null, from, to); 
     setListAdapter(contactAdapter); // setting adaptercontactView 
    }