在活動中創建DataBase類的構造函數

Question

請不要向我扔石頭，但我不能真正得到並找到答案爲什麼我不能從MainActivity中的類DB調用方法... 似乎我沒有做錯什麼，但是... NullPointerException每次我試圖，我做錯了什麼？ Thanksfull任何答案！

public class MainActivity extends ActionBarActivity{ 

     SQLiteDatabase db; 
    DBHelper dbHelper; 
    DB mDB; 

     @Override 
     protected void onCreate(Bundle savedInstanceState) { 
      super.onCreate(savedInstanceState); 
      setContentView(R.layout.activity_main); 

      dbHelper = new DBHelper(this); 
      db = dbHelper.getReadableDatabase(); 

      mDB = new DB(); 
      mDB.fillListView(); 

     } 

和第二類與方法

public class DB { 


    final String LOG = "phonebookLogs"; 
    private static final String TABLE_NAME = "contacts"; 
    private static final String COL_NAME = "name"; 
    private static final String COL_SURNAME = "surname"; 
    private static final String COL_BIRTHDAY = "birthday"; 
    private static final String COL_SEX = "sex"; 
    private static final String COL_ADRESS = "adress"; 
    private static final String COL_LINK = "link"; 
    private static final String ID = "_id"; 


    Cursor c; 

    public void fillListView(){ 
     c = db.query(TABLE_NAME, null, null, null, null, null, null); 
     if(c != null){ 
      if(c.moveToFirst()){ 
       do{ 
        String[] names = {c.getString(c.getColumnIndex(COL_SURNAME)) + " " 
          + c.getString(c.getColumnIndex(COL_NAME))}; 
        int[] links = {c.getInt(c.getColumnIndex(COL_LINK))}; 
        if(links[0] == 0){ 
         links[0] = R.drawable.default_contact; 
        } 


        Log.d(LOG, "\n" + links[0] + " " + names[0]); 
       }while(c.moveToNext()); 
      } 
     }else{ 
      Log.d(LOG, "Cursor is null!"); 
     } 

    } 
} 

登錄

07-06 23:37:09.929: E/AndroidRuntime(10952): FATAL EXCEPTION: main 
07-06 23:37:09.929: E/AndroidRuntime(10952): Process: com.nixsolutions.phonebook, PID: 10952 
07-06 23:37:09.929: E/AndroidRuntime(10952): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.nixsolutions.phonebook/com.nixsolutions.phonebook.MainActivity}: java.lang.NullPointerException 
07-06 23:37:09.929: E/AndroidRuntime(10952): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2202) 
07-06 23:37:09.929: E/AndroidRuntime(10952): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2252) 
07-06 23:37:09.929: E/AndroidRuntime(10952): at android.app.ActivityThread.access$800(ActivityThread.java:139) 
07-06 23:37:09.929: E/AndroidRuntime(10952): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1200) 
07-06 23:37:09.929: E/AndroidRuntime(10952): at android.os.Handler.dispatchMessage(Handler.java:102) 
07-06 23:37:09.929: E/AndroidRuntime(10952): at android.os.Looper.loop(Looper.java:136) 
07-06 23:37:09.929: E/AndroidRuntime(10952): at android.app.ActivityThread.main(ActivityThread.java:5103) 
07-06 23:37:09.929: E/AndroidRuntime(10952): at java.lang.reflect.Method.invokeNative(Native Method) 
07-06 23:37:09.929: E/AndroidRuntime(10952): at java.lang.reflect.Method.invoke(Method.java:515) 
07-06 23:37:09.929: E/AndroidRuntime(10952): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:790) 
07-06 23:37:09.929: E/AndroidRuntime(10952): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:606) 
07-06 23:37:09.929: E/AndroidRuntime(10952): at dalvik.system.NativeStart.main(Native Method) 
07-06 23:37:09.929: E/AndroidRuntime(10952): Caused by: java.lang.NullPointerException 
07-06 23:37:09.929: E/AndroidRuntime(10952): at com.nixsolutions.phonebook.DB.fillListView(DB.java:26) 
07-06 23:37:09.929: E/AndroidRuntime(10952): at com.nixsolutions.phonebook.MainActivity.onCreate(MainActivity.java:63) 
07-06 23:37:09.929: E/AndroidRuntime(10952): at android.app.Activity.performCreate(Activity.java:5275) 
07-06 23:37:09.929: E/AndroidRuntime(10952): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087) 
07-06 23:37:09.929: E/AndroidRuntime(10952): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2166) 
07-06 23:37:09.929: E/AndroidRuntime(10952): ... 11 more 

Answer 1

請dbHelper和DB類的分貝成員

變化fillListView

public void fillListView(Context context){ 
    dbHelper = new DBHelper(context); 
    db = dbHelper.getReadableDatabase(); 
    c = db.query(TABLE_NAME, null, null, null, null, null, null); 

稱其爲

mDB.fillListView(this)