訪問內部類的方法從根源NullPointerException異常

我想在我的app.I添加數據庫寫了這個類：訪問內部類的方法從根源NullPointerException異常

public class DatabaseHandler extends SQLiteOpenHelper { 

    private static final int DATABASE_VERSION = 1; 

    private static final String DATABASE_NAME = "testDb"; 

    private static final String TABLE_KISILER = "loginCre"; 

    private static final String KEY_ID = "id"; 
    private static final String KEY_NAME = "username"; 
    private static final String KEY_PASS = "password"; 

    public DatabaseHandler(Context context) { 
     super(context, DATABASE_NAME, null, DATABASE_VERSION); 
    } 

    // Database Oluşturma işlemi. 
    @Override 
    public void onCreate(SQLiteDatabase db) { 
     String CREATE_KISILER_TABLE = "CREATE TABLE " + TABLE_KISILER + "(" 
       + KEY_ID + " INTEGER PRIMARY KEY," 
       + KEY_NAME + " VARCHAR(50)," 
       + KEY_PASS + " VARCHAR(50)" + ")"; 
     db.execSQL(CREATE_KISILER_TABLE); 
    } 

    @Override 
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) { 
     db.execSQL("DROP TABLE IF EXISTS " + TABLE_KISILER); 
     onCreate(db); 
    } 

    //CRUD 

    // Yeni Kayıt Eklemek. 
    public void addLoginInfo(String username,String password) { 
     /* 
     SQLiteDatabase db = this.getWritableDatabase(); 

     ContentValues values = new ContentValues(); 
     values.put(KEY_NAME, username); // Kisi Adı Getir 
     values.put(KEY_PASS, password); // Kisi Soyadı Getir 

     // Ekleme işlemi... 
     db.insert(TABLE_KISILER, null, values); 
     db.close(); // Açık olan database i kapat. 
     */ 
    } 

}

我想從主要活動訪問addLoginInfo方法，然後我用這個線onCreate

private DatabaseHandler _db; 
_db.addLoginInfo("test", "test");

但我的應用程序是crashing.There是logcat的：

12-05 11:24:55.802: E/AndroidRuntime(2841): FATAL EXCEPTION: main 
12-05 11:24:55.802: E/AndroidRuntime(2841): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.impact.xxx/com.impact.xxx.MainActivity}: java.lang.NullPointerException 
12-05 11:24:55.802: E/AndroidRuntime(2841):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211) 
12-05 11:24:55.802: E/AndroidRuntime(2841):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261) 
12-05 11:24:55.802: E/AndroidRuntime(2841):  at android.app.ActivityThread.access$600(ActivityThread.java:141) 
12-05 11:24:55.802: E/AndroidRuntime(2841):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256) 
12-05 11:24:55.802: E/AndroidRuntime(2841):  at android.os.Handler.dispatchMessage(Handler.java:99) 
12-05 11:24:55.802: E/AndroidRuntime(2841):  at android.os.Looper.loop(Looper.java:137) 
12-05 11:24:55.802: E/AndroidRuntime(2841):  at android.app.ActivityThread.main(ActivityThread.java:5103) 
12-05 11:24:55.802: E/AndroidRuntime(2841):  at java.lang.reflect.Method.invokeNative(Native Method) 
12-05 11:24:55.802: E/AndroidRuntime(2841):  at java.lang.reflect.Method.invoke(Method.java:525) 
12-05 11:24:55.802: E/AndroidRuntime(2841):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737) 
12-05 11:24:55.802: E/AndroidRuntime(2841):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553) 
12-05 11:24:55.802: E/AndroidRuntime(2841):  at dalvik.system.NativeStart.main(Native Method) 
12-05 11:24:55.802: E/AndroidRuntime(2841): Caused by: java.lang.NullPointerException 
12-05 11:24:55.802: E/AndroidRuntime(2841):  at com.impact.xxx.MainActivity.onCreate(MainActivity.java:164) 
12-05 11:24:55.802: E/AndroidRuntime(2841):  at android.app.Activity.performCreate(Activity.java:5133) 
12-05 11:24:55.802: E/AndroidRuntime(2841):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087) 
12-05 11:24:55.802: E/AndroidRuntime(2841):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175) 
12-05 11:24:55.802: E/AndroidRuntime(2841):  ... 11 more

爲什麼我得到nullPointerException？

來源

2014-12-05 Okan

你有數據庫處理器的初始化的對象，比如'_db =新DatabaseHandler（this）'在使用之前？ – 2014-12-05 11:52:24

@Okan看到我的答案，並使用它將起作用 – 2014-12-05 11:58:07

你沒有初始化_db object.try這樣的：

private DatabaseHandler _db = new DatabaseHandler(this); 
_db.addLoginInfo("test", "test");

來源

2014-12-05 11:52:48

在你的活動將這些行，我希望它會工作

DatabaseHandler _db = new DatabaseHandler(this); 
_db.getWritableDatabase(); 
_db.addLoginInfo("test", "test");

來源

2014-12-05 11:56:57

訪問內部類的方法從根源NullPointerException異常

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