从中访问内部类方法会导致nullpointerexception

Question

我正在尝试在我的应用程序中添加数据库。我编写了此类：

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.
        */
    }

}

我想从主要活动中访问addLoginInfo方法，然后在onCreate使用了以下addLoginInfo行

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

但是我的应用程序崩溃了。

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

为什么我会得到nullPointerException？

Answer 1

您没有初始化_db object.try这个：

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

Answer 2

在您的活动中写上这些线，希望它能起作用

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