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以下源碼簡化了我的源代碼。什麼是getDatabaseLocked（）的空指針異常，我該如何解決它？
public class exfragment extends Fragment implements FragmentLifeCycle{
Context mContext;
DB db;
public exfragment(){
}
public exfragment(Context context){
mContext = context;
}
public void onCreate(Bundle savedInstanceState){
super.onCreate(savedInstanceState)
db = new DB(mContext, "name.db", null, 1);
}
public void selectName(){
db = DB.getReadableDatabase();
}
}
此源代碼使一個錯誤，因爲有沒有參數 沒有構造函數的話，添加一個構造函數沒有參數。
當我執行包括我的源在內的應用程序後，單擊home按鈕以在後臺執行。 然後我執行了一些應用程序，直到它們導致內存不足。當我用我的源代碼重新啓動應用程序時，應用程序使得FATAL EXCEPTION。
db = DB.getReadableDatabase() method.
我認爲這是由上下文問題引起的。因爲我沒有參數插入構造函數，所以上下文變爲空。
請參考下面的logcat消息
04-15 18:47:23.969 24132-24132/sslab.com.sms_service E/AndroidRuntime: Caused by: java.lang.NullPointerException
04-15 18:47:23.969 24132-24132/sslab.com.sms_service E/AndroidRuntime: at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:224)
04-15 18:47:23.969 24132-24132/sslab.com.sms_service E/AndroidRuntime: at android.database.sqlite.SQLiteOpenHelper.getReadableDatabase(SQLiteOpenHelper.java:188)
04-15 18:47:23.969 24132-24132/sslab.com.sms_service E/AndroidRuntime: at sslab.com.sms_service.SmsReceiveListFragment.selectSmsList(SmsReceiveListFragment.java:494)
04-15 18:47:23.969 24132-24132/sslab.com.sms_service E/AndroidRuntime: at sslab.com.sms_service.SmsReceiveListFragment.callSmsList(SmsReceiveListFragment.java:189)
04-15 18:47:23.969 24132-24132/sslab.com.sms_service E/AndroidRuntime: at sslab.com.sms_service.SmsReceiveListFragment.onCreateView(SmsReceiveListFragment.java:103)
04-15 18:47:23.969 24132-24132/sslab.com.sms_service E/AndroidRuntime: at android.support.v4.app.Fragment.performCreateView(Fragment.java:1962)
04-15 18:47:23.969 24132-24132/sslab.com.sms_service E/AndroidRuntime: at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1067)
04-15 18:47:23.969 24132-24132/sslab.com.sms_service E/AndroidRuntime: at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1248)
04-15 18:47:23.969 24132-24132/sslab.com.sms_service E/AndroidRuntime: at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1230)
04-15 18:47:23.969 24132-24132/sslab.com.sms_service E/AndroidRuntime: at android.support.v4.app.FragmentManagerImpl.dispatchActivityCreated(FragmentManager.java:2042)
04-15 18:47:23.969 24132-24132/sslab.com.sms_service E/AndroidRuntime: at android.support.v4.app.FragmentController.dispatchActivityCreated(FragmentController.java:165)
04-15 18:47:23.969 24132-24132/sslab.com.sms_service E/AndroidRuntime: at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:543)
04-15 18:47:23.969 24132-24132/sslab.com.sms_service E/AndroidRuntime: at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1163)
04-15 18:47:23.969 24132-24132/sslab.com.sms_service E/AndroidRuntime: at android.app.Activity.performStart(Activity.java:5018)
04-15 18:47:23.969 24132-24132/sslab.com.sms_service E/AndroidRuntime: at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2032)
04-15 18:47:23.969 24132-24132/sslab.com.sms_service E/AndroidRuntime: at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
04-15 18:47:23.969 24132-24132/sslab.com.sms_service E/AndroidRuntime: at android.app.ActivityThread.access$600(ActivityThread.java:130)
04-15 18:47:23.969 24132-24132/sslab.com.sms_service E/AndroidRuntime: at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
04-15 18:47:23.969 24132-24132/sslab.com.sms_service E/AndroidRuntime: at android.os.Handler.dispatchMessage(Handler.java:99)
04-15 18:47:23.969 24132-24132/sslab.com.sms_service E/AndroidRuntime: at android.os.Looper.loop(Looper.java:137)
04-15 18:47:23.969 24132-24132/sslab.com.sms_service E/AndroidRuntime: at android.app.ActivityThread.main(ActivityThread.java:4745)
04-15 18:47:23.969 24132-24132/sslab.com.sms_service E/AndroidRuntime: at java.lang.reflect.Method.invokeNative(Native Method)
04-15 18:47:23.969 24132-24132/sslab.com.sms_service E/AndroidRuntime: at java.lang.reflect.Method.invoke(Method.java:511)