我在另一个应用程序中使用相同的代码，它工作正常，我不明白为什么在这里我收到NullPointerException。
这是我与Fragments的第一次，也许我写错了！
代码下方。
这是我的Thrd Fragment
import android.app.Activity;
import android.content.Context;
import android.content.Intent;
import android.content.SharedPreferences;
import android.net.Uri;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.ListView;
import android.widget.Toast;
import java.io.IOException;
import java.util.List;
public class ThreeFragment extends Fragment {
public MySQLiteHelper db = new MySQLiteHelper(getContext());
public List<VotiEsami> esami;
public VotiEsamiAdapter adapter; //Adapter per caricare la listview di voti
public ThreeFragment() {
// Required empty public constructor
}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// Inflate the layout for this fragment
View view = inflater.inflate(R.layout.fragment_three, container, false);
//Find the listview
final ListView listView = (ListView)view.findViewById(R.id.voti_listView);
//Get all exams inside database
esami = db.getVotiEsami();
adapter = new VotiEsamiAdapter(getContext(), R.layout.listview_voti_line, null);
listView.setAdapter(adapter);
//TODO gestire cancellazione del voto dal DB e dalla listview
Button nuovo_voto = (Button) view.findViewById(R.id.button_add_new_voto);
nuovo_voto.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
//Start Dialog for input the new vote
Intent myIntent = new Intent(getActivity(), DialogAddVotoEsame.class);
ThreeFragment.this.startActivityForResult(myIntent, 1);
}
});
return view;
}
public void onActivityResult(int requestCode, int resultCode, Intent data) {
if(requestCode == 1) {
if (resultCode == Activity.RESULT_OK) {
String result_nome = data.getStringExtra("result_name_esame"); //Take the materia from Dialog
int result_cfu = data.getIntExtra("result_cfu_esame", 1); //Take the materia from Dialog
int result_voto = data.getIntExtra("result_voto_esame", 1); //Take the materia from Dialog
//Add exam with vote to DB
db.addEsameVoto(new VotiEsami(result_nome, result_cfu, result_voto));
//Refresh list
List<VotiEsami> newesame = db.getVotiEsami();
//Aggiorno la Listview dell'activity con il nuovo inserimento
esami.clear();
esami.addAll(newesame);
adapter.notifyDataSetChanged();
}
}
}//onActivityResult
}
在这里，我有如何管理数据库
import android.app.Activity;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import java.util.ArrayList;
import java.util.List;
public class MySQLiteHelper extends SQLiteOpenHelper {
//Database version
private static final int DATABASE_VERSION = 1;
//Database name
private static final String DATABASE_NAME = "agendaDB";
//Materie table name
public static final String TABLE_VOTI_ESAMI = "esami";
//Table_voti_esami columns
public static final String COLUMN_NOME_ESAME = "nome_esame";
public static final String COLUMN_CFU_ESAME = "cfu_esame";
public static final String COLUMN_VOTO_ESAME = "voto_esame";
//Stringa di creazione tabelle per database
//Tabella voti esami
private static final String CREATE_VOTO_ESAME_TABLE = "CREATE TABLE "
+ TABLE_VOTI_ESAMI + "("
+ COLUMN_NOME_ESAME + " TEXT,"
+ COLUMN_CFU_ESAME + " INTEGER,"
+ COLUMN_VOTO_ESAME + " INTEGER" + ")";
public MySQLiteHelper(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
// Creating Tables
@Override
public void onCreate(SQLiteDatabase db) {
db.execSQL(CREATE_VOTO_ESAME_TABLE);
}
// Upgrading database
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
// Drop older table if existed
db.execSQL("DROP TABLE IF EXISTS " + TABLE_VOTI_ESAMI);
// Create tables again
onCreate(db);
}
//Restituisco tutti gli esami dati
public List<VotiEsami> getVotiEsami(){
List<VotiEsami> esamiList = new ArrayList<>();
// Select All Query
String selectQuery = "SELECT * FROM " + TABLE_VOTI_ESAMI;
SQLiteDatabase db = this.getWritableDatabase();
Cursor cursor = db.rawQuery(selectQuery, null);
if(cursor != null) {
// looping through all rows and adding to list
if (cursor.moveToFirst()) {
do {
VotiEsami esami = new VotiEsami();
esami.set_nome_esame(cursor.getString(0));
esami.set_cfu(Integer.parseInt(cursor.getString(1)));
esami.set_voto(Integer.parseInt(cursor.getString(2)));
// Adding materia activity voti to list
esamiList.add(esami);
} while (cursor.moveToNext());
}
}else{
return null;
}
// return contact list
return esamiList;
}
// Aggiunta nuova materia sull activity voti
public void addEsameVoto(VotiEsami esam) {
SQLiteDatabase db = this.getWritableDatabase();
ContentValues values = new ContentValues();
values.put(COLUMN_NOME_ESAME, esam.get_nome_esame());
values.put(COLUMN_CFU_ESAME, esam.get_cfu());
values.put(COLUMN_VOTO_ESAME, esam.get_voto());
// Inserting Row
db.insert(TABLE_VOTI_ESAMI, null, values);
db.close(); // Closing database connection
}
}//Chiudo classe
使用相同的数据库，相同的方法在另一个应用程序中使用NullPointerException而无效{简单Activity s）
另一个谜（对我来说）：
我有5 Fragment s，对于First，没问题我能看到它，但是如果我去其他人（2,3,4或5）我会得到同样的错误，你可以看到记录它总是写：
E/AndroidRuntime: FATAL EXCEPTION: main
java.lang.NullPointerException
at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:224)
at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:164)
at com.application.ddz.agendauniversitaria.MySQLiteHelper.getVotiEsami(MySQLiteHelper.java:70)
at com.application.ddz.agendauniversitaria.ThreeFragment.onCreateView(ThreeFragment.java:48)
at android.support.v4.app.Fragment.performCreateView(Fragment.java:1974)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1067)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1252)
at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:738)
at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1617)
at android.support.v4.app.FragmentManagerImpl.executePendingTransactions(FragmentManager.java:570)
at android.support.v4.app.FragmentPagerAdapter.finishUpdate(FragmentPagerAdapter.java:141)
at android.support.v4.view.ViewPager.populate(ViewPager.java:1177)
at android.support.v4.view.ViewPager.populate(ViewPager.java:1025)
at android.support.v4.view.ViewPager$3.run(ViewPager.java:254)
at android.view.Choreographer$CallbackRecord.run(Choreographer.java:791)
at android.view.Choreographer.doCallbacks(Choreographer.java:591)
at android.view.Choreographer.doFrame(Choreographer.java:560)
at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:777)
at android.os.Handler.handleCallback(Handler.java:730)
at android.os.Handler.dispatchMessage(Handler.java:92)
at android.os.Looper.loop(Looper.java:176)
at android.app.ActivityThread.main(ActivityThread.java:5419)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:525)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1046)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:862)
at dalvik.system.NativeStart.main(Native Method)
有人能告诉我史诗失败在哪里吗？ 感谢。
答案 0 :(得分：0)
我认为您可能需要在db方法中初始化onCreate()对象。
onCreate()
系统在创建片段时调用此方法。内 你的实现，你应该初始化的基本组件 片段暂停时要保留的片段 停了，然后又恢复了。
所以你可以这样做：
public class ThreeFragment extends Fragment {
public MySQLiteHelper db;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
db = new MySQLiteHelper(getContext());
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
// Your code
esami = db.getVotiEsami();
}
}