我正在申请5 Fragment s。在Fragment s的活动中，我从SharedPreferences获取数据并更改活动标题。但是在Db.java文件中，我连接到数据库并执行查询，我无法从SharedPreferences获取数据并收到错误：
java.lang.NullPointerException：尝试调用接口方法＆＃39; java.lang.String android.content.SharedPreferences.getString（java.lang.String，java.lang.String）＆＃39;在null对象引用上 at com.smartfon_apps.timetable_sch32.Db.getAllItemsFri（Db.java:77） at com.smartfon_apps.timetable_sch32.FridayFragment.onCreateView（FridayFragment.java:30） 在android.support.v4.app.Fragment.performCreateView（Fragment.java:2074） 在android.support.v4.app.FragmentManagerImpl.moveToState（FragmentManager.java:1104） 在android.support.v4.app.FragmentManagerImpl.moveToState（FragmentManager.java:1286） 在android.support.v4.app.BackStackRecord.run（BackStackRecord.java:758） 在android.support.v4.app.FragmentManagerImpl.execSingleAction（FragmentManager.java:1632） 在android.support.v4.app.BackStackRecord.commitNowAllowingStateLoss（BackStackRecord.java:637） 在android.support.v4.app.FragmentPagerAdapter.finishUpdate（FragmentPagerAdapter.java:143） 在android.support.v4.view.ViewPager.populate（ViewPager.java:1237） 在android.support.v4.view.ViewPager.setCurrentItemInternal（ViewPager.java:668） 在android.support.v4.view.ViewPager.setCurrentItemInternal（ViewPager.java:630） 在android.support.v4.view.ViewPager.setCurrentItem（ViewPager.java:611） 在android.support.design.widget.TabLayout $ ViewPagerOnTabSelectedListener.onTabSelected（TabLayout.java:2156） 在android.support.design.widget.TabLayout.dispatchTabSelected（TabLayout.java:1138） 在android.support.design.widget.TabLayout.selectTab（TabLayout.java:1132） 在android.support.design.widget.TabLayout.selectTab（TabLayout.java:1105） 在android.support.design.widget.TabLayout $ Tab.select（TabLayout.java:1396） 在android.support.design.widget.TabLayout $ TabView.performClick（TabLayout.java:1501） 在android.view.View $ PerformClick.run（View.java:19844） 在android.os.Handler.handleCallback（Handler.java:739） 在android.os.Handler.dispatchMessage（Handler.java:95） 在android.os.Looper.loop（Looper.java:135） 在android.app.ActivityThread.main（ActivityThread.java:5349） at java.lang.reflect.Method.invoke（Native Method） 在java.lang.reflect.Method.invoke（Method.java:372） 在com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run（ZygoteInit.java:908） 在com.android.internal.os.ZygoteInit.main（ZygoteInit.java:703）
这是我的Db.java文件：
package x.x.x;
import java.io.IOException;
import org.xmlpull.v1.XmlPullParser;
import org.xmlpull.v1.XmlPullParserException;
import android.app.Activity;
import android.content.ContentValues;
import android.content.Context;
import android.content.SharedPreferences;
import android.content.res.Resources;
import android.content.res.XmlResourceParser;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.preference.PreferenceManager;
import android.util.Log;
public class Db {
public static final String _ID_column = "_id";
public static final String CLASS_COLUMN = "clas";
public static final String NUMBER_COLUMN = "number";
public static final String NAZVANIE_COLUMN = "nazvanie";
public static final String DAY_COLUMN = "day";
public static final String TIME_COLUMN = "time";
public static final String ROOM_COLUMN = "room";
public static final String TEACHER_COLUMN = "teacher";
public static final String DATABASE_NAME = "lessons_database.db";
public static final String TABLE_NAME = "lessonstable";
private static final int DATABASE_VERSION = 1;
private DbHelper mDbHelper;
private SQLiteDatabase mDb;
private SharedPreferences sp;
public Db(Context context) {
mDbHelper = new DbHelper(context);
}
public Cursor getAllItemsFri() {
mDb = mDbHelper.getReadableDatabase();
sp = PreferenceManager.getDefaultSharedPreferences();
String select_class = sp.getString("select_class", "");
//String select_class = "1А";
String where = "clas = '" + select_class + "' AND day = 'Пятница'";
return mDb.query(TABLE_NAME, null, where, null, null, null, null);
}
public void close() {
if (mDbHelper != null) mDbHelper.close();
if (mDb != null) mDb.close();
}
public class DbHelper extends SQLiteOpenHelper {
public Context fContext;
public DbHelper(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
fContext = context;
}
@Override
public void onCreate(SQLiteDatabase db) {
// TODO Auto-generated method stub
db.execSQL("CREATE TABLE " + TABLE_NAME + " ( "
+ _ID_column + " INTEGER PRIMARY KEY AUTOINCREMENT, " + CLASS_COLUMN + " TEXT, " + NUMBER_COLUMN + " TEXT,"
+ NAZVANIE_COLUMN + " TEXT, " + DAY_COLUMN + " TEXT,"
+ TIME_COLUMN + " TEXT, " + ROOM_COLUMN + " TEXT,"
+ TEACHER_COLUMN + " TEXT" + ");");
// Добавляем записи в таблицу
ContentValues values = new ContentValues();
// Получим файл из ресурсов
Resources res = fContext.getResources();
// Открываем xml-файл
XmlResourceParser _xml = res.getXml(R.xml.lessons_records);
try {
// Ищем конец документа
int eventType = _xml.getEventType();
while (eventType != XmlPullParser.END_DOCUMENT) {
// Ищем теги record
if ((eventType == XmlPullParser.START_TAG)
&& (_xml.getName().equals("record"))) {
// Тег Record найден, теперь получим его атрибуты и
// вставляем в таблицу
String clas = _xml.getAttributeValue(1);
String number = _xml.getAttributeValue(2);
String nazv = _xml.getAttributeValue(3);
String day = _xml.getAttributeValue(4);
String time = _xml.getAttributeValue(5);
String room = _xml.getAttributeValue(6);
String teacher = _xml.getAttributeValue(7);
values.put("clas", clas);
values.put("number", number);
values.put("nazvanie", nazv);
values.put("day", day);
values.put("time", time);
values.put("room", room);
values.put("teacher", teacher);
db.insert(TABLE_NAME, null, values);
}
eventType =
_xml.next();
}
}
// Catch errors
catch (XmlPullParserException | IOException e) {
Log.e("Test", e.getMessage(), e);
} finally {
// Close the xml file
_xml.close();
}
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
// TODO Auto-generated method stub
//Log.w("TestBase", "Upgrading database from version " + oldVersion + " to " + newVersion + ", which will destroy all old data");
db.execSQL("DROP TABLE IF EXISTS " + TABLE_NAME);
onCreate(db);
}
}
}
以下是我在Fragment s的活动，其中一切正常：
import android.content.SharedPreferences;
import android.database.sqlite.SQLiteDatabase;
import android.os.Bundle;
import android.preference.PreferenceManager;
import android.support.design.widget.TabLayout;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import java.util.ArrayList;
import java.util.List;
import com.smartfon_apps.timetable_sch32.R;
import com.smartfon_apps.timetable_sch32.MondayFragment;
import com.smartfon_apps.timetable_sch32.TuesdayFragment;
import com.smartfon_apps.timetable_sch32.WednesdayFragment;
import com.smartfon_apps.timetable_sch32.ThursdayFragment;
import com.smartfon_apps.timetable_sch32.FridayFragment;
import static com.smartfon_apps.timetable_sch32.Db.*;
public class TimetableActivity extends AppCompatActivity {
private Toolbar toolbar;
private TabLayout tabLayout;
private ViewPager viewPager;
SharedPreferences sp;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_timetable);
sp = PreferenceManager.getDefaultSharedPreferences(this);
String select_class = sp.getString("select_class", "");
toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
//getSupportActionBar().setDisplayHomeAsUpEnabled(true);
setTitle("Расписание " + select_class + " класса");
viewPager = (ViewPager) findViewById(R.id.viewpager);
setupViewPager(viewPager);
tabLayout = (TabLayout) findViewById(R.id.tabs);
tabLayout.setupWithViewPager(viewPager);
}
private void setupViewPager(ViewPager viewPager) {
ViewPagerAdapter adapter = new ViewPagerAdapter(getSupportFragmentManager());
adapter.addFragment(new MondayFragment(), "ПН");
adapter.addFragment(new TuesdayFragment(), "ВТ");
adapter.addFragment(new WednesdayFragment(), "СР");
adapter.addFragment(new ThursdayFragment(), "ЧТ");
adapter.addFragment(new FridayFragment(), "ПТ");
viewPager.setAdapter(adapter);
}
class ViewPagerAdapter extends FragmentPagerAdapter {
private final List<Fragment> mFragmentList = new ArrayList<>();
private final List<String> mFragmentTitleList = new ArrayList<>();
public ViewPagerAdapter(FragmentManager manager) {
super(manager);
}
@Override
public Fragment getItem(int position) {
return mFragmentList.get(position);
}
@Override
public int getCount() {
return mFragmentList.size();
}
public void addFragment(Fragment fragment, String title) {
mFragmentList.add(fragment);
mFragmentTitleList.add(title);
}
@Override
public CharSequence getPageTitle(int position) {
return mFragmentTitleList.get(position);
}
}
}
有关如何解决此问题的任何建议？