我正在开发一个包含5个片段的应用。在所有片段中，我有从sqlite检索的数据的列表视图。我做到了这一点，但我遇到了错误：
09-04 23:13:22.226 20797-20797/? E/AndroidRuntime: FATAL EXCEPTION: main
java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference
at com.smartfon_apps.timetable_sch32.Db$DbHelper.onCreate(Db.java:68)
at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:251)
at android.database.sqlite.SQLiteOpenHelper.getReadableDatabase(SQLiteOpenHelper.java:187)
at com.smartfon_apps.timetable_sch32.Db.getAllItems(Db.java:38)
at com.smartfon_apps.timetable_sch32.MondayFragment.onCreateView(MondayFragment.java:33)
at android.support.v4.app.Fragment.performCreateView(Fragment.java:2074)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1104)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1286)
at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:758)
at android.support.v4.app.FragmentManagerImpl.execSingleAction(FragmentManager.java:1632)
at android.support.v4.app.BackStackRecord.commitNowAllowingStateLoss(BackStackRecord.java:637)
at android.support.v4.app.FragmentPagerAdapter.finishUpdate(FragmentPagerAdapter.java:143)
at android.support.v4.view.ViewPager.populate(ViewPager.java:1237)
at android.support.v4.view.ViewPager.populate(ViewPager.java:1085)
at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1611)
at android.view.View.measure(View.java:17496)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5466)
at android.support.design.widget.CoordinatorLayout.onMeasureChild(CoordinatorLayout.java:671)
at android.support.design.widget.HeaderScrollingViewBehavior.onMeasureChild(HeaderScrollingViewBehavior.java:90)
at android.support.design.widget.AppBarLayout$ScrollingViewBehavior.onMeasureChild(AppBarLayout.java:1319)
at android.support.design.widget.CoordinatorLayout.onMeasure(CoordinatorLayout.java:736)
at android.view.View.measure(View.java:17496)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5466)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
at android.support.v7.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:135)
at android.view.View.measure(View.java:17496)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5466)
at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1438)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:724)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:615)
at android.view.View.measure(View.java:17496)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5466)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
at android.view.View.measure(View.java:17496)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5466)
at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1438)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:724)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:615)
at android.view.View.measure(View.java:17496)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5466)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2636)
at android.view.View.measure(View.java:17496)
at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2031)
at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1193)
at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1400)
at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1078)
at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5875)
at android.view.Choreographer$CallbackRecord.run(Choreographer.java:767)
at android.view.Choreographer.doCallbacks(Choreographer.java:580)
at android.view.Choreographer.doFrame(Choreographer.java:550)
at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java
我的DB.java
package com.xxx.xxx;
import java.io.IOException;
import org.xmlpull.v1.XmlPullParser;
import org.xmlpull.v1.XmlPullParserException;
import android.content.ContentValues;
import android.content.Context;
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.util.Log;
public class Db {
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;
public Db(Context context) {
mDbHelper = new DbHelper(context);
}
public Cursor getAllItems() {
mDb = mDbHelper.getReadableDatabase();
String where = "clas = " + "'1А'";
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);
}
@Override
public void onCreate(SQLiteDatabase db) {
// TODO Auto-generated method stub
db.execSQL("CREATE TABLE " + TABLE_NAME + " ("
+ 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(0);
String number = _xml.getAttributeValue(1);
String nazv = _xml.getAttributeValue(2);
String day = _xml.getAttributeValue(3);
String time = _xml.getAttributeValue(4);
String room = _xml.getAttributeValue(5);
String teacher = _xml.getAttributeValue(6);
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);
}
}
}
答案 0 :(得分：2)
在DBHelper类中，变量fContext永远不会被初始化。您应该在DBHelper构造函数中设置其值：
public DbHelper(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
fContext = context;
}