我有一个使用数据库的片段类（SQLiteOpenHelper）和3个不同的标签，包含3个不同的片段。现在，当我想在启动应用程序时在我的片段中使用我的数据库时，我得到一个NullPointerException。谁能告诉我为什么会这样？
我的TabsPageAdapter的代码：
public class TabsPagerAdapter extends FragmentPagerAdapter {
List<MyFragment> myTabs = new ArrayList<MyFragment>();
public TabsPagerAdapter(FragmentManager fm){
super(fm);
myTabs.add(new AllShowsFragment());
myTabs.add(new UnwatchedShowsFragment());
myTabs.add(new WatchedShowsFragment());
}
@Override
public Fragment getItem(int index){
switch(index) {
case 0:
return myTabs.get(0);
case 1:
return myTabs.get(1);
case 2:
return myTabs.get(2);
}
return null;
}
@Override
public int getCount() {
return 3;
}
}
我主要活动中最重要的部分：
public class MainActivity extends FragmentActivity implements
ActionBar.TabListener {
public static boolean simple_progress;
public static int editmode;
private ViewPager viewPager;
private TabsPagerAdapter mAdapter;
private ActionBar actionBar;
private DatabaseHandler db;
private Show current_show;
private int current_tab = 0;
private String[] tabs = { "All Shows", "Unwatched", "Watched" };
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
db = new DatabaseHandler(this);
viewPager = (ViewPager) findViewById(R.id.pager);
actionBar = getActionBar();
mAdapter = new TabsPagerAdapter(getSupportFragmentManager());
viewPager.setAdapter(mAdapter);
actionBar.setHomeButtonEnabled(false);
actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
for (String tab_name : tabs){
actionBar.addTab(actionBar.newTab().setText(tab_name)
.setTabListener(this));
}
viewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {
@Override
public void onPageSelected(int position) {
actionBar.setSelectedNavigationItem(position);
}
@Override
public void onPageScrolled(int arg0, float arg1, int arg2) {
}
@Override
public void onPageScrollStateChanged(int arg0) {
}
});
}
@Override
public void onTabReselected(Tab tab, FragmentTransaction ft) {
}
@Override
public void onTabSelected(Tab tab, FragmentTransaction ft) {
viewPager.setCurrentItem(tab.getPosition());
current_tab = tab.getPosition();
update();
}
@Override
public void onTabUnselected(Tab tab, FragmentTransaction ft) {
}
public void update(){
mAdapter.myTabs.get(current_tab).update();
}
}
我获得NullPointerException的片段类的大部分代码：
public class AllShowsFragment extends MyFragment {
private SQLiteDatabase newDB;
private ListView lv;
private List<Show> showList = new ArrayList<Show>();
private int position;
private DatabaseHandler db;
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_shows, container, false);
lv = (ListView) rootView.findViewById(R.id.mainlist);
lv.setOnItemLongClickListener(new OnItemLongClickListener() {
public boolean onItemLongClick(AdapterView<?> arg0, View arg1,
int pos, long id) {
position = pos;
createClickMenu();
return true;
}
});
return rootView;
}
public void update(){
populateListfromDB();
}
public void populateListfromDB() {
db = new DatabaseHandler(getActivity());
newDB = db.getWritableDatabase(); // here I am getting the NullPointerException
showList.clear();
Cursor cursor = newDB.rawQuery("SELECT id as _id, title, cseason, cepisode, watched FROM "
+ db.TABLE_SHOWS, null);
cursor.moveToFirst();
if (cursor != null & cursor.moveToFirst()) {
cursor.moveToFirst();
do {
Show show = new Show(Integer.parseInt(cursor.getString(0)), cursor.getString(1),
cursor.getString(2), cursor.getString(3),
Integer.parseInt(cursor.getString(4)));
showList.add(show);
} while (cursor.moveToNext());
}
myCursorAdapter = new CustomAdapter(this, showList,((MainActivity)this.getActivity()).editmode,
((MainActivity)this.getActivity()).simple_progress);
lv.setAdapter(myCursorAdapter);
}
}
DatabaseHandler的第一部分代码：
public class DatabaseHandler extends SQLiteOpenHelper{
public DatabaseHandler(Context context){
super(context, DATABASE_NAME, null, DATABASE_VERSION);
};
private static final int DATABASE_VERSION = 1;
private static final String DATABASE_NAME = "showManager";
public static final String TABLE_SHOWS = "shows";
public static final String KEY_ID = "id";
public static final String KEY_TITLE = "title";
public static final String KEY_CURRENT_SEASON = "cseason";
public static final String KEY_CURRENT_EPISODE = "cepisode";
public static final String KEY_WATCHED = "watched";
@Override
public void onCreate(SQLiteDatabase db) {
String CREATE_SHOWS_TABLE = "CREATE TABLE " + TABLE_SHOWS + "("
+ KEY_ID + " INTEGER PRIMARY KEY," + KEY_TITLE + " TEXT,"
+ KEY_CURRENT_SEASON + " TEXT," + KEY_CURRENT_EPISODE +
" TEXT," + KEY_WATCHED + " INTEGER" + ")";
db.execSQL(CREATE_SHOWS_TABLE);
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion){
db.execSQL("DROP TABLE IF EXISTS " + TABLE_SHOWS);
}
}
堆栈跟踪：
java.lang.NullPointerException
at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:224)
at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:164)
at wrodarczyk.com.showtracker.AllShowsFragment.populateListfromDB(AllShowsFragment.java:55)
at wrodarczyk.com.showtracker.AllShowsFragment.update(AllShowsFragment.java:50)
at wrodarczyk.com.showtracker.MainActivity.update(MainActivity.java:173)
at wrodarczyk.com.showtracker.MainActivity.onTabSelected(MainActivity.java:124)
at com.android.internal.app.ActionBarImpl.selectTab(ActionBarImpl.java:577)
at com.android.internal.app.ActionBarImpl.addTab(ActionBarImpl.java:506)
at com.android.internal.app.ActionBarImpl.addTab(ActionBarImpl.java:492)
at wrodarczyk.com.showtracker.MainActivity.onCreate(MainActivity.java:51)
at android.app.Activity.performCreate(Activity.java:5312)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1111)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2552)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2653)
at android.app.ActivityThread.access$800(ActivityThread.java:156)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1355)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:157)
at android.app.ActivityThread.main(ActivityThread.java:5872)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:515)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:858)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:674)
at dalvik.system.NativeStart.main(Native Method)