Android 在FragmentStatePagerAdapter NPE中运行数据库查询

我正试图对我的数据库运行一个查询，以从我的数据库中获取一些数据（会逐页更改）。我在

Double balance=db.getSum（）处得到一个NullPointerException
import android.content.Context;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentStatePagerAdapter;


public class MyFragmentPagerAdapter extends FragmentStatePagerAdapter {

    int listSize;
    String[] tabNames;
    String tabName;
    protected Context mContext;

    /** Constructor of the class */
    public MyFragmentPagerAdapter(FragmentManager fm, Context c) {
        super(fm);
        mContext = c;
    }

    /** This method will be invoked when a page is requested to create */
    @Override
    public Fragment getItem(int arg0) {

        MainFragment mainFragment = new MainFragment();
        final DatabaseHelper db = new DatabaseHelper(mContext);
        Double balance = db.getSum(); //NPE here*******************
        tabName = "[" + getPageTitle(arg0) + "]";
        Bundle data = new Bundle();
        data.putInt("current_page", arg0 + 1);
        data.putString("tabName", tabName);
        mainFragment.setArguments(data);
        return mainFragment;
    }

    /** Returns the number of pages */
    @Override
    public int getCount() {
        return listSize;
    }

    @Override
    public CharSequence getPageTitle(int position) {
        return tabNames[position];
    }

    public void setCount(int count){
        listSize = count;
    }

    public void setNames(String[] names) {
        tabNames = names;
    }

    @Override
    public int getItemPosition(Object object) {
        return POSITION_NONE;
    }
}

这是我的数据库getSum（）

这是我得到的错误

07-09 15:26:52.861  14874-14874/com.rcd.simpleregister D/AndroidRuntime﹕ Shutting down VM
07-09 15:26:52.865  14874-14874/com.rcd.simpleregister E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.rcd.simpleregister, PID: 14874
    java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.sqlite.SQLiteDatabase android.content.Context.openOrCreateDatabase(java.lang.String, int, android.database.sqlite.SQLiteDatabase$CursorFactory, android.database.DatabaseErrorHandler)' on a null object reference
            at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:224)
            at android.database.sqlite.SQLiteOpenHelper.getReadableDatabase(SQLiteOpenHelper.java:188)
            at com.rcd.simpleregister.DatabaseHelper.getSum(DatabaseHelper.java:90)
            at com.rcd.simpleregister.MyFragmentPagerAdapter.getItem(MyFragmentPagerAdapter.java:31)
            at android.support.v4.app.FragmentStatePagerAdapter.instantiateItem(FragmentStatePagerAdapter.java:105)
            at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:832)
            at android.support.v4.view.ViewPager.populate(ViewPager.java:982)
            at android.support.v4.view.ViewPager.populate(ViewPager.java:914)
            at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1436)
            at android.view.View.measure(View.java:16537)
            at android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java:719)
            at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:455)
            at android.view.View.measure(View.java:16537)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5135)
            at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
            at android.view.View.measure(View.java:16537)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5135)
            at com.android.internal.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:327)
            at android.view.View.measure(View.java:16537)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5135)
            at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
            at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2291)
            at android.view.View.measure(View.java:16537)
            at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:1947)
            at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1137)
            at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1326)
            at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1024)
            at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5796)
            at android.view.Choreographer$CallbackRecord.run(Choreographer.java:761)
            at android.view.Choreographer.doCallbacks(Choreographer.java:574)
            at android.view.Choreographer.doFrame(Choreographer.java:544)
            at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:747)
            at android.os.Handler.handleCallback(Handler.java:733)
            at android.os.Handler.dispatchMessage(Handler.java:95)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5102)
            at java.lang.reflect.Method.invoke(Native Method)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)

有人知道我为什么会得到这个吗

谢谢
 传递给
SQLiteOpenHelper
超级构造函数的
Context
为空

您发布的代码没有准确显示
mContext
的值的来源。在任何情况下，在片段中，您实际上应该只有一个无参数构造函数，并且当片段附加到活动时，您可以对
上下文使用
getActivity（）
。
您是对的。。。不知道那是怎么从我身边溜走的。我更改了我在活动中投入的值，现在它工作正常。谢谢你。。有时候我需要被揍一顿！一旦它允许我，我会接受答案。
07-09 15:26:52.861  14874-14874/com.rcd.simpleregister D/AndroidRuntime﹕ Shutting down VM
07-09 15:26:52.865  14874-14874/com.rcd.simpleregister E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.rcd.simpleregister, PID: 14874
    java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.sqlite.SQLiteDatabase android.content.Context.openOrCreateDatabase(java.lang.String, int, android.database.sqlite.SQLiteDatabase$CursorFactory, android.database.DatabaseErrorHandler)' on a null object reference
            at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:224)
            at android.database.sqlite.SQLiteOpenHelper.getReadableDatabase(SQLiteOpenHelper.java:188)
            at com.rcd.simpleregister.DatabaseHelper.getSum(DatabaseHelper.java:90)
            at com.rcd.simpleregister.MyFragmentPagerAdapter.getItem(MyFragmentPagerAdapter.java:31)
            at android.support.v4.app.FragmentStatePagerAdapter.instantiateItem(FragmentStatePagerAdapter.java:105)
            at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:832)
            at android.support.v4.view.ViewPager.populate(ViewPager.java:982)
            at android.support.v4.view.ViewPager.populate(ViewPager.java:914)
            at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1436)
            at android.view.View.measure(View.java:16537)
            at android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java:719)
            at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:455)
            at android.view.View.measure(View.java:16537)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5135)
            at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
            at android.view.View.measure(View.java:16537)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5135)
            at com.android.internal.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:327)
            at android.view.View.measure(View.java:16537)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5135)
            at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
            at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2291)
            at android.view.View.measure(View.java:16537)
            at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:1947)
            at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1137)
            at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1326)
            at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1024)
            at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5796)
            at android.view.Choreographer$CallbackRecord.run(Choreographer.java:761)
            at android.view.Choreographer.doCallbacks(Choreographer.java:574)
            at android.view.Choreographer.doFrame(Choreographer.java:544)
            at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:747)
            at android.os.Handler.handleCallback(Handler.java:733)
            at android.os.Handler.dispatchMessage(Handler.java:95)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5102)
            at java.lang.reflect.Method.invoke(Native Method)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)