Android 第二次使用DatabaseHelper时出现NullPointerException

我想更新数据并在片段列表中显示，数据已经传递到db中，但在使用getWritableDatabase（）进行更新时得到NullPointerException

现在我知道它不能再调用getWritableDatabase（）和getReadableDatabase（），也不需要关闭数据库

我该怎么做才能完成它

计算器

    public class CalcFragment extends Fragment {

    OnEqualSelectedListener mCallback;

    public interface OnEqualSelectedListener {

        public void updateResult();
    }
    ...
    private void getEqualResult() {

        textview_result.setText(mResult + sum);

        mDbHelper = new DB(getActivity());
        mDbHelper.open();
        mDbHelper.create(mResult + sum);
        mDbHelper.close();

        mCallback.updateResult();
...
}

主课

public class MainActivity extends FragmentActivity implements CalcFragment.OnEqualSelectedListener {

    View view;
    ...
    @Override
    public void updateResult() {
        NoteFragment fragmentResult = new NoteFragment();
        fragmentResult.updateView();
        if(Debug.log){ Log.d("MainActivity", "updateResult");}
    }

DB.class

private static class DatabaseHelper extends SQLiteOpenHelper {

    ...

    public DB(Context context) {
    this.mContext = context;

    if (Debug.log) {
        Log.d("DB", "DB");
    }

}

public DB open() throws SQLException {

        dbHelper = new DatabaseHelper(mContext);
        db = dbHelper.getWritableDatabase();

    if (Debug.log) {Log.d("DB", "SQLException");
    }
    return this;
}

public void close() {

    dbHelper.close();
    if (Debug.log) {
        Log.d("DB", "close");
    }

}

public Cursor getAll() {
    mCursor = db.rawQuery("SELECT * FROM history ORDER BY created DESC",
            null);

    return mCursor;
}

清单说明

public class NoteFragment extends ListFragment {


private DB mDbHelper;
private Cursor mCursor;
private SimpleCursorAdapter adapter;

    ...

public void setAdapter(){

    mDbHelper = new DB(getActivity());
    mDbHelper.open();
    mCursor = mDbHelper.getAll();
    getActivity().startManagingCursor(mCursor);

    String[] from_column = new String[]{DB.KEY_ITEM, DB.KEY_CREATED};
    int[] to_layout = new int[]{android.R.id.text1, android.R.id.text2};

    adapter = new SimpleCursorAdapter(getActivity(), android.R.layout.simple_list_item_2,
            mCursor, from_column, to_layout);

    setListAdapter(adapter);

    if(Debug.log){ Log.d("NoteFragment", "setAdapter");}

}

public void updateView(){

    mDbHelper = new DB(getActivity());
    mDbHelper.open();
    mCursor = mDbHelper.getAll();
    getActivity().startManagingCursor(mCursor);

    String[] from_column = new String[]{DB.KEY_ITEM, DB.KEY_CREATED};
    int[] to_layout = new int[]{android.R.id.text1, android.R.id.text2};

    adapter = new SimpleCursorAdapter(getActivity(), android.R.layout.simple_list_item_2,
            mCursor, from_column, to_layout);

    adapter.changeCursor(mCursor);
    adapter.notifyDataSetChanged();

    if(Debug.log){ Log.d("NoteFragment", "updateView");}
}

日志

传递给sqlite帮助程序的

getActivity（）

上下文是
null
。这是因为您刚刚在
updateResult（）
中创建了带有
new
的片段，它尚未附加到任何活动

通常，您不应该像这样直接在片段上调用
updateView（）
之类的方法。相反，等待框架调用片段生命周期回调，例如
onCreateView（）
 调用
fragmentResult.updateView（）之前执行在活动中添加/替换片段

只有在将片段附加到活动之后，才能从片段内部的
getActivity（）
获得上下文。
谢谢！它起作用了！我使用FragmentTransaction替换片段。
03-27 06:09:05.550: E/AndroidRuntime(2652): FATAL EXCEPTION: main
03-27 06:09:05.550: E/AndroidRuntime(2652): java.lang.NullPointerException
03-27 06:09:05.550: E/AndroidRuntime(2652):     at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:224)
03-27 06:09:05.550: E/AndroidRuntime(2652):     at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:164)
03-27 06:09:05.550: E/AndroidRuntime(2652):     at com.example.mycalc.DB.open(DB.java:85)
03-27 06:09:05.550: E/AndroidRuntime(2652):     at com.example.mycalc.NoteFragment.updateView(NoteFragment.java:85)
03-27 06:09:05.550: E/AndroidRuntime(2652):     at com.example.mycalc.MainActivity.updateResult(MainActivity.java:34)
03-27 06:09:05.550: E/AndroidRuntime(2652):     at com.example.mycalc.CalcFragment.getEqualResult(CalcFragment.java:258)
03-27 06:09:05.550: E/AndroidRuntime(2652):     at com.example.mycalc.CalcFragment.access$1(CalcFragment.java:239)
03-27 06:09:05.550: E/AndroidRuntime(2652):     at com.example.mycalc.CalcFragment$1.onClick(CalcFragment.java:171)