java - fragment 中的CRUD SQLite

我正在活动中设置CRUD SQLite及其成功，但是当我尝试迁移到片段时，在放入数据时出现了一些错误

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object android.content.Context.getSystemService(java.lang.String)' on a null object reference
    at android.widget.ArrayAdapter.init(ArrayAdapter.java:310)
    at android.widget.ArrayAdapter.<init>(ArrayAdapter.java:128)
    at com.example.wildangamin.pasarwildan.DaganganFragment.RefreshList(DaganganFragment.java:65)
    at com.example.wildangamin.pasarwildan.BuatBiodata$1.onClick(BuatBiodata.java:52)

我尝试了一些东西，但没有一个起作用
有代码（DaganganFragment）

public void RefreshList() {
    db = dbcenter.getReadableDatabase();
    cursor = db.rawQuery("SELECT * FROM biodata", null);
    daftar = new String[cursor.getCount()];
    cursor.moveToFirst();
    for (int cc = 0; cc < cursor.getCount(); cc++) {
        cursor.moveToPosition(cc);
        daftar[cc] = cursor.getString(1).toString();
    }
    listView01.setAdapter(new ArrayAdapter<>(mContext, android.R.layout.simple_list_item_1, daftar));
    listView01.setSelected(true);
    listView01.setOnItemClickListener(new AdapterView.OnItemClickListener() {


        public void onItemClick(AdapterView arg0, View arg1, int arg2, long arg3) {
            final String selection = daftar[arg2]; //.getItemAtPosition(arg2).toString();
            final CharSequence[] dialogitem = {"Lihat Data", "Update Data", "Hapus Data"};
            AlertDialog.Builder builder = new AlertDialog.Builder(mContext);
            builder.setTitle("Pilihan");
            builder.setItems(dialogitem, new DialogInterface.OnClickListener() {
                public void onClick(DialogInterface dialog, int item) {
                    switch (item) {
                        case 0:
                            Intent i = new Intent(mContext, LihatBiodata.class);
                            i.putExtra("nama", selection);
                            startActivity(i);
                            break;
                        case 1:
                            Intent in = new Intent(mContext, UpdateBiodata.class);
                            in.putExtra("nama", selection);
                            startActivity(in);
                            break;
                        case 2:
                            SQLiteDatabase db = dbcenter.getWritableDatabase();
                            db.execSQL("delete from biodata where namabarang = '" + selection + "'");
                            RefreshList();
                            break;
                    }
                }
            });
            builder.create().show();
        }
    });
    ((ArrayAdapter) listView01.getAdapter()).notifyDataSetInvalidated();
}

和（BuatBiodata）

ton1.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View arg0) {
            // TODO Auto-generated method stub
            SQLiteDatabase db = dbHelper.getWritableDatabase();
            db.execSQL("insert into biodata(kode, namabarang, harga, hargasatuan, deskripsi) values('" +
                    text1.getText().toString() + "','" +
                    text2.getText().toString() + "','" +
                    text3.getText().toString() + "','" +
                    text4.getText().toString() + "','" +
                    text5.getText().toString() + "')");
            Toast.makeText(getApplicationContext(), "Berhasil", Toast.LENGTH_LONG).show();
            DaganganFragment.df.RefreshList();
            finish();
        }
    });

当处于活动状态时，它可以很好地工作，但是在某些情况下，它不能工作

最佳答案

检查mContext的值是否正确设置。那应该是问题所在。

编辑：我检查了本教程中DaganganFragment的实现。您正在访问对象df而不进行初始化。