Java getReadableDatabase（）'；关于空对象引用

我是新来的，我正在做一些调试。我有一些难以表达的困难

我试图从另一个活动中创建的SQLite数据库中读取数据

我目前的问题似乎是：

SQLiteDatabase db = myOpenHelper.getReadableDatabase();

它应该指向我的数据库助手类

public class SecondActivity extends ListActivity {

    CustomOpenHelper myOpenHelper;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_second);

        displayDataInTable();

    }

        void displayDataInTable() {
        List<String> values = queryTable();

        if (values != null) {
            ArrayAdapter<String> adapter = new ArrayAdapter<String>(SecondActivity.this, android.R.layout.simple_list_item_1, values);
            setListAdapter(adapter);


        }

    }

    List<String> queryTable() {
        List<String> player = new ArrayList<String>();

        SQLiteDatabase db = myOpenHelper.getReadableDatabase();
        Cursor cursor = db.query(true, CustomOpenHelper.TABLE_NAME, new String[]{"_id, name, score"}, null, null, null, null, null, null, null);

        while (cursor.moveToNext()) {
            int id = cursor.getInt(cursor.getColumnIndex("_id"));
            String name = cursor.getString(cursor.getColumnIndex("name"));
            int score = cursor.getInt(cursor.getColumnIndex("score"));
            player.add(id + " --> the player " + name + " has got a score of " + score + "s");
        }

        return player;
    }



    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_second, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

---

你觉得怎么样？

试试这个，你可以创建这样合适的对象

CustomOpenHelper myOpenHelper = new CustomOpenHelper(this);

然后写下面的陈述

SQLiteDatabase db = myOpenHelper.getReadableDatabase();

---

不必打电话：

SQLiteDatabase db = myOpenHelper.getReadableDatabase();

如果出现错误，请改用此选项，这样就不需要sqlitedatabase对象：

Cursor cursor = getActivity().getContentResolver().query(Uri, 
projection,selection,selectionArgs,sortOrder);

---

你能发送完整的代码吗？你实例化myOpenHelper了吗？是的，先生，已编辑，请告知你是否想查看其他类？你应该实例化

CustomOpenHelper

并将其分配给

myOpenHelper

。嗨，mixel，我想我已经完成了？在代码的顶部？：CustomOpenHelper myOpenHelper；谢谢你们，谢谢甘帕特·卡莉娅。这花了我一段时间，但已经奏效了。

Cursor cursor = getActivity().getContentResolver().query(Uri, 
projection,selection,selectionArgs,sortOrder);