android的数据库帮助类getWritableDatabase，空指针错误

[xxxxsong]

DBHelp

public class DbHelper extends SQLiteOpenHelper {

    private final static String DB_NAME = "HKC";

    public DbHelper(Context context) {
        super(context, DB_NAME, null, 1);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        //创建了一个名为person的表
        String sql = "CREATE TABLE product_info(_id INTEGER PRIMARY KEY AUTOINCREMENT," +
                "iccid varchar(10)," +
                "name varchar(50)," +
                "directions varchar(100)," +
                "type varchar(1)," +
                "price varchar(10)," +
                "old_price varchar(10))";
        db.execSQL(sql);
        Log.d("SQLiteDatabaseTest", sql);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

    }
}

MainActivity 代码

public class MainActivity extends Activity {

    private static final String TAG = "MainActivity";

    DbHelper dbHelper=new DbHelper(this);
    SQLiteDatabase db=dbHelper.getWritableDatabase();


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

    public void onClick(View view) {
        switch (view.getId()) {
            case R.id.insert:
                break;
            case R.id.select:
                break;
        }
    }
}

异常
Caused by: 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

[Jing丶無雙]

这样就行了，原因new DbHelper(this)这里的this在onCreate的时候才会被创建

public class MainActivity extends Activity {
 
    private static final String TAG = "MainActivity";
 
    DbHelper dbHelper;
    SQLiteDatabase db;
 
 
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        dbHelper = new DbHelper(this);
        db = dbHelper.getWritableDatabase();
    }
 
    public void onClick(View view) {
        switch (view.getId()) {
            case R.id.insert:
                break;
            case R.id.select:
                break;
        }
    }
}

[Jing丶無雙]

或者你自己断点看下。你这样写 断点看到ContextWrapper为null
你的写法：

我的写法：

[xxxxsong]

引用 3 楼 xj396282771 的回复:

或者你自己断点看下。你这样写 断点看到ContextWrapper为null 你的写法： 我的写法：

ok了

[xxxxsong]

为什么会在getWritableDatabase处报异常呢，查了有一会都没解决 请教一下各位