Question

我正在尝试访问存储在sqlite数据库中的数据。我正在尝试使用Cursor访问数据。我已经编写了函数getInformation（）来访问数据。我的问题是当我试图调用此函数时它会抛出java.lang .NullPointerException错误。我不知道为什么会这样做。有人可以帮我解决这个错误。

E / AndroidRuntime：FATAL EXCEPTION：main进程：com.edkul.vimal.edkul，PID：2006 java.lang.NullPointerException：尝试调用虚方法＆＃39; android.database.Cursor com.edkul.vimal。 edkul.DatabaseHandler.getInformation（com.edkul.vimal.edkul.DatabaseHandler）＆＃39;在空对象引用上

这是我从Db获取数据的功能：

<pre><code>
    public Cursor getInformation(DatabaseHandler db){
    SQLiteDatabase sq = db.getWritableDatabase();
    String[] columnNames ={"StudentName","StudentEmail","StudentPassword",        
    "StudentConfirmPassword","InstituteName","ContactInfo"};
    String tableName = "StudentRecords";
    Cursor cr = sq.query(tableName,columnNames,null,null,null,null,null);
    return cr;
    }

</code></pre>

我试图调用此函数的部分代码：

<pre><code>


 btn = (Button) findViewById(R.id.loginButton);
 userName = (EditText) findViewById(R.id.userName);
 passWord = (EditText) findViewById(R.id.passWord);
 btn.setOnClickListener(new View.OnClickListener() {
 @Override
 public void onClick(View view) {
   String username = userName.getText().toString();
   String userpass = passWord.getText().toString();
   boolean login_status = false;
   Cursor cr1 = dbHandler.getInformation(dbHandler);
   String name = "";
   cr1.moveToFirst();                       
   do {
     if(username.equals(cr1.getString(0))
     &&userpass.equals(cr1.getString(2))){
     login_status = true;
     name = cr1.getString(0);
   }
 }while(cr1.moveToNext());
 if (login_status){
   Intent intentMain = new Intent(welcomepage.this,
   UserProfile.class);
   startActivity(intentMain);
   finish();
 }
}while(cr1.moveToNext());
 if (login_status){
   Intent intentMain = new Intent(welcomepage.this,
   UserProfile.class);
   startActivity(intentMain);
   finish();
 }

</code></pre>

Answer 1

您可以通过这种方式从SQLite数据库访问数据 -

try
    {
        AndroidOpenDbHelper queryDbHelperObj = new AndroidOpenDbHelper(this);
        SQLiteDatabase sqLiteDatabase = queryDbHelperObj.getReadableDatabase();
        Cursor cursor = sqLiteDatabase.rawQuery(SQL_SELECT_QUERY,null);

        if(cursor != null)
        {
            if(cursor.moveToFirst())
            {
                do
                {
                    String OriginDB = cursor.getString(cursor.getColumnIndex("Origin"));
                    String DestinationDB = cursor.getString(cursor.getColumnIndex("Destination"));

                }while(cursor.moveToNext());
            }
        }
    }
    catch(Exception err)
    {
        err.printStackTrace();
    }

希望这有帮助！

如何从sqlite数据库访问数据？

1 个答案: