使用游标检索数据时出现 NullPointerException [重复]

Question

我正在尝试使用光标从表中检索一些数据以将其显示在 TextView 中，但出现错误。

请帮助我。
这是我的代码

public class DisplayBudget extends AppCompatActivity implements  View.OnClickListener {
  SQLiteDatabase db;
  TextView bname,name,limit;
  Button del;
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.budget_details);
    bname = (TextView) findViewById(R.id.textViewbname);
    name = (TextView) findViewById(R.id.textViewname);
    limit = (TextView) findViewById(R.id.textViewlimit);
    del = (Button) findViewById(R.id.button2);
    del.setOnClickListener(this);
    DBController dbController = new DBController(this);
    dbController.openToDisplay();
    Cursor cursor = db.rawQuery(" select *from "+DBHelper.TABLE_NAME+ " where id = 1 ",null);
    cursor.moveToFirst();
    name.append(cursor.getString(cursor.getColumnIndex(DBHelper.OWNER_NAME)));
    bname.append(cursor.getString(cursor.getColumnIndex(DBHelper.BUDGET_NAME)));
    limit.append(cursor.getString(cursor.getColumnIndex(DBHelper.BUDGET_LIMIT)));
    cursor.close();
}

这是 LogCat

12-25 00:39:46.440  30123-30123/com.example.rajkumar.budget W/dalvikvm﹕ threadid=1: thread exiting with uncaught exception (group=0xb0d40b20)
12-25 00:39:46.440  30123-30123/com.example.rajkumar.budget E/AndroidRuntime﹕    FATAL EXCEPTION: main
Process: com.example.rajkumar.budget, PID: 30123
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.rajkumar.budget/com.example.rajkumar.budget.DisplayBudget}: java.lang.NullPointerException
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2184)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2233)
        at android.app.ActivityThread.access$800(ActivityThread.java:135)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:136)
        at android.app.ActivityThread.main(ActivityThread.java:5001)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:515)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
        at dalvik.system.NativeStart.main(Native Method)
 Caused by: java.lang.NullPointerException
        at com.example.rajkumar.budget.DisplayBudget.onCreate(DisplayBudget.java:28)
        at android.app.Activity.performCreate(Activity.java:5231)

Answer 1

你得到了NPE，因为db=null在

 Cursor cursor = db.rawQuery(" select * from "+DBHelper.TABLE_NAME+ " where id = 1 ",null);

您在哪里初始化 db ？也请参考Nils Patel 回答未来的问题。

Answer 2

在* and from 之间留出空格，这样您的查询看起来像，

Cursor cursor = db.rawQuery("select * from "+DBHelper.TABLE_NAME+ " where id = 1 ",null);

Answer 3

你只是改变它，所以你不能得到错误：-

String dataquery="select * from table name where id="+id;

Cursor cursor = db.rawQuery(dataquery,null);

很有帮助

Answer 4

试试这个。

DBController dbController = new DBController(this);
    SQLiteDatabase db = createDatabase.open();
 Cursor cursor = db.rawQuery(" select * from "+DBHelper.TABLE_NAME+ " where id = 1 ",null);
if(cursor.moveToFirst()){
 name.append(cursor.getString(cursor.getColumnIndex(DBHelper.OWNER_NAME)));
     bname.append(cursor.getString(cursor.getColumnIndex(DBHelper.BUDGET_NAME)));
    limit.append(cursor.getString(cursor.getColumnIndex(DBHelper.BUDGET_LIMIT)));     }