尝试调用空对象引用上的android.database.sqlite.SQLiteDatabase.query方法

Question

我有一个活动，在其中我正在尝试根据某些条件获取数据列表。这个selectsome()函数在SQLiteOpenHelper子类中。

MyDBHandler dbhelper = new MyDBHandler(this);
Cursor cursor;
cursor = dbhelper.selectSome(value);

在类MyDBHelper.java中

public Cursor selectSome(String arg)

{

    String columns[]=new String[]{"_id","_hotelname","_city","_imageSrc","_rent","_address"};
    String selarg[]={arg};
    Cursor c=db.query(TABLE_NAME,columns,"city = ?",selarg, null,null,null);
   return c;
}

我遇到了这个错误，请帮忙解决。

09-01 18:08:00.394 E/AndroidRuntime( 6246): FATAL EXCEPTION: main

09-01 18:08:00.394 E/AndroidRuntime( 6246): Process: com.example.hari.testapp, PID: 6246

09-01 18:08:00.394 E/AndroidRuntime( 6246): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.hari.testapp/com.example.hari.testapp.HotelList}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.Cursor android.database.sqlite.SQLiteDatabase.query(java.lang.String, java.lang.String[], java.lang.String, java.lang.String[], java.lang.String, java.lang.String, java.lang.String)' on a null object reference

09-01 18:08:00.394 E/AndroidRuntime( 6246):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)

09-01 18:08:00.394 E/AndroidRuntime( 6246):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)

09-01 18:08:00.394 E/AndroidRuntime( 6246):     at android.app.ActivityThread.access$800(ActivityThread.java:151)

09-01 18:08:00.394 E/AndroidRuntime( 6246):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)

09-01 18:08:00.394 E/AndroidRuntime( 6246):     at android.os.Handler.dispatchMessage(Handler.java:102)

09-01 18:08:00.394 E/AndroidRuntime( 6246):     at android.os.Looper.loop(Looper.java:135)

09-01 18:08:00.394 E/AndroidRuntime( 6246):     at android.app.ActivityThread.main(ActivityThread.java:5257)

09-01 18:08:00.394 E/AndroidRuntime( 6246):     at java.lang.reflect.Method.invoke(Native Method)

09-01 18:08:00.394 E/AndroidRuntime( 6246):     at java.lang.reflect.Method.invoke(Method.java:372)

09-01 18:08:00.394 E/AndroidRuntime( 6246):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)

09-01 18:08:00.394 E/AndroidRuntime( 6246):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

09-01 18:08:00.394 E/AndroidRuntime( 6246): Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.Cursor android.database.sqlite.SQLiteDatabase.query(java.lang.String, java.lang.String[], java.lang.String, java.lang.String[], java.lang.String, java.lang.String, java.lang.String)' on a null object reference

09-01 18:08:00.394 E/AndroidRuntime( 6246):     at com.example.hari.testapp.databaseFiles.MyDBHandler.selectSome(MyDBHandler.java:113)

- beginner

Answer 1

您的助手中的变量db未初始化。在查询之前调用getWritableDatabase()或getReadableDatabase()以获取SQLiteDatabase引用，然后将其分配给该变量：

db = getWritableDatabase();
Cursor c = db.query(...);

- laalto

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如何在我的示例中使用“??”？我正在尝试，但无法正确调用它。 - beginner

非常感谢，它对我起作用了。 - beginner