eclipse 尝试在空对象引用上调用虚拟方法 'android.database.Cursor android.database.sqlite.SQLiteDatabase.query
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Attempt to invoke virtual method 'android.database.Cursor android.database.sqlite.SQLiteDatabase.query on null object reference
提问by beginner
I have an activity in which i am trying to get the list of data based on some condition . This selectsome() function is in the SQLiteOpenHelper subclass.
我有一项活动，我试图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);
In class MyDBHelper.java
在 MyDBHelper.java 类中
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;
}
I am getting this error please help.
我收到此错误，请帮忙。
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.access0(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)
回答by laalto
Your variable dbin the helper is not initialized. Call getWritableDatabase()or getReadableDatabase()to obtain a SQLiteDatabasereference you can then assign to that variable before the query:
您db在帮助程序中的变量未初始化。调用getWritableDatabase()或getReadableDatabase()获取SQLiteDatabase引用，然后您可以在查询之前分配给该变量：
db = getWritableDatabase();
Cursor c = db.query(...);
