當新按鈕按下時，Android應用程序崩潰

Question

我正在製作一個Android應用程序，該應用程序包含SQLite數據庫中的國家/地區列表。初始屏幕顯示列表。當你按「編輯列表」時，一個新的活動開始你可以進入一個新的國家或刪除整個列表。該應用程序完美工作，直到我點擊我的第二​​個活動中的按鈕。如果我進入我的新國家並點擊添加，該應用程序給我一個消息說它已經停止，但該國仍然添加到我的數據庫。我的全部刪除按鈕也一樣。我的logcat出現錯誤，但仍然刪除了所有內容。這是我對於發生錯誤的類代碼：

public class EditList extends Activity 
{ 
    private DBManager db; 
    Cursor cursor; 
    EditText editCountry, editYear; 
    Button add, delete, back; 

    /** Called when the activity is first created. */ 
    @Override 
    public void onCreate(Bundle savedInstanceState) 
    { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 

     editCountry = (EditText)findViewById(R.id.editcountry); 
     editYear = (EditText)findViewById(R.id.edityear); 
     add = (Button)findViewById(R.id.add); 
     delete = (Button)findViewById(R.id.delete); 
     back = (Button)findViewById(R.id.backmain); 

     //Open database and fill it with content, then close it 
     db = new DBManager(this); 
     db.openToWrite(); 

     add.setOnClickListener(buttonAddOnClickListener); 
     delete.setOnClickListener(buttonDeleteAllOnClickListener); 

     //------ 
     Log.i("Test", "back to main"); 

     //go back to the main list screen 
     back = (Button)findViewById(R.id.backmain); 
     back.setOnClickListener(new View.OnClickListener() 
     { 
      public void onClick(View v) 
      { 
       db.close(); 
       finish(); 
      } 
     }); 
    } 

    //insert new country button 
    Button.OnClickListener buttonAddOnClickListener = new Button.OnClickListener() 
    { 
     @Override 
     public void onClick(View arg0) 
     { 
      int year = Integer.parseInt(editYear.getText().toString()); 
      String country = editCountry.getText().toString(); 

      db.insert(year, country); 

      updateList(); 

      //clear text fields after use 
      editYear.setText(null); 
      editCountry.setText(null); 
     } 
    }; 

    //delete all button 
    Button.OnClickListener buttonDeleteAllOnClickListener = new Button.OnClickListener() 
    { 
     @Override 
     public void onClick(View arg0) 
     { 
      db.deleteAll(); 

      updateList(); 
     } 
    }; 

    private void updateList() 
    { 
     cursor.requery(); 
    } 
} 

這裏發生的logcat的錯誤：

02-07 22:32:31.117: D/AndroidRuntime(2633): Shutting down VM 
02-07 22:32:31.117: W/dalvikvm(2633): threadid=1: thread exiting with uncaught exception (group=0xb4e35908) 
02-07 22:32:31.158: E/AndroidRuntime(2633): FATAL EXCEPTION: main 
02-07 22:32:31.158: E/AndroidRuntime(2633): java.lang.NullPointerException 
02-07 22:32:31.158: E/AndroidRuntime(2633):  at com.mypackage.msdassignment.EditList.updateList(EditList.java:90) 
02-07 22:32:31.158: E/AndroidRuntime(2633):  at com.mypackage.msdassignment.EditList.access$1(EditList.java:88) 
02-07 22:32:31.158: E/AndroidRuntime(2633):  at com.mypackage.msdassignment.EditList$1.onClick(EditList.java:68) 
02-07 22:32:31.158: E/AndroidRuntime(2633):  at android.view.View.performClick(View.java:4202) 
02-07 22:32:31.158: E/AndroidRuntime(2633):  at android.view.View$PerformClick.run(View.java:17340) 
02-07 22:32:31.158: E/AndroidRuntime(2633):  at android.os.Handler.handleCallback(Handler.java:725) 
02-07 22:32:31.158: E/AndroidRuntime(2633):  at android.os.Handler.dispatchMessage(Handler.java:92) 
02-07 22:32:31.158: E/AndroidRuntime(2633):  at android.os.Looper.loop(Looper.java:137) 
02-07 22:32:31.158: E/AndroidRuntime(2633):  at android.app.ActivityThread.main(ActivityThread.java:5039) 
02-07 22:32:31.158: E/AndroidRuntime(2633):  at java.lang.reflect.Method.invokeNative(Native Method) 
02-07 22:32:31.158: E/AndroidRuntime(2633):  at java.lang.reflect.Method.invoke(Method.java:511) 
02-07 22:32:31.158: E/AndroidRuntime(2633):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793) 
02-07 22:32:31.158: E/AndroidRuntime(2633):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560) 
02-07 22:32:31.158: E/AndroidRuntime(2633):  at dalvik.system.NativeStart.main(Native Method) 

Answer 1

你永遠實例光標。你聲明它然後你使用它。那會失敗。