小编sad*_*mkp的帖子

我的程序包含一个Activity class和一个数据库class.我用来保存数据库值的代码ListView有一些问题.下面是一个inner class在Activity:

class getclicker extends ListActivity implements Button.OnClickListener {
    public void onClick(View v) {

        String datevalue = date.getText().toString();
        String Userselectvalue = userSelection.getText().toString();
        cursor1 = eventsData.getContact(datevalue, Userselectvalue);
        String[] fromColumns = { classdbOpenHelper.KEY_EVENT };
        int[] toViews = { R.id.event };
        SimpleCursorAdapter adapter = new SimpleCursorAdapter(this, R.layout.events, cursor1, fromColumns, toViews, 0);
        listView = getListView();
        listView.setAdapter(adapter);

    }

    public void onDestroy() {
        eventsData.close();
    }
}

sqlite类包含

public Cursor getContact(String datevalue, String Userselectvalue) {
    String …

我的程序包含两个activity.i通过方法showevent调用第二个活动.但是发生错误,nullpointer异常.这是什么？我的程序包含两个activity.i通过方法showevent调用第二个活动.但是发生错误,nullpointer异常.为什么这个

FirstActivity.java

public class FirstActivity extends FragmentActivity implements OnItemSelectedListener {
    /** Called when the activity is first created. */
    public final static String EXTRA_MESSAGE = "com.example.myfirstapp.MESSAGE";
    public final static String EXTRA_MESSAGE1 = "com.example.myfirstapp.MESSAGE";


    classdbOpenHelper eventsData;
     TextView userSelection;
     Button okButton;
     Button addButton;

     Button change_date_but;
     TextView date;
     TextView show;
     EditText edittext; 
      public static final int Date_dialog_id = 1;
      private int mYear;
      private int mMonth;
      private int mDay;

    private static final String[] items={"Yalahanka","Rajaji nagar","Sivaji Nagar","Koramangala","RT Nagar", "Banashankari","Yashwanthpura","Hebbal"};



    @Override
    public void onCreate(Bundle savedInstanceState) { …