空数据库报出错误信息

Question

public boolean isEmpty()
{
    if (database == null) {
        database = dbHelper.getReadableDatabase(); // or similar method that   
    }
    Cursor cursor = database.rawQuery("SELECT COUNT(*) FROM " + DatabaseHelper.TABLE_ENCOURAGEMENTS, null);
    if (cursor != null) {
        return false;
    } else {
        return true;
    }
}

将上面这段代码在一个空数据库中运行时报错，信息如下：

java.lang.IllegalStateException: Could not execute method of the activity
at android.view.View$1.onClick(View.java:2144)
Caused by: android.database.CursorIndexOutOfBoundsException: Index 0 requested, 
with a size of 0

谢谢！

Answer 1

public boolean isEmpty() {
   int count = -1;
   if (cursor != null && cursor.moveToFirst()) {
      count = cursor.getInt(0);
      if (count > 0) {
         return false;
      }
      else {
         return true;
      }
   }
   return true;
}

Explanation:

moveToFirst() is handy method that returns false if Cursor is empty. Checking whether Cursor is assigned to NULL isn't enough. You need to call it always you want to check status of your Cursor because also each Cursor is implicitly positioned before first row so you need to call it always.

Since you are selecting COUNT(*) so it always return at least one row with count value. You need to check this value whether is greather than zero or not. If yes, db is not empty.

问题原址:http://stackoverflow.com/questions/15888695/checking-for-an-empty-database-raises-an-error