How to check if an item is in a database before trying to add it

Question

I have an android app that is adding to and updating a database. The student ID number is the primary key. I'm trying to have the app display a message when the user attempts to add a duplicate. This is my latest attempt; it's triggering the catch clause.

In my main activity, I have this conditional statement:

 if (!dbHandler.checkStudentID(studentID)){
    Toast.makeText(MainActivity.this, "Student ID is a Duplicate",
         Toast.LENGTH_SHORT).show();
 }
 dbHandler.addNewStudent(studentID, firstName, lastName, grade);

In my handler the method is this:

public boolean checkStudentID (String studentID) {
   SQLiteDatabase db = this.getReadableDatabase();
   Cursor checkID = db.rawQuery("SELECT student_id FROM " + TABLE_NAME +
                   " where student_id=?", new String[]{studentID});
   String x = checkID.getString(0);
   checkID.close();
   return x.isEmpty();
}

Logcat isn't showing any issues and there aren't any syntax problems shown, so I'm at a loss as to what's not correct.

Answer 1

The problem is if the studentID could not be found, String x = checkID.getString(0) contains null. And with that, calling x.isEmpty() will throw a NullPointerException. So you should intercept that by:

public static boolean checkStudentID (String studentID) {
   SQLiteDatabase db = this.getReadableDatabase();
   Cursor checkID = db.rawQuery("SELECT student_id FROM " + TABLE_NAME +
                   " where student_id=?", new String[]{studentID});
   String x = checkID.getString(0);
   checkID.close();
   if (x == null || x.isEmpty()) {
       return false;
   }
   else {
       return true;
   }
}

I think you could even leave x.isEmpty() out altogether.

If the Toast.makeText().show() doesn't terminate the program, you should better wrap that addNewStudent part in an else-case. Otherwise, it will still be executed!

if (!dbHandler.checkStudentID(studentID)){
   Toast.makeText(MainActivity.this, "Student ID is a Duplicate",
        Toast.LENGTH_SHORT).show();
}
else{
   dbHandler.addNewStudent(studentID, firstName, lastName, grade);
}