Basic Query about Spinner INVALID_POSITION

Question

As per Android Docs,

public int getSelectedItemPosition ()

Since: API Level 1 Return the position of the currently selected item within the adapter's data set Returns int Position (starting at 0), or INVALID_POSITION if there is nothing selected.

Now i want to know in which case/situation INVALID_POSITION is returned , since when spinner is initialized it default position is set to 0 .

  Spinner spinner=(Spinner)findViewById(R.id.spinner1);
    ArrayAdapter<String> adapterVehicleList = new ArrayAdapter<String>(this, android.R.layout.simple_spinner_item, vehicleTypeList);
    adapterVehicleList.setDropDownViewResource(android.R.layout.select_dialog_singlechoice);
    spinner.setAdapter(adapterVehicleList);
    spinner.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
        @Override
        public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {
            // TODO Auto-generated method stub

        }

        @Override
        public void onNothingSelected(AdapterView<?> parent) {
            // TODO Auto-generated method stub

        }
    });

    int position =spinner.getSelectedItemPosition();
             position =spinner.getSelectedItemPosition();
    Toast.makeText(this,String.valueOf(position),Toast.LENGTH_LONG).show();code here