// this is your main activity
public class BluetoothTool extends Activity {
   // this is the static property
   // BluetoothConnection - is whatever class you use, doesn't matter for us
   public static BluetoothConnection btc;
   public void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      btc = new BluetoothConnection();
   }
}

// this is another file, another activity,
// but it can access directly static property of the main public
public class MenuItem1Activity extends Activity {
    public someMethodWhereINeedBlueTooth(String stuffToSend) {
       BluetoothTool.btc.send(stuffToSend);
    }
}