public GSM(string manufacturer, string model, int? price, string owner, Battery battery, Display display)
 {
     this.manufacturer = manufacturer;
     this.model = model;
     this.owner = owner;
     this.price = price;
     this.battery = new Battery("NA", 0f, 0f);
     this.display = new Display();
 }
 public GSM(string manufacturer, string model, int price, string owner, Battery battery)
     : this(manufacturer, model, price, owner, battery, new Display())
 {
 }