public void addInstrument(String serialNumber, double price, InstrumentSpec spec) {
	Instrument instrument  = null ;
	if (spec instanceof GuitarSpec ) {
		instrument  = new Guitar(serialNumber, price , (GuitarSpec) spec);
	}
	
	if (spec instanceof MandolinSpec ) {
		instrument  = new Mandolin(serialNumber, price , (MandolinSpec) spec);
	}
	inventory.add(instrument);
}