class CarDemo {
	public static void main(String[] args) {
		//Born with 20 gallons, gets 10 mpg.
		Car ford = new Car("Ford", 20, 10);
		System.out.println(ford);

		ford.drive(200);
		System.out.println(ford);

		Car chevy = new Car("Chevy", 20, 15);

		if (ford.isEmpty()) {
			ford.siphon(chevy, 30);	//will only get 20
		}

		System.out.println(ford);
		System.out.println(chevy);

		Car.swap(ford, chevy);
		System.out.println(ford);
		System.out.println(chevy);

		System.exit(0);
	}
}

class Car {
	private String name;
	private double gallons;
	private double mpg;	//miles per gallon

	public Car(String name, double gallons, double mpg) {	//constructor
		if (gallons < 0) {
			System.err.println("gallons " + gallons + " can't be negative");
			System.exit(1);
		}

		if (mpg < 0) {
			System.err.println("mpg " + mpg + " can't be negative");
			System.exit(1);
		}

		this.name = name;
		this.gallons = gallons;
		this.mpg = mpg;
	}

	public String toString() {
		return name + " has " + gallons + " gallons, gets " + mpg
			+ " mpg, could go " + gallons * mpg + " more miles";
	}

	public boolean isEmpty() {
		return gallons <= 0;	//should never be negative
	}

	public void drive(double miles) {	//how many miles we want to go

		//How many miles can we go?
		double will = Math.min(miles, gallons * mpg);

		gallons -= will / mpg;
	}

	//Siphon gas from the other Car into this one.
	//g is the number of gallons we want to siphon.

	public void siphon(Car other, double g) {
		if (this == other) {   //test two objects for identity
			System.err.println(name + " can't siphon from itself");
			System.exit(1);
		}

		//How many gallons can we siphon?
		double can = Math.min(g, other.gallons);

		gallons += can;
		other.gallons -= can;
	}

	//Swap the entire contents of the two cars--the engines, gas tanks, etc.

	static public void swap(Car car1, Car car2) {

		if (car1 == car2) {
			System.err.println("Swapping " + car1.name
				+ " with itself would do nothing.");
			System.exit(1);
		}

		double temp = car1.gallons;
		car1.gallons = car2.gallons;
		car2.gallons = temp;

		temp = car1.mpg;
		car1.mpg = car2.mpg;
		car2.mpg = temp;
	}
}