public class MultiSales {

public static void main(String[] args) {

doStuff(new Pen(10.00), 5);

doStuff(new Book(250.00, 5.00), 2);

}

public static void doStuff(Sellable item, int count) {

double income = item.sell(count);

System.out.println("Income: $" + income);

}

}

interface Sellable {

double sell();

default double sell(int count) {

double total = 0.0;

for(int i = 1; i <= count; i++) {

total += this.sell();

}

return total;

}

default String toString() {

return "df";

}

}

class Book implements Sellable {

private double price;

private double discount;

public Book(double price, double discount) {

this.price = price;

this.discount = discount;

}

@Override

public double sell() {

return price - (price * discount / 100.00);

}

}

class Pen implements Sellable {

private double price;

public Pen(double price) {

this.price = price;

}

@Override

public double sell() {

return price;

}

}