Week 6: Methods of OO Programming/10A. Interface Class as Variable Type+++

Although an object cannot be formed from an interface, an 'interface' can be used as a 'variable type'. 
Such a variable can store a REFERENCE to 'ANY OBJECT THAT IMPLEMENTS the INTERFACE'. 
However, note that ONLY OPERATIONS defined IN the INTERFACE can be called through such a variable.


Let’s examine a method named ‘mostExpensive’, which takes a list of ‘Priced’ type objects as a parameter. 
The method finds the most expensive among these.

Below is a refresher on the interface class ‘Priced’:

interface Priced {
    int givePrice();
}




 

The static method 'mostExpensive' now returns the element with the highest price from the given elements:

public static Priced mostExpensive(ArrayList<Priced> list) {

    Priced mostExpensive = list.get(0);
    for (Priced product : list) {

        // 'product' object is from a class implementing the 'Priced' interface
        // which can use the 'givePrice' method from the interface
        if (product.givePrice() > mostExpensive.givePrice()) {
            mostExpensive = product;
        }
    }
    return mostExpensive;
}
 







The objects in the list can be of different types, the only REQUIREMENT is that EACH IMPLEMENTS the "Priced" interface.
Examples include the classes "Fruit", "Car" and "MovieTicket":

class Fruit implements Priced {
    private String name;
    
    public Fruit(String name) {
        this.name = name;
    }

    @Override
    public int givePrice() {
        return 10;
    }
}



class Car implements Priced {
    private String brand;
    private int price;
    
    public Car(String brand, int price) {
        this.brand = brand;
        this.price = price;
    }

    @Override
    public int givePrice() {
        return price;
    }
}



class MovieTicket implements Priced {
    private String movie;
    private int ageLimit;
    private int price;
    
    public MovieTicket(String movie, int ageLimit, int price) {
        this.movie = movie;
        this.ageLimit = ageLimit;
        this.price = price;
    }

    @Override
    public int givePrice() {
        return price;
    }  
}





Let's create a few objects and call the method:
public static void main(String[] args) {
    ArrayList<Priced> products = new ArrayList<>();
    products.add(new Car("Mercedes", 15000));
    products.add(new MovieTicket("Rambo 6", 21, 13));
    products.add(new Fruit("Banana"));
    
    Priced mostExpensive = mostExpensive(products);
}