Example to demonstrate Open-Closed Principle

OpenClosedPrincipleExample.java

import java.util.ArrayList;
import java.util.List;

class Item {
    private final String type;
    private final double price;
    private DiscountStrategy discountStrategy;
    
    public Item(String type, double price, DiscountStrategy discountStrategy) {
        this.type = type;
        this.price = price;
        this.discountStrategy = discountStrategy;
    }

    public String getType() {
        return type;
    }

    public double getPrice() {
        return price;
    }
    
    public DiscountStrategy getDiscountStrategy(){
        return discountStrategy;
    }
}

interface DiscountStrategy {
    double applyDiscount(double price);
}

class BookDiscountStrategy implements DiscountStrategy {
    @Override
    public double applyDiscount(double price) {
        return price * 0.9; // 10% discount on books
    }
}

class ElectronicMarkupStrategy implements DiscountStrategy {
    @Override
    public double applyDiscount(double price) {
        return price * 1.2; // 20% markup on electronics
    }
}

class ShoppingCart {
    private final List<Item> items;
    
    public ShoppingCart(List<Item> items){
        this.items = items;
    }

    public double calculateItemPrice(Item item) {
        // Dynamic behaviour basis DiscountStrategy types
        DiscountStrategy strategy = item.getDiscountStrategy();
        return strategy.applyDiscount(item.getPrice());
    }
    
    public double calculateTotalPrice(){
        double totalPrice = 0.0;
        for(Item item: items)
            totalPrice += calculateItemPrice(item);
        return totalPrice;
    }
}

public class Main {
    public static void main(String[] args) {
        // List of items
        List<Item> items = new ArrayList<Item>();
        
        // Creating Book items and discount
        DiscountStrategy bookDiscount = new BookDiscountStrategy();
        Item book = new Item("Book", 50.0, bookDiscount);
        items.add(book);
        
        // Creating Electronic items and discount
        DiscountStrategy electronicStrategy = new ElectronicMarkupStrategy();
        Item tv = new Item("Electronic", 100.0, electronicStrategy);
        items.add(tv);
        
        // Building a shopping cart
        ShoppingCart shoppingCart = new ShoppingCart(items);

        // Calculating total price for items
        double totalPriceForBook = shoppingCart.calculateItemPrice(book);
        double totalPriceForTv   = shoppingCart.calculateItemPrice(tv);
        double totalCartPrice    = shoppingCart.calculateTotalPrice();

        // Displaying results
        System.out.println("Total Price for Book    : $" + totalPriceForBook);  // Should be 45
        System.out.println("Total Price for TV      : $" + totalPriceForTv);    // Should be 120
        System.out.println("Total Price entire cart : $" + totalCartPrice);     // Should be 165
    }
}