Smart Billing & Discount Engine

I built this small Java console project to practise OOP and the Strategy pattern. It adds up a sample shopping cart, applies a discount, and prints the bill summary.

What it does

The demo uses three strategies:

• Percentage discount: takes a percentage off the cart total.
• Fixed discount: subtracts a set amount, without letting the final amount go below zero.
• BuyOneGetOneDiscount: item-based buy-one-get-one. For every 2 units of an item, 1 is free (so 3 pens means 1 free, 5 pencils means 2 free). It needs the cart items, so call getFinalAmount(cart.getItems(), strategy).

ShoppingCart holds the items, DiscountStrategy defines the discount method, and BillingEngine uses whichever strategy it receives.

Run it

You need a JDK (Java 8 or newer), with both javac and java available. Run these commands from the project folder:

mkdir -p build
javac -encoding UTF-8 -d build App.java BillingEngine.java cart/*.java discount/*.java
java -cp build App

The entry file is App.java, with a capital A. Compiled files go into build/, which is ignored by Git.

Sample result

The sample cart contains 3 pens at Rs 20 each, 2 notebooks at Rs 50 each, and 5 pencils at Rs 10 each. Its total is Rs 210.

Strategy	Discount	Final amount
10% off	Rs 21	Rs 189
Rs 40 off	Rs 40	Rs 170
Buy one get one	Rs 90	Rs 120

These are three separate examples on the same cart, not discounts stacked together.

Project structure

java-oop-billing-engine/
├── App.java
├── BillingEngine.java
├── cart/
│   ├── Item.java
│   └── ShoppingCart.java
├── discount/
│   ├── DiscountStrategy.java
│   ├── PercentageDiscount.java
│   ├── FixedAmountDiscount.java
│   └── BuyOneGetOneDiscount.java
├── .gitignore
└── README.md

Scope

This is a learning project, not a production billing system. It currently uses double for money, and the billing summary does not handle a zero cart total separately. The next improvements would be money handling and tests.

Add a discount

Implement discount.DiscountStrategy, define applyDiscount(double amount) (override applyDiscount(List<Item>) too if it depends on items), and use the new strategy in App.java.

Author

Mitesh Anshu - @miteshanshu