main.dart

import 'dart:io';
/*
 * An implementation of shopping cart and checkout in Dart
 *
 */

class Product {
  const Product({required this.id, required this.name, required this.price});
  final int id;
  final String name;
  final double price;

  String get displayNames => '($initial)${name.substring(1)}, price \$$price';
  String get initial => name.substring(0, 1);
}

class Item {
  const Item(this.product, this.quantity);
  final Product product;
  final int quantity;
  double get price => product.price * quantity;

  @override
  String toString() => "$quantity x ${product.name} \$$price";
}

class Cart {
  // private value
  final Map<int, Item> _items = {};

  void addProduct(Product product) {
    final item = _items[product.id];
    if (item == null) {
      _items[product.id] = Item(product, 1);
    } else {
      _items[product.id] = Item(product, item.quantity + 1);
    }
  }

  bool get isEmpty => _items.isEmpty;

  // get total
  double total() => _items.values
      .map((item) => item.price)
      .reduce((value, element) => value + element);

  @override
  String toString() {
    if (_items.isEmpty) {
      return 'Cart is empty';
    }
    final itemizedList =
        _items.values.map((item) => item.toString()).join('\n');
    return "------\n$itemizedList\nTotal: \$${total()}\n------";
  }
}

const List<Product> allProducts = [
  Product(id: 1, name: 'apple', price: 1.1),
  Product(id: 2, name: 'banana', price: 2.2),
  Product(id: 3, name: 'milk', price: 3.3),
];


// loop (Pseudocode)
// prompt: view cart / add item / checkout
// if selection == add item
//    choose a product
//    add it to cart
//    print cart
// else if selection == view cart
//    print cart
// else if selection == checkout
//    do checkout
// end

void main() {
  final cart = Cart();
  while (true) {
    stdout.write(
        'What do you want to do? (v)iew items, (a)dd items, (c)heckout: ');
    final line = stdin.readLineSync();
    if (line == 'a') {
      final product = chooseProduct();
      if (product != null) {
        cart.addProduct(product);
        print(cart);
      }
    } else if (line == 'v') {
      print(cart);
    } else if (line == 'c') {
      if (checkout(cart)) {
        break;
      }
    }
  }
}

Product? chooseProduct() {
  final productList = allProducts.map((e) => e.displayNames).join('\n');
  stdout.write('Available products: \n$productList\nYour Choice: ');
  final line = stdin.readLineSync();
  for (var product in allProducts) {
    if (product.initial == line) {
      return product;
    }
  }
  print('Not found');
  return null;
}

bool checkout(Cart cart) {
  if (cart.isEmpty) {
    print('Cart is empty');
    return false;
  }
  final total = cart.total();
  print('Total: \$$total');
  stdout.write('Payment in cash: ');
  final line = stdin.readLineSync();

  if (line == null || line.isEmpty) {
    return false;
  }

  final paid = double.tryParse(line);
  if (paid == null) {
    return false;
  }

  if (paid >= total) {
    final change = paid - total;
    print('Change: \$${change.toStringAsFixed(2)}');
    return true;
  } else {
    print('Not enough cash');
    return false;
  }
}

The interactive output in console