ProductServiceImpl.java

@Service
public class ProductServiceImpl implements ProductService {

    @Autowired
    ProductRepository productRepository;

    @Autowired
    CoinAmount coinAmount;

    @Override
    public ProductDispenseResponse dispense(ProductRequest request) {
        Optional<Product> product = productRepository.findById(request.getId());

        if (product.get().getStock() < 1) {
            return ProductDispenseResponse.builder()
                    .responseMessage("SOLD OUT")
                    .currentBalance(coinAmount.getBalance())
                    .build();
        }

        if (coinAmount.getBalance() < product.get().getPrice()) {
            return ProductDispenseResponse.builder()
                    .responseMessage("Unfortunately your balance is not sufficient.")
                    .currentBalance(coinAmount.getBalance())
                    .build();
        }

        updateStock(product.get());
        coinAmount.setBalance(coinAmount.getBalance() - product.get().getPrice());
        return ProductDispenseResponse.builder()
                .productName(product.get().getName())
                .responseMessage("THANK YOU")
                .currentBalance(coinAmount.getBalance())
                .build();
    }

    private void updateStock(Product product) {
        product.setStock(product.getStock()-1);
        productRepository.save(product);
    }
}