How can I make the "Product added to cart!" show once only when I input the product ID and the quantity

Question

This is the code that I have been working on, and it is really a huge problem, I cant use any statement only those you see here, I need help :D thank you

if (choice == 1) {
    buyer.listProducts(seller);
    
    int productId, quantity;
    boolean validInput = false;

    while (!validInput) {
        System.out.print("Enter the ID of the product to add to the cart: ");
        if (scanner.hasNextInt()) {
            productId = scanner.nextInt();
            Product productToAdd = findProductById(productId, seller);

            if (productToAdd != null) {
                validInput = true;

                // Prompt for the quantity
                validQuantity = false;

                while (!validQuantity) {
                    System.out.print("Enter quantity: ");
                    if (scanner.hasNextInt()) {
                        quantity = scanner.nextInt();
                        if (quantity > 0 && quantity <= productToAdd.quantity) {
                            validQuantity = true;
                        } else {
                            System.out.println("Invalid quantity or not enough stock.");
                        }
                    } else {
                        System.out.println("Invalid input. Please enter a valid quantity (a positive integer).");
                        scanner.next(); // Clear the input buffer
                    }
                }

                if (validQuantity) {
                    buyer.addToCart(productToAdd, quantity);
                    System.out.println("Product added to cart!");
                }
            } else {
                System.out.println("Product not found. Please enter a valid product ID.");
            }
        } else {
            System.out.println("Invalid input. Please enter a valid product ID (a number).");
            scanner.next(); // Clear the input buffer
        }
    }
}

This is the code that I was working on and I am still confuse on what I am gonna do, I need help from you guys

I wanted to make the "Product added to cart!" only show once