如何从ArrayList中获得总价值，其中一个是产品，另一个是价格？

Question

I am trying to get the total value from all the products purchased of the Cart. This information came from an ArrayList but I am not sure if the code I am doing is correct.

​public class Cart {

    // creating a new list every time so need to modify..it will hold the list
    private List<CartLine> cartLineList = new ArrayList<>();

    /**
     *
     * @return the actual cartline list
     */
    public List<CartLine> getCartLineList() {
            return cartLineList;
    }

    public double getTotalValue(List<CartLine> cartLineList)
    {
         //TODO implement the method
        //return Products*Price
        double results=0;
    //  for(CartLine cartLine: getCartLineList()){
            //results += (cartLine.getQuantity()* cartLine.getProduct().getPrice());
        //}

        return results;

    }


   //more code here...

}

this what CartLine looks like

public class CartLine {

private Product product;
private int quantity;

public CartLine(Product product, int quantity) {
    this.product = product;
    this.quantity = quantity;
}

public double getSubtotal() {
    return quantity * product.getPrice();
}

public Product getProduct() {
    return product;
}

public void setProduct(Product product) {
    this.product = product;
}

public int getQuantity() {
    return quantity;
}

public void setQuantity(int quantity) {
    this.quantity = quantity;
}

}

Answer 1

If getQuantity does returns the amount purchased, and .getProduct().getPrice() returns the price of the item, then the summation code looks fine (only commented out). Here's what it should look like:

public double getTotalValue(List<CartLine> cartLineList) {
    double results = 0;
    for(CartLine cartLine: getCartLineList()){
        results += (cartLine.getQuantity() * cartLine.getProduct().getPrice());
    }
    return results;
}

It would be helpful if you showed us what CartLine is. Could there be different products with different prices in the same CartLine instance?

Answer 2

The same answer but using lambdas

 Collector<CartLine, double[], double[]> productSumCollector = Collector.of(
        () -> new double[1],
        (result, cartLine) -> result[0] += (cartLine.getQuantity() * cartLine.getProduct().getPrice()),
        (result1, result2) -> {
             result1[0] += result2[0];
             return result1;
        }
    );

return Arrays.stream(cartLines).collect(productSumCollector)[0];

For reference see https://www.deadcoderising.com/2017-03-07-java-8-creating-a-custom-collector-for-your-stream/