How to multiply a BigDecimal by an integer in Java

You have a lot of type-mismatches in your code such as trying to put an int value where BigDecimal is required. The corrected version of your code:

public class Payment
{
    BigDecimal itemCost  = BigDecimal.ZERO;
    BigDecimal totalCost = BigDecimal.ZERO;

    public BigDecimal calculateCost(int itemQuantity, BigDecimal itemPrice)
    {
        itemCost  = itemPrice.multiply(BigDecimal.valueOf(itemQuantity));
        totalCost = totalCost.add(itemCost);
        return totalCost;
    }
}