Adding BigDecimal.ZERO to another BigDecimal value

Question

I'm still learning Java, and I've been reading articles on several sites. I found an article at Java Code Geeks that I have a question about. The article is explaining open/closed principle. The article uses a scenario of applying a discount to a company's product for it's example. The first part of the code is as follows:

import java.math.BigDecimal;
import java.math.RoundingMode;

public class Discount {

    public BigDecimal apply(BigDecimal price) {

        BigDecimal percent = new BigDecimal("0.10");
        BigDecimal discount = price.multiply(percent);
        return price.subtract(discount.setScale(2, RoundingMode.HALF_UP));
    }
}

The second part of the code is as follows:

import java.math.BigDecimal;

public class DiscountService {

    public BigDecimal applyDiscounts(BigDecimal price,Discount discount) {

        BigDecimal discountPrice = price.add(BigDecimal.ZERO);
        discountPrice = discount.apply(discountPrice);
        return discountPrice;
   }
}

On Oracle's site, it says that the ZERO in BigDecimal has a a value of 0 and a scale of zero. Does this mean that in price.add(BigDecimal.ZERO) we are simply adding 0 to the price that's brought in? If so, why? Or is it there simply to drop the decimal places from the price? Or is there some other purpose for it?

Thanks!

Answer 1

price.add(BigDecimal.ZERO) is being used to create a new BigDecimal since BigDecimal doesn't have a copy constructor.

As to why, this is presumably a defensive copy in case the BigDecimal being passed in is actually a "sabotaged" version (because BigDecimal isn't final) whose value can be changed after the fact. This is talked about in depth in Effective Java as the item "Make defensive copies when needed;" Item 39 in 2nd edition or Item 50 in 3rd Edition.