I have a function discount that takes in a 'rate' parameter (a double) and updates a private variable netPrice within my class (also a double) to a reduced price. The calculation seems to work but there is a problem; it produces inaccuracies.
public class Good{
private String name;
private double netPrice;
final static double VAT_RATE = 0.2;
/**
* The constructor instantiates an object representing a priced good.
* @param name
* @param netPrice
*/
public Good(String name, double netPrice){
this.name = name;
this.netPrice = netPrice;
}
/**
* Retrieves and reads the value of name.
* @return the name of the good.
*/
public String getName(){
return name;
}
/**
* Updates the value name.
* @param name
*/
public void setName(String name){
this.name = name;
}
/**
* Retrieves and reads the value of netPrice
* @return the net price of the good.
*/
public double getNetPrice(){
return netPrice;
}
/**
* Updates the value netPrice
* @param netPrice
*/
public void setNetPrice(double netPrice){
this.netPrice = netPrice;
}
/**
* @return netprice adjusted for VAT
*/
public double grossPrice(){
return netPrice + (netPrice * VAT_RATE);
}
public void discount(double rate){
double discount = (netPrice/100) * rate;
netPrice = netPrice - discount;
}
/**
* @return formatted string stating the name and the gross price of the good.
*/
public String toString(){
return "This " + name + " has a gross price of \u00A3 " + grossPrice();
}
public static void main(String[] args){
Good milk = new Good("milk", 0.50);
System.out.println(milk);
milk.discount(20);
System.out.println(milk);
}
}
In the example above I'm expecting the discounted price to be £ 0.48 but I get £ 0.48000000000000004. I have no idea why this is off, so if anybody could shed some light on why this is happening that would be great.
As for the fix, I initially thought I could circumvent this problem by using the following method for rounding to 2 d.p.:
public void discount(double rate){
double discount = (netPrice/100) * rate;
netPrice = netPrice - discount;
netPrice = Math.round(netPrice * 100.0) / 100.0
However this doesn't seem to work, in fact it gives me the same answer as above. Another fix could involve using BigDecimal but I don't know exactly how I could get this to work cleanly without changing my netPrice variable to a different type (something that I don't want to do). Any ideas?
