问‘'discount’函数不能正常工作

EN

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); 
  }
} 

public void discount(double rate){
        double discount = (netPrice/100) * rate;
        netPrice = netPrice - discount;
        netPrice = Math.round(netPrice * 100.0) / 100.0