if if else語句中的意外結果

Question

我是初學者，所以這個結果我得到了我的if else語句的一個條件，正在讓我大開眼界。除了QtyCalc變量大於等於100的情況外，一切正常。Finprice變量被列爲光盤變量，我找不到原因。幫幫我？

import javax.swing.JOptionPane; 
public class SoftwareSales { 
    public static void main(String[] args) { 
     final int price = 99; 
     String Qty; 
     double QtyCalc, preprice, Finprice, disc; 

     Qty = JOptionPane.showInputDialog(null, "How many packages will you buy?"); 
     QtyCalc = Double.parseDouble(Qty); 
     preprice = QtyCalc * price; 

     if (QtyCalc >= 100) { 
      disc = (preprice * (0.5)); 
      Finprice = (preprice - disc); 
      JOptionPane.showMessageDialog(null, "Your discount is: " + disc + ".\n" + "Your final price is: " + Finprice + ". "); 
     } else if (QtyCalc >= 50 && QtyCalc <= 99) { 
      disc = (preprice * 0.4); 
      Finprice = (preprice - disc); 
      JOptionPane.showMessageDialog(null, "Your discount is: " + disc + ".\n" + "Your final price is: " + Finprice + ". "); 
     } else if (QtyCalc >= 20 && QtyCalc <= 49) { 
      disc = (preprice * 0.3); 
      Finprice = (preprice - disc); 
      JOptionPane.showMessageDialog(null, "Your discount is: " + disc + ".\n" + "Your final price is: " + Finprice + ". "); 
     } else if (QtyCalc >= 10 && QtyCalc <= 19) { 
      disc = (preprice * 0.2); 
      Finprice = (preprice - disc); 
      JOptionPane.showMessageDialog(null, "Your discount is: " + disc + ".\n" + "Your final price is: " + Finprice + ". "); 
     } else if (QtyCalc < 10 && QtyCalc >= 1) { 
      disc = 0; 
      Finprice = 0; 
      JOptionPane.showMessageDialog(null, "Sorry, there is no discount for purchases less than 10." + " Your price is: " + preprice); 

     } else { 
      JOptionPane.showMessageDialog(null, "You have entered an invalid number."); 
     } 
     disc = 0; 
     Finprice = 0; 

     System.exit(0); 
    } 
} 

Answer 1

最終價格等於數量> = 100時的折扣，因爲折扣是50％...查看您發佈的代碼。特別是：

if (QtyCalc >= 100) { 
    disc = (preprice * (0.5)); 
    Finprice = (preprice - disc); 
    JOptionPane.showMessageDialog(null, "Your discount is: " + disc + ".\n" + "Your final price is: " + Finprice + ". "); 
} 

Answer 2

有沒有奇怪的行爲，只是簡單的數學：

 disc = (preprice * (0.5)); 
     Finprice = (preprice - disc); 

你也應該考慮消除冗餘代碼：

import javax.swing.JOptionPane; 
public class SoftwareSales { 
    public static void main(String[] args) { 
     final int PRICE = 99; 
     String qty; 
     double qtyCalc, preprice, finprice, disc; 
     disc = 0; 
     qtyCalc = 0; 

     boolean invalid = false; 
     do{ 
      invalid = false; 
      qty = JOptionPane.showInputDialog(null, "How many packages will you buy?"); 
      if (qty == null){ 
       return; 
      } 
      try{ 
       qtyCalc = Double.parseDouble(qty);   
      }catch(NumberFormatException e){ 
       invalid = true; 
      } 
     }while(invalid); 

     preprice = qtyCalc * PRICE; 
     if (qtyCalc >= 100) { 
      disc = (preprice * (0.5)); 
     } else if (qtyCalc >= 50 && qtyCalc <= 99) { 
      disc = (preprice * 0.4); 
     } else if (qtyCalc >= 20 && qtyCalc <= 49) { 
      disc = (preprice * 0.3); 
     } else if (qtyCalc >= 10 && qtyCalc <= 19) { 
      disc = (preprice * 0.2); 
     } else if (qtyCalc < 10 && qtyCalc >= 1) { 
      disc = 0; 
      finprice = 0; 
      JOptionPane.showMessageDialog(null, "Sorry, there is no discount for purchases less than 10. Your price is: " + preprice); 
      return; 
     } else { 
      JOptionPane.showMessageDialog(null, "You have entered an invalid number."); 
      return; 
     } 

     finprice = (preprice - disc); 
     JOptionPane.showMessageDialog(null, String.format("Your discount is: %.2f.\nYour final price is: %.2f. ",disc,finprice)); 

    } 
} 

還記得Java的約定變量：始終以小寫開頭。 決賽等級大寫