Java JDK-从double到int的可能有损转换

Question

So I have recently written the following code:

    import java.util.Scanner;

public class TrainTicket
{
      public static void main (String args[])
      {

         Scanner money = new Scanner(System.in);
         System.out.print("Please type in the type of ticket you would like to buy.\nA. Child B. Adult C. Elder.");
         String type = money.next();
         System.out.print("Now please type in the amount of tickets you would like to buy.");
         int much = money.nextInt();
         int price = 0;
         switch (type)
          {
            case "A":
            price = 10;
            break;
            case "B":
            price = 60;
            break;
            case "C":
            price = 35;
            break;
            default:
            price = 0;
            System.out.print("Not a option ;-;");
           }
          if (price!=0)
          {
            int total2 = price* much* 0.7;
            System.out.print("Do you have a coupon code? Enter Y or N");
            String YN = money.next();
            if (YN.equals("Y"))
            {
             System.out.print("Please enter your coupon code.");
             int coupon = money.nextInt();
             if(coupon==21)
             {
              System.out.println("Your total price is " + "$" + total2 + ".");
             }
             else
             {
              System.out.println("Invalid coupon code, your total price is " + "$" + price* much + ".");
             }
            }
            else
            {
            System.out.println("Your total price is " + "$" + price* much + "." ); 
            }
          }

       money.close();
      }
}

However, it keeps displaying this:

TrainTicket.java:31: error: incompatible types: possible lossy conversion from double to int
            int total2 = price* much* 0.7;

When I try and run it with cmd.

Can someone help and explain the error that I have made? Any help is appreciated :). Thanks!

Answer 1

When you convert double to int ,the precision of the value is lost. For example, When you convert 4.8657 (double) to int.The int value will be 4.Primitive int does not store decimal numbers.So you will lose 0.8657.

In your case,0.7 is a double value(floating point treated as double by default unless mentioned as float-0.7f). When you calculate price*much*0.7 ,the answer is a double value and so the compiler wouldn't allow you to store it in a type integer since there could be a loss of precision.So that's what is possible lossy conversion ,you may lose precision.

So what could you do about it? You need to tell the compiler that you really want to do it.You need to tell it that you know what you are doing. So explicitly convert double to int using the following code:

int total2= (int) price*much*0.7;
 /*(int) tells compiler that you are aware      of what you are doing.*/
 //also called as type casting

In your case,since you are calculating the cost,I'll suggest you to declare variable total2 as the type double or float.

double total2=price*much*0.7;
 float total2=price*much*0.7;
 //will work

Answer 2

You are trying to assign price* much* 0.7 , which is a floating point value (a double ), to an integer variable. A double is not an exact integer, so in general an int variable cannot hold a double value.

For instance, suppose the result of your calculation is 12.6 . You can't hold 12.6 in an integer variable, but you could cast away the fraction and just store 12 .

If you are not worried about the fraction you will lose, cast your number to an int like this:

int total2 = (int) (price* much* 0.7);

Or you could round it to the nearest integer.

int total2 = (int) Math.round(price*much*0.7);