Question

我无法在我的个人项目中使用if和else if语句。

在询问用户输入时，我的程序似乎在第一行正常工作，但代码中存在问题。我的编译器要求我使用switch方法。

我也遇到过编译器告诉我无法将String转换为double的问题，这是我已经使用搜索找到的内容。

我知道这可能有很多要问，但我真的很感谢你的帮助。

/**
* This application executes number of gallons purchased, car wash if the 
* customer desires.  
* There will be four options, Regular, Premium, Super,
* or None.  A car wash is $1.25 if purchased with $10.00 or more.  If it is 
* anything equal or below $9.99 then the car wash fee is $3.00.  
* Regular per gallon is $2.89
* Premium per gallon is $3.09
* Super per gallon is $3.39
* 
* 
* @author Christian Guerra
*/
package finalflight;


//The line below is preparing the system to ask the user for inputs
import java.util.Scanner;


public class ExxonCarServices {

public static void main(String[] args) {


       String gasType;
       String carWash;
       String gasPrice;
       String numGallons;
       double gasRegular = 2.89;
       double gasPremium = 3.09;
       double gasSuper = 3.39;
       double gasNone = 0;

       Scanner keyboard = new Scanner(System.in);

       System.out.print("Hello which type of gas would you like today?  "
               + "Please make the selection Regular, Premium, Super, or None" + " ");

       gasType = keyboard.nextLine();

       System.out.print("How many gallons would you like?" + " ");

       numGallons = keyboard.nextLine();

       System.out.print("Would you like to add a professional car wash cleaning today?"  
       + " " + "Please select Yes or No" + " ");

       carWash = keyboard.nextLine();



     if (gasType.equals("Regular")) { 

        gasRegular = Regular;

     } else if (gasType.equals ("Premium")) {

        gasPremium = Premium;
     } else if (gasType.equals("Super")) {

        gasSuper = Super;

     } else { 

        gasNone = 0;
 }



    if (numGallons * gasPrice <10) {

        carWash = 3;

    } else {

        carWash = 1.25;

    }
  }
}

Answer 1

编译器告诉你这段代码是正确的：

if (gasType.equals("Regular")) { 
  gasRegular = Regular;
} ...

但是从Java 7.0开始也可以使用switch语句编写：

switch (gasType) {
case "Regular":
  gasRegular = Regular;
  break;
case "Premium":
  gasPremium = Premium;
  break;
  ....
}

您遇到的错误＆＃34;无法将字符串转换为双倍＆＃34;可能是因为Double.parseDouble(someString)在String类型的变量中分配时遗失Double。

Answer 2

我怀疑编译器告诉你使用交换机案例，它可能只是建议它（对于多个项目，交换机案例几乎总是更好可读性选项。

要将String转换为Double，只需使用Double.parseDouble()。

在你的情况下，这看起来像这样：

double numGallonsDouble = Double.parseInt(numGallons);

如何正确使用if和else语句？

2 个答案: