java: double 不能被取消引用

Question

这是作业：

实现一个类 Car。一个 Car 对象应该有三个（私有）实例变量，一个用于燃油效率（代表每加仑英里数），一个用于燃油水平（代表加仑），以及一个用作里程表的变量（代表英里数）。汽车的燃油效率应在 Car 构造函数中指定为参数，并且构造函数应将燃油油位和里程表设置为零。还应该有第二个 Car 构造函数，它具有初始化燃油效率和燃油水平的两个参数（并且构造函数应该将里程表设置为零）。应该有 getFuelEfficiency()、getOdometer() 和 getFuelLevel() 方法。应该有一个方法 addFuel(double gallons) 将指定量添加到燃料水平，然后返回对当前 Car 对象的引用。应该有一个方法 drive(double miles) 来模拟驾驶汽车指定的距离。 drive() 方法应该通过使用的燃料量来调整燃料水平，通过行驶的英里数来调整里程表，并且它应该返回行驶的英里数，如果有，则可能小于指定的英里数没有足够的燃料。请注意，没有 setFuelEfficiency()、setOdometer() 和 setFuelLevel() 方法。燃油效率领域是不可变的；一旦由构造函数设置，就无法更改。里程表的值只能通过驾驶汽车来改变，就像在真正的汽车中一样。只能通过驾驶汽车或添加燃料来更改燃料水平的值。这里总结一下 Car 类的构造函数和方法（公共接口）。

这是我的代码：

public class Car {
    private double fuelEfficiency;
    private double fuelLevel;
    private double odometer;

    public Car(double fuelEfficiency) {
        this.fuelLevel = 0.0;
        this.odometer = 0.0;
    }

    public Car(double fuelEfficiency, double fuelLevel) {
        this.fuelLevel = fuelLevel;
        this.fuelEfficiency = fuelEfficiency;
        this.odometer = 0.0;
    }

    public double getFuelEfficiency() {
        return this.fuelEfficiency;
    }

    public double getFuelLevel() {
        return this.fuelLevel;
    }

    public double getOdometer() {
        return this.odometer;
    }

    public double addFuel(double gallons) {
        this.fuelLevel = this.fuelLevel + gallons;
        return this.fuelLevel;
    }

    public double drive(double miles) {

        double distance = (this.fuelLevel * this.fuelEfficiency);

        if (distance > miles) {
            distance = miles;
        }

        this.fuelLevel = (fuelLevel - (distance / fuelEfficiency));
        this.odometer = odometer + distance;
        return distance;
    }
    public String toString(){
        return "MPG: " + this.fuelEfficiency + "miles: " + this.odometer + "fuel: " + this.fuelLevel;
    }
}

这是我应该用来测试我的代码并且不应该更改的 PreTestDrive：

public class PreTestDrive
{
   public static void main( String[] args )
   {
      Car car1 = new Car(25.5);

      double fuelEfficiency = car1.getFuelEfficiency();
      double fuelLevel = car1.getFuelLevel();
      double odometer = car1.getOdometer();

      if (fuelLevel != 0.0 || fuelEfficiency != 25.5 || odometer != 0.0)
      {
         System.out.println("1. There is a problem with your constructors.");
         System.exit(0);
      }

      Car car2 = new Car(10.0, 5.0);

      fuelEfficiency = car2.getFuelEfficiency();
      fuelLevel = car2.getFuelLevel();
      odometer = car2.getOdometer();

      if (fuelLevel != 5.0 || fuelEfficiency != 10.0 || odometer != 0.0)
      {
         System.out.println("2. There is a problem with your constructors.");
         System.exit(0);
      }

      String s1 = car1.toString();

      if (! s1.equals("Car: mpg = 25.5, miles = 0.0, fuel = 0.0.") )
      {
         System.out.println("3. There is a problem with your toString() method.");
         System.exit(0);
      }

      String s2 = car2.toString();

      if (! s2.equals("Car: mpg = 10.0, miles = 0.0, fuel = 5.0.") )
      {
         System.out.println("4. There is a problem with your toString() method.");
         System.exit(0);
      }

      double newFuelLevel = car1.addFuel(12.5).getFuelLevel();

      fuelEfficiency = car1.getFuelEfficiency();
      odometer = car1.getOdometer();

      if(newFuelLevel != 12.5 || fuelEfficiency != 25.5 || odometer != 0.0)
      {
         System.out.println("5. There is a problem with your addFuel method.");
         System.exit(0);
      }

      double distance = car1.drive(76.5);

      if (distance != 76.5 || ! car1.toString().equals("Car: mpg = 25.5, miles = 76.5, fuel = 9.5."))
      {
         System.out.println("6. There is a problem with your drive method.");
         System.exit(0);
      }

      distance = car1.drive(1000);

      if (distance != 242.25 || ! car1.toString().equals("Car: mpg = 25.5, miles = 318.75, fuel = 0.0."))
      {
         System.out.println("7. There is a problem with your drive method.");
         System.exit(0);
      }

      distance = car1.drive(1);

      if (distance != 0.0 || ! car1.toString().equals("Car: mpg = 25.5, miles = 318.75, fuel = 0.0."))
      {
         System.out.println("8. There is a problem with your drive method.");
         System.exit(0);
      }

      System.out.println("Your Car class passed all of these tests.");
   }
}

当我运行 PreTestDrive 时，我收到错误 java: double cannot be dereferenced。有什么想法吗？

Answer 1

应该有一个方法 addFuel(double gallons) 将指定量添加到燃料水平，然后返回对当前 Car 对象的引用。

您的 addFuel 方法不返回对当前 Car 对象的引用。

看起来像这样：

public Car addFuel(double gallons) {
    this.fuelLevel += gallons;
    return this;
}

另外，您没有在 Car(double fuelEfficiency) 构造函数中设置 fuelEfficiency 字段。