Question

所以当我尝试运行用户类时，我不断收到错误，说需要double，没有找到参数。我在第17,40,42,44,46和48行遇到错误。他们认为所有错误都表示需要双打。用简单的英语答案将不胜感激。

我的主要课程：

import java.util.Scanner;

public class ElectricityCalculatorUser {

    //Main method

    public static void main (String [] args) {

        ElectricityCalculator myCalculator = new ElectricityCalculator();  

        Scanner input = new Scanner (System.in);

        //Input the initial reading 
        double initialReading;
        System.out.print ("What is the inital reading on your electricity meter in kwH? ");
        initialReading = input.nextDouble ();

        //Input the final reading
        double finalReading;
        System.out.print ("What is the final reading on your electricity meter in kwH? ");
        finalReading = input.nextDouble ();

        //Input the number of days
        //between readings
        double numberOfDays;
        System.out.print ("How many days have passed between your initial and final reading? ");
        numberOfDays = input.nextDouble ();

        //Calculations
        double totalElectricityUsed = myCalculator.totalElectricityUsed();
        System.out.print ("Total electricity used = " + totalElectricityUsed);
        double costOfElectricity = myCalculator.costOfElectricity();
        System.out.print ("Cost of electricity = " + costOfElectricity);
        double standingCharge = myCalculator.standingCharge();
        System.out.print ("Standing charge = " + standingCharge);
        double costBeforeVAT = myCalculator.costBeforeVAT();
        System.out.print ("Cost before VAT is added = " + costBeforeVAT);
        double VAT = myCalculator.VAT();
        System.out.print ("Cost of VAT = " + VAT);
        double totalCost = myCalculator.totalCost();
        System.out.print ("Total cost = " + totalCost);
        }   
}

我的课程包括所有方法：

public class ElectricityCalculator {

    //Attributes
    private double initialReading;
    private double finalReading;
    private double numberOfDays;

    //Constructors
    public ElectricityCalculator (double ir, double fr, double nod) {
        initialReading = ir;
        finalReading = fr;
        numberOfDays = nod;
        }

    //Calculating total electricity used
    public double totalElectricityUsed () {
        return finalReading - initialReading;
        }

    //Calculating cost of electricity
    public double costOfElectricity () {
        return totalElectricityUsed * 0.175;
        }

    //Calculating standing charge
    public double standingCharge (double numberOfDays) {
        return numberOfDays * 0.25;
        }

    //Calculating cost before VAT is added
    public double costBeforeVAT (double costOfElectricity, double standingCharge) {
        return costOfElectricity + standingCharge;
        }

    //Cost of VAT
    public double VAT (double costBeforeVAT) {
        return costBeforeVAT * 0.05;
        }

    //Total cost of electricity used 
    //including VAT  
    public double totalCost (double costBeforeVAT, double VAT) {
        return costBeforeVAT + VAT;
        }

}

Answer 1

在java中，如果你不编写构造函数，将自动为你添加一个默认构造函数，这个构造函数将是public并且不带参数。

如下所示：

public ElectricityCalculator () {

}

但是，在定义任何构造函数时，将删除默认构造函数。因此，你班上唯一的构造函数是

public ElectricityCalculator (double ir, double fr, double nod) {
  initialReading = ir;
  finalReading = fr;
  numberOfDays = nod;
}

因此

ElectricityCalculator myCalculator = new ElectricityCalculator();

与任何构造函数都不匹配。

您可以在获取构造对象所需的值之后简单地创建实例

ElectricityCalculator myCalculator = new ElectricityCalculator(initialReading, finalReading, numberOfDays);

Answer 2

除了Sleiman Jneidi的回答，你正在调用函数，但不提供任何参数，因为方法定义要求：

double standingCharge = myCalculator.standingCharge();

需要更改为：

double standingCharge = myCalculator.standingCharge(10.0); //example for 10 days

代码的第42,44,46,48行中的同样问题

Answer 3

public ElectricityCalculator (double ir, double fr, double nod);
public double standingCharge (double numberOfDays);
public double costBeforeVAT (double costOfElectricity, double standingCharge);
public double VAT (double costBeforeVAT);
public double totalCost (double costBeforeVAT, double VAT);

构造函数和这些方法接受参数，但是你试图像调用它们一样调用它们。

对于构造函数，您只需移动此行

即可

ElectricityCalculator myCalculator = new ElectricityCalculator();

在您从用户那里获取输入后，您可以传入参数。

//                                          pass arguments here
//                                     v                v              v
... = new ElectricityCalculator( initialReading , finalReading , numberOfDays );

对于其他方法，您需要传递临时计算的结果。例如，VAT(...)需要costBeforeVAT我认为应该是costBeforeVAT(... , ...)的返回值。

double costBeforeVAT = ...;

//                           pass argument here
//                                   v
double VAT = myCalculator.VAT( costBeforeVAT );

请注意，在某些情况下，您可能不需要这些方法来获取某些参数，例如

public double standingCharge () {
    return numberOfDays * 0.25;
}

因为numberOfDays已经是班级ElectricityCalculator和

的成员

public double costBeforeVAT () {
    return costOfElectricity() + standingCharge();
}

因为可以直接调用这些方法，而不是要求传递结果。

相关："Passing Information to a Method or a Constructor"。

使用构造函数和获取另一个类的方法的问题

3 个答案: