java - 尝试调用从另一个类定义变量的方法时出现编译错误

Question

这两个类在 javdocs 中有深入描述，但简而言之:它旨在计算电力使用产生的二氧化碳排放量。但是，我在尝试编译时返回错误，内容如下:

non-static method calcAverageBill(java.util.ArrayList<java.lang.Double>) cannot be referenced from a static context.

我不确定是什么问题，有人可以帮忙吗？

public class CO2fromElectricity
{
    private double monthBillAverage;                           
    private double monthPriceAverage;                          
    private double annualCO2emission;                         
    private double emissionFactor;                             
    private double months;                                    

    CO2fromElectricity() { }

    public double calcAverageBill(ArrayList<Double> monthlyBill)
    {
        monthBillAverage = 0;
        for(double billToken : monthlyBill)
        {
            monthBillAverage += billToken;
        }
        return monthBillAverage / monthlyBill.size();
    }


    public double calcAveragePrice(ArrayList<Double> monthlyPrice)
    {
        monthPriceAverage = 0;
        for(double billToken : monthlyPrice)
        {
            monthPriceAverage += billToken;
        }
        return monthPriceAverage / monthlyPrice.size();
    }


    public double calcElectricityCO2(double avgBill, double avgPrice)
    {
        emissionFactor = 1.37;
        months = 12;
        return annualCO2emission = avgBill / avgPrice * emissionFactor * months;
    }
}


public class CO2fromElectricityTester
{
    public static void main(String[] args)
    {

        ArrayList<Double> monthlyBill = new ArrayList<Double>();                                                
        ArrayList<Double> monthlyPrice = new ArrayList<Double>();                                               
        double averageMonthlyBill,                                                                               
               averageMonthlyPrice,                                                                             
               annualCO2emission;                                                                               
        double monthlyBillToken1[] = {192.14, 210.42, 231.25, 186.13},                                           
               monthlyPriceToken1[] = {.07, .06, .08, .06};                                                      

        for(int index = 0; index < monthlyBillToken1.length; index++)                                            
        {
            monthlyBill.add(monthlyBillToken1[index]);                                                           
            monthlyPrice.add(monthlyPriceToken1[index]);                                                        
        }

        ArrayList<CO2FootprintV1> electricCO2outputput = new ArrayList<CO2FootprintV1>();


        averageMonthlyBill = CO2fromElectricity.calcAverageBill(monthlyBill);                                    
        averageMonthlyPrice = CO2fromElectricity.calcAveragePrice(monthlyPrice);                                 
        annualCO2emission = CO2fromElectricity.calcElectricityCO2(averageMonthlyBill, averageMonthlyPrice);      


        System.out.println("Average Monthly Electricity Bill: " + averageMonthlyBill);                           
        System.out.println("Average Monthly Electricity Prince: " + averageMonthlyPrice);                        
        System.out.println("Annual CO2 Emissions from Electricity Useage: " + annualCO2emission + "pounds");     
    }
}

Answer 1

问题是 calcAverageBill 是一种对类型为 CO2fromElectricity 的对象进行操作的方法，但您调用它时就好像它是静态方法一样。也就是说，您必须在 CO2fromElectricity 实例上调用它，例如:

CO2fromElectricity inst = new CO2fromElectricity();
averageMonthlyBill = inst.calcAverageBill(monthlyBill);

或者，您可以通过在方法定义中添加 static 关键字来使方法静态化:

public static double calcAverageBill(ArrayList<Double> monthlyBill)

快速浏览一下 CO2fromElectricity，您可能希望定义的字段(例如 monthBillAverage)是每个方法中的局部变量，而不是作为字段或静态属性，因为您的类不会从实例范围或所有执行范围内定义这些受益(事实上，您可能会遇到当前定义它们的方式的一些麻烦)。