Arraylist程序

Question

我做了一個程序，但輸出顯示0。你能看到我做錯了嗎？ 我的評估指示詢問。

package snippet; 

import java.util.ArrayList; 

public class CO2FromElectricity { 
    // declaration of private instance variables 

    /** 
    * Default constructor to create an object from the CO2FromElectricity class. 
    */ 
    CO2FromElectricity() { 
    } 

    /** 
    * A mutator method which calculates the average annual electricity bill. 
    * 
    * @param monthlyBill 
    *   an ArrayList containing the monthly bills for home electricity use. 
    * @return the average monthly electricity bill. 
    */ 
    public double calcAverageBill(ArrayList<Double> monthlyBill) { 
     monthlyBill.add(279.41); 
     monthlyBill.add(238.03); 
     monthlyBill.add(248.64); 
     monthlyBill.add(258.73); 
     monthlyBill.add(395.48); 
     monthlyBill.add(419.91); 
     monthlyBill.add(431.15); 
     monthlyBill.add(407.56); 
     monthlyBill.add(417.14); 
     monthlyBill.add(308.35); 
     monthlyBill.add(337.91); 
     monthlyBill.add(320.77); 
     double sum = 0.0; 
     double monthlyTotal = 0.0; 

     for (int i = 0; i < monthlyBill.size(); i++) { 
      sum += monthlyBill.get(i); 
     } 

     return monthlyTotal/monthlyBill.size(); 
    } 

    /** 
    * A mutator method which calculates the average annual price of electricity. 
    * 
    * @param monthlyPrice 
    *   an ArrayList containing the monthly price of electricity per kilowatthour. 
    * @return the average monthly price of electricity. 
    */ 
    public double calcAveragePrice(ArrayList<Double> monthlyPrice) { 
     monthlyPrice.add(0.1117); 
     monthlyPrice.add(0.1107); 
     monthlyPrice.add(0.1110); 
     monthlyPrice.add(0.1113); 
     monthlyPrice.add(0.1135); 
     monthlyPrice.add(0.1138); 
     monthlyPrice.add(0.1217); 
     monthlyPrice.add(0.1215); 
     monthlyPrice.add(0.1216); 
     monthlyPrice.add(0.1228); 
     monthlyPrice.add(0.1209); 
     monthlyPrice.add(0.1192); 
     double sum = 0.0; 
     double monthlyTotal = 0.0; 

     for (int i = 0; i < monthlyPrice.size(); i++) { 
      sum += monthlyPrice.get(i); 
     } 
     return monthlyTotal/monthlyPrice.size(); 
    } 

    /** 
    * A mutator method which calculates the annual home CO2 emission from electricity. 
    * 
    * @param avgBill 
    *   the average monthly home electricity bill. 
    * @param avgPrice 
    *   the average monthly price of home electricity. 
    * @return the annual home CO2 emission from home electricity use. 
    */ 
    public double calcElectricityCO2(double avgBill, double avgPrice) { 
     return (avgBill/avgPrice) * 1.37 * 12; 
    } 
} 

輸出結果應該如下所示。請看我做錯了什麼。

Average Monthly Electricity Bill: 191.23 
Average Monthly Electricity Price: 0.11 
Annual CO2 Emissions from Electricity Usage: 28667.1 pounds 

Answer 1

每月總計始終是0.0。所以，你的函數返回0.0 /總和

Answer 2

輸出爲0，因爲你寫的：

double monthlyTotal = 0.0; 

我想你想

return sum/monthlyPrice.size();