Java开发：如何在while循环中计算用户输入的每日支出总和

首先你现有代码存在一个明显错误：最后遍历支出的for循环中，你误用了局部变量newExpense作为遍历对象，它仅在while循环内部有效，外部无法访问，应该改为遍历全局的expenses ArrayList。

下面提供两种实现支出总和统计的简单方案：

方案一：添加支出时实时累加总和 #

在main方法开头定义一个总和变量，每次新增支出时直接累加对应金额，最后直接打印结果：

import java.util.Scanner;
import java.util.ArrayList;

public class DailyExpensesMain {

    public static void main(String[] args) {
        
        ArrayList<DailyExpenses> expenses = new ArrayList<DailyExpenses>();
        Scanner sc = new Scanner(System.in);
        boolean isRunning = true;
        // 初始化总支出为0
        double totalExpenses = 0.0;       
        
        System.out.println("Enter the date for which you want to record the expenses : ");
        String date = sc.nextLine();

        while(isRunning) {          
            System.out.println("Enter category: (quit to exit)");  
            String category = sc.nextLine();
            if(category.equalsIgnoreCase("quit")) {   
                 break;
            }

            System.out.println("Enter price: ");
            double price = sc.nextDouble(); 
            // 实时累加当前支出金额
            totalExpenses += price;
         
            sc.nextLine();              
                                    
            System.out.println("Enter details: ");
            String detail = sc.nextLine();
             
            DailyExpenses newExpense = new DailyExpenses(date, category, price, detail);    
            expenses.add(newExpense);                   
        }
            
        sc.close();
        // 遍历全局的expenses列表打印明细
        for(DailyExpenses u: expenses) {      
            System.out.println("Date: " + u.getDate() + " Category: " + u.getExpenseCategory() + " Price: " + u.getExpensePrice() + 
                " Detail: " + u.getExpenseDetail());
        }
        // 格式化打印总支出，保留两位小数
        System.out.printf("Total Expenses: %.2f%n", totalExpenses);
    }
}

方案二：循环结束后遍历列表计算总和 #

如果不想在添加阶段累加，也可以在程序末尾遍历expenses列表，逐个取出金额求和：

import java.util.Scanner;
import java.util.ArrayList;

public class DailyExpensesMain {

    public static void main(String[] args) {
        
        ArrayList<DailyExpenses> expenses = new ArrayList<DailyExpenses>();
        Scanner sc = new Scanner(System.in);
        boolean isRunning = true;       
        
        System.out.println("Enter the date for which you want to record the expenses : ");
        String date = sc.nextLine();

        while(isRunning) {          
            System.out.println("Enter category: (quit to exit)");  
            String category = sc.nextLine();
            if(category.equalsIgnoreCase("quit")) {   
                 break;
            }

            System.out.println("Enter price: ");
            double price = sc.nextDouble(); 
         
            sc.nextLine();              
                                    
            System.out.println("Enter details: ");
            String detail = sc.nextLine();
             
            DailyExpenses newExpense = new DailyExpenses(date, category, price, detail);    
            expenses.add(newExpense);                   
        }
            
        sc.close();
        // 计算总支出
        double totalExpenses = 0.0;
        for(DailyExpenses expense : expenses) {
            totalExpenses += expense.getExpensePrice();
            // 同时打印支出明细
            System.out.println("Date: " + expense.getDate() + " Category: " + expense.getExpenseCategory() + " Price: " + expense.getExpensePrice() + 
                " Detail: " + expense.getExpenseDetail());
        }
        // 格式化打印总支出
        System.out.printf("Total Expenses: %.2f%n", totalExpenses);
    }
}

两种方案都能满足需求，第一种仅需一次遍历效率更高，第二种逻辑更直观，可根据个人习惯选择。

内容的提问来源于stack exchange，提问作者ProgrammingTillSuccess