Java嵌套switch语句输出错误求助（行星体重计算程序）

我正在完成CS101课程的作业，要求使用嵌套switch语句结合charAt方法判断用户选择的行星，计算并输出对应行星上的体重。但嵌套switch语句未按预期工作，出现以下错误：

• 选择Sun时，同时输出Sun和Uranus的体重结果；
• 选择Moon时，同时输出Moon和Uranus的体重结果；
• 选择Saturn时，同时输出Saturn、Mars和Uranus的体重结果；
• 选择Mercury时，同时输出Mercury和Uranus的体重结果；
• 选择Mars时，同时输出Mars和Mercury的体重结果；
• Uranus、Venus、Jupiter和Neptune的输出正常。

原程序代码：

import java.io.*;
import java.util.*;

public class Planets 
{

    public static void main(String[] args) throws IOException
    {
        Scanner in;
        in = new Scanner(System.in);
        boolean runAgain = true;
        System.out.println("Starting program...\n\n");
        
        double weight = getWeight();
        
        while(runAgain)
        {
            String planet = getPlanet();
            calculateWeight(weight, planet);
            
            System.out.println("\nWould you like to see your weight on a different planet? (Yes or No)");
            String input = in.nextLine();
            if(input.charAt(0)=='N' || input.charAt(0)=='n')
            {
                runAgain = false;
            }
        }
            

    } //end main
    
    public static double getWeight() throws IOException
    { //Prompts user to enter their weight
        Scanner in;
        in = new Scanner(System.in);
        System.out.println("Greetings! This program calculates how much you would weigh on other planets.\n\nPlease enter your weight (lbs.): ");
        double weight = in.nextDouble();
        return weight;
    }
    
    public static String getPlanet() throws IOException
    { //Prompts user to enter their planet
        Scanner in;
        in = new Scanner(System.in);
        System.out.println("\nChoose your planet:\nSun\nMoon\nSaturn\nMercury\nMars\nUranus\nVenus\nJupiter\nNeptune");
        String planet = in.nextLine();
        return planet;
    }
    
    public static void calculateWeight(double weight, String planet) throws IOException
    { //Outputs the weight of the given planet ; takes two parameters weight and planet
        switch(planet.charAt(0))
        {
            case 's':
            case 'S':
                switch(planet.charAt(1))
                {
                case 'u':
                case 'U':
                    System.out.println("Your weight on the Earth is " + weight + " lbs.\nYour weight on the Sun is " + 27.94*weight);
                    break;
                case 'a':
                case 'A':
                    System.out.println("Your weight on the Earth is " + weight + " lbs.\nYour weight on Saturn is " + .91*weight);
                    break;
                }
            case 'm':
            case 'M':
            switch(planet.charAt(1))
            {
                case 'o':
                case 'O':
                    System.out.println("Your weight on the Earth is " + weight + " lbs.\nYour weight on the Moon is " + .17*weight);
                    break;
                case 'e':
                case 'E':
                    System.out.println("Your weight on the Earth is " + weight + " lbs.\nYour weight on Mercury is " + .37*weight);
                    break;
                case 'a':
                case 'A':
                    System.out.println("Your weight on the Earth is " + weight + " lbs.\nYour weight on Mars is " + .38*weight);
                    break;
            } 
            case 'u':
            case 'U':
                System.out.println("Your weight on Earth is " + weight + " lbs.\nYour weight on Uranus is " + .88*weight);
                break;
            case 'v':
            case 'V':
                System.out.println("Your weight on Earth is " + weight + " lbs.\nYour weight on Venus is " + .90*weight);
                break;
            case 'j':
            case 'J':
                System.out.println("Your weight on Earth is " + weight + " lbs.\nYour weight on Jupiter is " + 2.64*weight);
                break;
            case 'n':
            case 'N':
                System.out.println("Your weight on Earth is " + weight + " lbs.\nYour weight on Neptune is " + 1.12*weight);
                break;
        } 
    } 
    
} //end class

错误原因分析 #

问题出在switch语句的贯穿（fall-through）特性：

• 外层switch中，case 's'/'S'执行完内部嵌套switch后，没有添加break语句，导致程序会继续执行后续的case 'm'/'M'代码块；
• 同样，case 'm'/'M'执行完内部switch后也没有break，会继续执行case 'u'/'U'的代码块；
• 这就解释了为什么选择Sun、Moon等行星时，会额外输出后续case对应的结果（比如Uranus）。

修复方案 #

在case 's'/'S'的嵌套switch结束后添加break，在case 'm'/'M'的嵌套switch结束后也添加break，阻止程序贯穿到下一个case。

修正后的代码 #

import java.io.*;
import java.util.*;

public class Planets 
{

    public static void main(String[] args) throws IOException
    {
        Scanner in;
        in = new Scanner(System.in);
        boolean runAgain = true;
        System.out.println("Starting program...\n\n");
        
        double weight = getWeight();
        
        while(runAgain)
        {
            String planet = getPlanet();
            calculateWeight(weight, planet);
            
            System.out.println("\nWould you like to see your weight on a different planet? (Yes or No)");
            String input = in.nextLine();
            if(input.charAt(0)=='N' || input.charAt(0)=='n')
            {
                runAgain = false;
            }
        }
            

    } //end main
    
    public static double getWeight() throws IOException
    { //Prompts user to enter their weight
        Scanner in;
        in = new Scanner(System.in);
        System.out.println("Greetings! This program calculates how much you would weigh on other planets.\n\nPlease enter your weight (lbs.): ");
        double weight = in.nextDouble();
        return weight;
    }
    
    public static String getPlanet() throws IOException
    { //Prompts user to enter their planet
        Scanner in;
        in = new Scanner(System.in);
        System.out.println("\nChoose your planet:\nSun\nMoon\nSaturn\nMercury\nMars\nUranus\nVenus\nJupiter\nNeptune");
        String planet = in.nextLine();
        return planet;
    }
    
    public static void calculateWeight(double weight, String planet) throws IOException
    { //Outputs the weight of the given planet ; takes two parameters weight and planet
        switch(planet.charAt(0))
        {
            case 's':
            case 'S':
                switch(planet.charAt(1))
                {
                case 'u':
                case 'U':
                    System.out.println("Your weight on the Earth is " + weight + " lbs.\nYour weight on the Sun is " + 27.94*weight);
                    break;
                case 'a':
                case 'A':
                    System.out.println("Your weight on the Earth is " + weight + " lbs.\nYour weight on Saturn is " + .91*weight);
                    break;
                }
                // 添加break阻止贯穿到case 'm'
                break;
            case 'm':
            case 'M':
                switch(planet.charAt(1))
                {
                    case 'o':
                    case 'O':
                        System.out.println("Your weight on the Earth is " + weight + " lbs.\nYour weight on the Moon is " + .17*weight);
                        break;
                    case 'e':
                    case 'E':
                        System.out.println("Your weight on the Earth is " + weight + " lbs.\nYour weight on Mercury is " + .37*weight);
                        break;
                    case 'a':
                    case 'A':
                        System.out.println("Your weight on the Earth is " + weight + " lbs.\nYour weight on Mars is " + .38*weight);
                        break;
                } 
                // 添加break阻止贯穿到case 'u'
                break;
            case 'u':
            case 'U':
                System.out.println("Your weight on Earth is " + weight + " lbs.\nYour weight on Uranus is " + .88*weight);
                break;
            case 'v':
            case 'V':
                System.out.println("Your weight on Earth is " + weight + " lbs.\nYour weight on Venus is " + .90*weight);
                break;
            case 'j':
            case 'J':
                System.out.println("Your weight on Earth is " + weight + " lbs.\nYour weight on Jupiter is " + 2.64*weight);
                break;
            case 'n':
            case 'N':
                System.out.println("Your weight on Earth is " + weight + " lbs.\nYour weight on Neptune is " + 1.12*weight);
                break;
        } 
    } 
    
} //end class

内容的提问来源于stack exchange，提问作者b-rad90