使用用户输入的子字符串切换语句

Question

目标：我正在尝试根据用户的输入输出一条消息。

情况：基本上用户会输入几个字符，例如 5r（5 Red）或 8y（8 Yellow）。预期的符号是先有数字，后有字符，但用户可能不遵守规则。

我使用了 switch 语句（练习的一部分），但输出没有给我预期的结果

代码：

        String input = reader.nextLine().toLowerCase();
        String firstChar = input.substring(0,1);
        String secondChar = input.substring(1);
        String answer = "";

        switch(firstChar)
        {
            case "0":
            {
                switch(secondChar)
                {
                    case "b":
                        answer = "Zero Blue";
                    case "r":
                        answer = "Zero Red";
                    case "g":
                        answer = "Zero Green";
                    case "y":
                        answer = "Zero Yellow";
                }
            }
            case "1":
            {
                switch(secondChar)
                {
                    case "b":
                        answer = "One Blue";
                    case "r":
                        answer = "One Red";
                    case "g":
                        answer = "One Green";
                    case "y":
                        answer = "One Yellow";
                }
            }
           ... other case statements....
           default:
              answer = "not valid";

当用户写入 0b 时，我的输出应该是“零蓝”，但目前它显示的是“无效”。为什么？

Answer 1

正如@markspace 提到的，您缺少一个 break 语句。以下是 Java documentation 关于 switch 语句的简短摘录：

每个 break 语句都会终止封闭的 switch 语句。控制流继续 switch 块之后的第一条语句。 break 语句是必要的，因为没有它们，switch 块中的语句就会失败。

我还建议使用专用方法和for 循环以便于调试。

下面的代码会产生你想要的结果：

public static String getAnswer() {

        Scanner reader = new Scanner(System.in);
        String input = reader.nextLine().toLowerCase();
        String firstChar = input.substring(0, 1);
        String secondChar = input.substring(1);

        String[] chars = new String[] { "b", "r", "g", "y" };
        String[] answers1 = new String[] { "Zero Blue", "Zero Red", "Zero Green", "Zero Yellow" };
        String[] answers2 = new String[] { "One Blue", "One Red", "One Green", "One Yellow" };

        if ((firstChar.equals("0"))) {
            for (int i = 0; i < chars.length; i++) {
                if (secondChar.equals(chars[i])) {
                    return answers1[i];
                }
            }
        }
        else if ((firstChar.equals("1"))) {
            for (int i = 0; i < chars.length; i++) {
                if (secondChar.equals(chars[i])) {
                    return answers2[i];
                }
            }
        }
        return "not valid";
    }

    public static void main(String[] args) throws IOException {

        System.out.println(getAnswer());
    }

编辑：除了上面的解决方案，我想提供一个更复杂的解决方案，它涉及使用枚举器。它应该让你的代码感觉更干净、更切题，并且更模块化：

public enum Answer {

    BLUE('b',"Blue"), RED('r',"Red"),
    GREEN('g',"Green"), YELLOW('y',"Yellow");

    private static final String[] prefix = new String[] {"Zero", "One"};

    char letter;
    String name;

    Answer(char letter, String name) {
        this.letter = letter;
        this.name = name;
    }

    public static String getForInput(char input, int index) {

        if (index > prefix.length)
            return "value out of range " + index;

        for (Answer answer : Answer.values()) {
            if (input == answer.letter)
                return prefix[index] + " " + answer.name;
        }
        return "unable to find answer for input " + input;
    }
}

public static String getAnswer() {

    System.out.println("Enter your input: ");
    Scanner reader = new Scanner(System.in);
    String input = reader.nextLine().toLowerCase();
    int number = Integer.valueOf(input.substring(0, 1));
    char letter = input.substring(1).charAt(0);

    return Answer.getForInput(letter, number);

}

Answer 2

我建议根本不使用 swich case，使用哈希映射：

package mypackage;

import java.util.*;

public class MyComputingClass {

    private static Map<Character, String> firstCharMapping;
    private static Map<Character, String> secondCharMapping;

    public static void main(String[] args) {
        initCharMappers();

        System.out.println("Mapping of 0Y : " + mapStuff("0Y"));
        System.out.println("Mapping of 0y : " + mapStuff("0y"));
        System.out.println("Mapping of 0YJ : " + mapStuff("0YJ"));
        System.out.println("Mapping of 2B : " + mapStuff("2B"));
        System.out.println("Mapping of eB : " + mapStuff("eB"));
    }

    private static void initCharMappers() {
        firstCharMapping = new HashMap<>();
        secondCharMapping = new TreeMap<>(Comparator.comparing(Character::toLowerCase));

        firstCharMapping.put('0', "Zero");
        firstCharMapping.put('1', "One");
        firstCharMapping.put('2', "Two");

        secondCharMapping.put('Y', "Yellow");
        secondCharMapping.put('B', "Blue");
        secondCharMapping.put('G', "Green");
    }

    private static String mapStuff(String str) {
        String INVALID_INPUT = "Invalid input";
        if (!str.matches("\\d[a-zA-Z]")) {
            return INVALID_INPUT;
        }

        Character firstChar = str.toCharArray()[0];
        Character secondChar = str.toCharArray()[1];

        if (!firstCharMapping.containsKey(firstChar) || !secondCharMapping.containsKey(secondChar)) {
            return INVALID_INPUT;
        }

        return firstCharMapping.get(firstChar) + " " + secondCharMapping.get(secondChar);
    }
}

输出：

Mapping of 0Y : Zero Yellow
Mapping of 0y : Zero Yellow
Mapping of 0YJ : Invalid input
Mapping of 2B : Two Blue
Mapping of eB : Invalid input