开关中的中断标签

Question

已编辑：谢谢大家的帮助。我能够使用我在前几章中学到的技能和你的建议来让它工作。非常感谢！

我决定尝试通过创建一个简单的文字冒险来巩固我从 Java 中学到的东西：初学者指南。我即将开始涉及 classes 和方法的第 4 章。前三章处理了 if、for、while、do-while、switch、简单的键盘交互和 break/continue。

我计划在每一章之后回去编辑它以使用我学到的新技能。我几乎没有触及表面，我运行遇到了一个问题。

// A basic, but hopefully, lengthy text adventure.

class TextAdventure
{
    public static void main(String args[])
    throws java.io.IOException
    {
        System.out.println("\t\t BASIC TEXT ADVENTURE");


        // variables I need, attributes, classes, character name, player's choice, gold
        int str = 0, inte = 0, chr = 0, con = 0, dex = 0, gold;
        char charName, choice;

        System.out.println("Welcome player! You are about to embark upon a quest in the form of a text adventure.");
        System.out.println("You will make choices, fight monsters, and seek treasure. Come back victorious and you");
        System.out.println("could quite possibly buy your way into nobility!");
        System.out.println();


caseChoice: {       
        System.out.println("Please select your class:");
        System.out.println("1. Warrior");
        System.out.println("2. Mage");
        System.out.println("3. Rogue");
        System.out.println("4. Archer");

        choice = (char) System.in.read(); // Get players choice of class



        switch(choice)
        {
        case '1': 
            System.out.println("You have chosen the Warrior class!");
            System.out.println("You're stats are as followed:");
            System.out.println("Str: 16");
            System.out.println("Int: 11");
            System.out.println("Chr: 14");
            System.out.println("Con: 15");
            System.out.println("Dex: 9");
            str = 16; 
            inte = 11;
            chr = 14;
            con = 15;
            dex = 9;
            break;

        case '2':
            System.out.println("You have chosen the Mage class!");
            System.out.println("You're stats are as followed:");
            System.out.println("Str: 16");
            System.out.println("Int: 11");
            System.out.println("Chr: 14");
            System.out.println("Con: 15");
            System.out.println("Dex: 9");
            str = 9; 
            inte = 16;
            chr = 14;
            con = 15;
            dex = 11;
            break;

        case '3':
            System.out.println("You have chosen the Rogue class!");
            System.out.println("You're stats are as followed:");
            System.out.println("Str: 16");
            System.out.println("Int: 11");
            System.out.println("Chr: 14");
            System.out.println("Con: 15");
            System.out.println("Dex: 9");
            str = 15; 
            inte = 11;
            chr = 14;
            con = 9;
            dex = 16;
            break;

        case '4':
            System.out.println("You have chosen the Archer class!");
            System.out.println("You're stats are as followed:");
            System.out.println("Str: 16");
            System.out.println("Int: 11");
            System.out.println("Chr: 14");
            System.out.println("Con: 15");
            System.out.println("Dex: 9");
            str = 9; 
            inte = 11;
            chr = 14;
            con = 15;
            dex = 16;
            break;

            default:
                System.out.println("Not a valid choice, please enter a digit 1-4");
                break caseChoice;

        }

}

    }
}

开关中默认语句的目的是将代码流带回 class 选择。我没有收到编译错误或运行时间错误。当您 select 除了 1、2、3 或 4 之外的任何内容时。它会说“不是有效的选择，请输入数字 1-4”，就像它假设的那样，但程序结束了。

我不能在开关中使用这样的标签吗？还是它不起作用，因为它在技术上位于代码块之外？

Answer 1

我相信您在问题中所描述的是某种 goto 功能，而不是 Java 中标签的工作方式。

Java 不幸的是 支持标签。 this article from Oracle.

中对此进行了描述

所以，基本上你可以有带标签的循环，你可以使用关键字 continue、break 等等来控制循环的流程。

以下示例说明了如何使用带有 break 关键字的循环。当调用 break 时，它会终止带标签的语句，即 someLabel 之后的语句。它不会返回到指定标签的位置执行。

someLabel:
    for (i = 0; i < 100; i++) {
        for (j = 0; j < 100; j++) {
            if (i % 20 == 0) {
                break someLabel;
            }
        }
    }

continue 关键字以相同的方式处理标签。当您调用例如continue someLabel; 外循环将继续。

作为per this SO-question你也可以这样构造：

BlockSegment:
if (conditionIsTrue) {
    doSomeProcessing ();
    if (resultOfProcessingIsFalse()) break BlockSegment;
    otherwiseDoSomeMoreProcessing();
    // These lines get skipped if the break statement
    // above gets executed
}
// This is where you resume execution after the break
anotherStatement();

所以，如果您 break 到 switch 中的标签，基本上会发生什么，您将破坏整个语句（而不是跳转到语句的开头）。

您可以通过运行下面的程序进一步测试标签。如果您输入 "quit"，它会中断 while 循环，否则它只会中断开关。

public static void main(String... args) {
    programLoop:
    {
        while (true) {
            Scanner scanner = new Scanner(System.in);
            final String input = scanner.next();
            switch (input) {
                case "quit":
                    break programLoop; // breaks the while-loop
                default:
                    break; // break the switch
            }
            System.out.println("After the switch");
        }
    }
}

就个人而言，我 推荐使用标签 需要一个非常特殊的案例。我发现如果您重新排列代码以便不需要标签（例如通过将复杂的代码分解为更小的函数），代码会更容易理解。

Answer 2

您可以将代码包含在一个 while 循环中，如下所示以完成任务：

boolean validChoice=false;
while(!validChoice){
switch(choice){
    case 1:
        //logic
        validChoice=true;
    case 2:
        //logic
        validChoice=true;
    case 3:
        //logic
        validChoice=true;
    default:
        //print "invalid choice" and ask to reenter

}

Answer 3

我认为将标签与 break 语句结合使用会使您误入歧途。您可以简单地在开关中使用 break 语句，如果您想避免程序退出，只需使用 while。在更新的代码下方。

// A basic, but hopefully, lengthy text adventure.

import java.util.Scanner;

class TextAdventure
{
    public static void main(String args[])
    {
        System.out.println("\t\t BASIC TEXT ADVENTURE");


        // variables I need, attributes, classes, character name, player's choice, gold
        int str = 0, inte = 0, chr = 0, con = 0, dex = 0, gold;
        char charName, choice;

        System.out.println("Welcome player! You are about to embark upon a quest in the form of a text adventure.");
        System.out.println("You will make choices, fight monsters, and seek treasure. Come back victorious and you");
        System.out.println("could quite possibly buy your way into nobility!");
        System.out.println();

        boolean toEnd = false;
        while(!toEnd) {

            {
                System.out.println("Please select your class:");
                System.out.println("1. Warrior");
                System.out.println("2. Mage");
                System.out.println("3. Rogue");
                System.out.println("4. Archer");

                Scanner scanner = new Scanner(System.in);
                choice = scanner.next().charAt(0); // Get players choice of class

                toEnd = true;

                switch (choice) {
                    case '1':
                        System.out.println("You have chosen the Warrior class!");
                        System.out.println("You're stats are as followed:");
                        System.out.println("Str: 16");
                        System.out.println("Int: 11");
                        System.out.println("Chr: 14");
                        System.out.println("Con: 15");
                        System.out.println("Dex: 9");
                        str = 16;
                        inte = 11;
                        chr = 14;
                        con = 15;
                        dex = 9;

                        break;

                    case '2':
                        System.out.println("You have chosen the Mage class!");
                        System.out.println("You're stats are as followed:");
                        System.out.println("Str: 16");
                        System.out.println("Int: 11");
                        System.out.println("Chr: 14");
                        System.out.println("Con: 15");
                        System.out.println("Dex: 9");
                        str = 9;
                        inte = 16;
                        chr = 14;
                        con = 15;
                        dex = 11;

                        break;

                    case '3':
                        System.out.println("You have chosen the Rogue class!");
                        System.out.println("You're stats are as followed:");
                        System.out.println("Str: 16");
                        System.out.println("Int: 11");
                        System.out.println("Chr: 14");
                        System.out.println("Con: 15");
                        System.out.println("Dex: 9");
                        str = 15;
                        inte = 11;
                        chr = 14;
                        con = 9;
                        dex = 16;

                        break;

                    case '4':
                        System.out.println("You have chosen the Archer class!");
                        System.out.println("You're stats are as followed:");
                        System.out.println("Str: 16");
                        System.out.println("Int: 11");
                        System.out.println("Chr: 14");
                        System.out.println("Con: 15");
                        System.out.println("Dex: 9");
                        str = 9;
                        inte = 11;
                        chr = 14;
                        con = 15;
                        dex = 16;

                        break;

                    default:
                        System.out.println("Not a valid choice, please enter a digit 1-4");
                        toEnd = false;
                        break;// caseChoice;

                }

            }
        }

    }
}

允许在 Java 中使用标签，但不是好的做法。再想一想，避免使用像 IOException 这样不必要的异常，因为它不会被您的代码抛出。

开关中的中断标签

Break label in switch

java

switch-statement

java-8

break