在交换机中打破标签 [英] Break label in switch

问题描述

编辑：
谢谢大家的帮助。我能够使用我在前几章中学到的技巧和你的建议来使用它。非常感谢你！

我决定尝试通过创建简单的文本冒险来巩固我从Java学到的东西：初学者指南。我即将开始涉及类和方法的第4章。前三章讨论了if，for，while，do-while，switch，简单的键盘交互以及break / continue。

I decided to try cementing the things I have learned from Java: A Beginner's Guide by creating a simple text adventure. I am about to start Chapter 4 which involves classes and methods. The first three chapters have dealt with, if, for, while, do-while, switch, simple keyboard interaction, and break/continue.

我计划在每章后回去并编辑它以使用我学到的新技能。我几乎没有触及表面，我遇到了一个问题。

I plan on going back after every chapter and editing it to use the new skills that I have learned. I have barely scratched the surface and I am running into a problem.

// 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;

        }

}

    }
}

交换机中默认语句的意图是将代码流返回到类选择中。我没有收到编译错误或运行时错误。当您选择除1,2,3或4之外的任何内容时。它表示不是一个有效的选择，请输入数字1-4，就像它想象的那样，但程序结束。

The intent of the default statement in the switch is to bring the flow of code back to the class choice. I DO NOT receive a compile error or run time error. When you select anything besides 1, 2, 3, or 4. It says " Not a valid choice, please enter a digit 1-4" like it's suppose to, but the program ends.

我不允许在交换机中使用这样的标签吗？或者它不起作用，因为它在技术上超出了代码块？

Am I not allowed to use a label like that in a switch? Or does it not work because it's technically outside the block of code?

推荐答案

我相信你在问题中描述的是某种 goto 功能，而不是Java中标签的工作原理。

I believe that what you are describing in your problem is some kind of goto functionality and that is not how labels in Java works.

Java 不幸支持标签。这在 Oracle的这篇文章中有描述。

Java unfortunately supports labels. This is described in this article from Oracle.

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

So, basically you can have loops with labels and you can use keyword continue, break and so on to control the flow of the loop.

以下示例说明如何使用 break 关键字。当调用 break 时，它会终止带标签的语句，即 someLabel 之后的语句。它不会返回执行指定标签的位置。

The following sample illustrates how to use the loop with the break keyword. When break is invoked it terminates the labeled statement i.e. the statement following someLabel. It does NOT go back to execute where the label was specified.

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。当你调用例如继续someLabel; 外部循环将继续。

The continue keyword handles labels the same way. When you invoke e.g. continue someLabel; the outer loop will be continued.

As 根据这个SO问题你也可以做这样的结构：

As per this SO-question you can also do constructs like this:

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，基本上如果你中断会发生什么对于开关中的标签，您将破坏整个语句（而不是跳转到语句的开头）。

So, basically what happens if you break to a label in your switch you will break that entire statement (and not jump to the beginning of the statement).

您可以通过运行以下程序进一步测试标签。如果你输入quit，它会打破while循环，否则它只会破坏开关。

You can test labels further by running the program below. It breaks the while-loop if you enter "quit", otherwise it simply breaks the switch.

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，需要一个非常特殊的情况，以便我建议使用标签。我发现如果您重新安排代码以便不需要标签（例如将复杂代码分解为较小的函数），代码就会更容易理解。

Personally, it would take a very special case in order for me to ever recommend using labels. I find that the code gets easier to follow if you instead rearrange your code so that labels are not needed (by e.g. break out complex code to smaller functions).

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