Java如何在switch语句下打破while循环？

Posted 2023-02-16

【中文标题】Java如何在switch语句下打破while循环？

【英文标题】：Java How can I break a while loop under a switch statement?

【发布时间】：2014-05-14 10:35:22

【问题描述】：

我有一个作业要实现一个简单的测试应用程序，下面是我当前的代码：

import java.util.*;

public class Test

private static int typing;

public static void main(String argv[])
    Scanner sc = new Scanner(System.in);
    System.out.println("Testing starts");
    while(sc.hasNextInt())
        typing = sc.nextInt();
        switch(typing)
            case 0:
              break; //Here I want to break the while loop
            case 1:
              System.out.println("You choosed 1");
              break;
            case 2:
              System.out.println("You choosed 2");
              break;
            default:
              System.out.println("No such choice");
        
    
      System.out.println("Test is done");
    

我现在要做的是，当按下0时，表示用户想退出测试，然后我打破while loop并打印Test is done，但它不会那样工作，我知道原因可能是"break"打破了switch，我怎么能让它打破while loop呢？

【参考方案1】：

如何终止内部菜单？

示例代码：

import java.util.Scanner;

public class Example 

    public static void main(String[] args) 
        Scanner input = new Scanner(System.in); //used to get input
        int option1, option2 = 0;
        boolean loop_terminate = true; //flag used to terminate inner while loop

        //Main Menu
        while (true) 
            //Main Menu options
            System.out.println("1.Option 1");
            System.out.println("2.Option 2");
            System.out.println("3.Option 3");
            System.out.println("4.Option 4");
            System.out.println("5.Exit main menu");

            System.out.print("Please enter your choice : ");
            option1 = input.nextInt();

            switch (option1) 

                case 1:
                       //do something here    
                    break;
                case 2:
                       //do something here 
                    break;
                case 3:

                    while (loop_terminate) 
                        //Inner menu options
                        System.out.println("1.Inner Menu option 1");
                        System.out.println("2.Inner Menu option 2");
                        System.out.println("3.Inner Menu option 3");
                        System.out.println("4.Return to Main Menu");

                        System.out.print("Please enter your choice : ");
                        option2 = input.nextInt();
                        switch (option2) 

                            case 1:
                                break;
                            case 2:
                                break;
                            case 3:
                                break;
                            case 4:
                                loop_terminate = false; //this will terminate inner menu
                                break;
                            default:
                                System.out.println("Invalid option");
                                break;
                        
                    
                    break; //never forget to add this break statement
                case 4:
                      break;
                case 5:
                    return; //terminate outer menu

                default:
                    System.out.println("Invalid option");
            
        

     

【参考方案2】：

将 while 放入函数中，当您按下 0 而不是 break 时，只需 return。例如：

    import java.util.*;

public class Test

private static int typing;

public static void main(String argv[])
    Scanner sc = new Scanner(System.in);
    func(sc);
      System.out.println("Test is done");
    


public static void func(Scanner sc) 


    System.out.println("Testing starts");
    while(sc.hasNextInt())
        typing = sc.nextInt();
        switch(typing)
            case 0:
              return; //Here I want to break the while loop
            case 1:
              System.out.println("You choosed 1");
              break;
            case 2:
              System.out.println("You choosed 2");
              break;
            default:
              System.out.println("No such choice");
        
    

【参考方案3】：

你可以label你的while循环，breaklabeled loop，应该是这样的：

loop: while(sc.hasNextInt())
    typing = sc.nextInt();
    switch(typing)
        case 0:
          break loop; 
        case 1:
          System.out.println("You choosed 1");
          break;
        case 2:
          System.out.println("You choosed 2");
          break;
        default:
          System.out.println("No such choice");
    

label 可以是任何你想要的词，例如"loop1"。

【讨论】：

"loop" 是一个糟糕/无聊的标签选择。更有趣的选择包括“up”、“out”、“stuff”、“theBank”和“dance”。

玩笑不谈，给循环一个信息量更大的标签（在你的情况下，可能是scanner），让中断看起来像break scanner（停止扫描）。

@noamtm 这并不明显，所以感谢您指出这一点。

【参考方案4】：

您需要一个布尔变量，例如shouldBreak.

    boolean shouldBreak = false;
    switch(typing)
        case 0:
          shouldBreak = true;
          break; //Here I want to break the while loop
        case 1:
          System.out.println("You choosed 1");
          break;
        case 2:
          System.out.println("You choosed 2");
          break;
        default:
          System.out.println("No such choice");
    
    if (shouldBreak) break;