switch语句循环

Question

我正在尝试使用 switch 语句制作一个简单的菜单。但是我遇到了开关问题：

public class main {

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

         printMenu();

    }

    public static void printMenu() throws IOException{

    char selection = 0;
    do{
        System.out.println("Choose option: ");
        System.out.println("1. Option 1");
        System.out.println("2. Option 2");
        System.out.println("3. QUIT");
        System.out.println("\t\t\t");

        selection = (char)System.in.read();

        switch(selection){          
            case '1':

                    System.out.printf("opt1 chosen\n");

                break;

            case '2':

                System.out.printf("opt2 chosen\n");

                break;                  

            case '3':

                break;

            }
        }
        while(selection != '3'); 

    } 
}

For some reason, when selecting either one or two, the result is that print menu gets printed twice, like this:

程序输出：

Choose option: 
1. opt1.
2. opt2.
3. opt3.

1
opt1 chosen
Choose option: 
1. opt1.
2. opt2.
3. opt3.

Choose option: 
1. opt1.
2. opt2.
3. opt3.

问题是，是什么导致了这个问题？

Answer 1

当您按一个数字时 <Enter> 这是两个字符，而不是一个。即您正在输入

1\n

这是不可避免的，但您可以选择使用以不同方式处理此问题的 Scanner 以不同方式解析输入，或者您可以忽略它。（或者您可以预期用户必须在数字后键入 \n...

Answer 2

Peter Lawrey 是对的

我建议使用扫描仪 class :

public static void printMenu() throws IOException {
    Scanner scanner = new Scanner(System.in);
    int selection = 0;
    do{
        System.out.println("Choose option: ");
        System.out.println("1. Option 1");
        System.out.println("2. Option 2");
        System.out.println("3. QUIT");
        System.out.println("\t\t\t");

        selection = (char) scanner.nextInt();

        switch(selection){
        case 1:

            System.out.printf("opt1 chosen\n");

            break;

        case 2:

            System.out.printf("opt2 chosen\n");

            break;

        case 3:

            break;

        }
        scanner.nextLine();

    }
    while(selection != '3');

}

Answer 3

正如彼得指出的那样，问题的出现是因为您阅读 'selection' 输入的方式。您可以按如下方式更正功能：

public class main {

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

    public static void printMenu() throws IOException {
        char selection = '0';
        while (selection != '3') {
            if (selection != '\n') {
                System.out.println("Choose option: ");
                System.out.println("1. Option 1");
                System.out.println("2. Option 2");
                System.out.println("3. QUIT");
                System.out.println("\t\t\t");
            }
            selection = (char) System.in.read();
            switch (selection) {
                case '1':
                    System.out.printf("opt1 chosen\n");
                    break;
                case '2':
                    System.out.printf("opt2 chosen\n");
                    break;
                case '3':
                    break;
                default:
                    break;
            }
        }
    }
}

switch语句循环

Switch statement loops through

java

switch-statement