java - switch 语句中的 .hasNextInt()

Question

我基本上是在尝试验证，以便您只能输入整数。这就是我目前所拥有的，但如果我输入字母，它会通过switch并把结果留为空白。

我想要它，这样如果输入的不是整数，它就会进入default开关。

任何帮助都会很棒。谢谢！

while(loop && kb.hasNextInt())
{   

    choice = kb.nextInt();
    switch(choice)
    {
    case 1 :
            language = "FRENCH";
            loop = false;
            break;
    case 2 : 
            language = "GERMAN";
            loop = false;
            break;
    case 3 :
            language = "SPANISH";
            loop = false;
            break;
    default :
            System.out.println("That is not a correct choice. Please try again!");
            break;

    }
}   

Answer 1

如果下一个输入不是整数，则.hasNextInt()返回false，因此循环将提前终止。

如果要允许文本输入并对其进行响应，则需要逐行读取文本而不是数字，并解析读取的行Integer.parseInt。如果无法解析该行，您将获得一个NumberFormatException. 你可以抓住它，并妥善处理。

    while (loop && scanner.hasNextLine()) {
        String line = scanner.nextLine();
        try {
            choice = Integer.parseInt(line);
        } catch (NumberFormatException e) {
            System.out.println("That is not an integer. Please try again!");
            continue;
        }

        switch (choice) {
            case 1:
                language = "FRENCH";
                loop = false;
                break;
            case 2:
                language = "GERMAN";
                loop = false;
                break;
            case 3:
                language = "SPANISH";
                loop = false;
                break;
            default:
                System.out.println("That is not a correct choice. Please try again!");
                break;
        }
    }

Answer 2

这是因为一封信会导致你while(loop && kb.hasNextInt())成为false。我建议在循环内添加一个if语句。hasNextInt()while

---

示例（使用while循环而不是if语句来真正尝试获取数字）：

while(loop)
{
    // validate int using while loop
    while(!kb.hasNextInt())                             
    {
        System.out.println("you must enter a number! ");
        kb.next();
    }

    choice = kb.nextInt();

    switch(choice)
    {
    case 1 :
            language = "FRENCH";
            loop = false;
            break;
    case 2 : 
            language = "GERMAN";
            loop = false;
            break;
    case 3 :
            language = "SPANISH";
            loop = false;
            break;
    }
}   

System.out.println("Thank You " + studentID + " you have been registered for " + language);

Answer 3

如果用户没有输入数字，则此代码甚至会在它开始之前爆炸，因为同时需要 kb.hasNextInt() 为真（有一个数字）才能运行。

我所做的是我通常将验证放在接收输入的位置：

int choice;
Boolean retry = null;
while(retry == null) {
    try{
        String input = scanner.nextLine();
        choice = Integer.parseInt(input);
        retry = false;
    }catch(NumberFormatException e){
        System.out.println("Please enter a number from 1 to 4.");
    }
}

switch(choice){
    case 1:
        // Do stuff
        break;
    case 2:
        // Do stuff
        break;
    case 3:
        // Do stuff
        break;
    case 4:
        // Do stuff
        break;
    default:
        System.out.println("Something went wrong!");
}