Question

我在下面编写的程序是这样设计的，如果用户在提示时没有输入整数，程序将循环直到它们为止。这适用于初始检查，但第二次检查不起作用。代码如下：

import java.util.Scanner;
public class SafeInput
{
    public static void main(String[]args)
    {
        System.out.println("Please enter any integer. If you want to exit, enter -1>");
        Scanner scan = new Scanner(System.in);

        while(!scan.hasNextInt())
        {
            String garbage = scan.nextLine();
            System.out.println("You've entered garbage.");
        }

        int input = scan.nextInt();

        while(input != -1)
        {           
            System.out.println("\nYou entered: "+input);
            System.out.println("Please enter any integer. If you want to exit, enter -1>");

            while(!scan.hasNextInt())
            {
                System.out.println("You've entered garbage.");
                String garbage1 = scan.nextLine();
            }

            input = scan.nextInt();
        }

        System.out.println("-1 entered. Goodbye");
    }
}

以下是我执行程序时会发生的事情：

Please enter any integer. If you want to exit, enter -1>
this is a string
You've entered garbage.
1

You entered: 1
Please enter any integer. If you want to exit, enter -1>
2

You entered: 2
Please enter any integer. If you want to exit, enter -1>
this is also a string
You've entered garbage.
You've entered garbage.
string
You've entered garbage.
1

You entered: 1
Please enter any integer. If you want to exit, enter -1>
2

You entered: 2
Please enter any integer. If you want to exit, enter -1>
-1
-1 entered. Goodbye

为什么当我第二次检查整数失败时，程序输出：

You've entered garbage.
You've entered garbage.

而不是：

You've entered garbage.

谢谢！

Answer 1

第二个while循环应该如下更新。

        while(input != -1)
        {           
            System.out.println("\nYou entered: "+input);
            System.out.println("Please enter any integer. If you want to exit, enter -1>");
            scan.nextLine();
            while(!scan.hasNextInt())
            {
                System.out.println("You've entered garbage");
                String garbage1 = scan.nextLine();
            }

            input = scan.nextInt();
        }

Answer 2

使用next（）而不是nextLine（）。

     while(input != -1) {           
                System.out.println("\nYou entered: "+input);
                System.out.println("Please enter any integer. If you want to exit, enter -1>");

                while(!scan.hasNextInt())
                {
                    System.out.println("You've entered garbage.");
                    String garbage1 = scan.next();
                }

                input = scan.nextInt();
}

查看Scanner＃nextLine（）java doc，

此方法返回当前行的其余部分，不包括任何行最后的分隔符。该位置设置为下一个的开头线。

参见代码

调试：

while(input != -1)
    {           
        System.out.println("\nYou entered: "+input);
        System.out.println("Please enter any integer. If you want to exit, enter -1>");

        while(!scan.hasNextInt())
        {
            System.out.println("-- Second -- ");
            System.out.println("You've entered garbage.");
            String garbage1 = scan.nextLine();
            System.out.println(" garbage1 -> " + garbage1);
        }

        input = scan.nextInt();
    }

输出：

Please enter any integer. If you want to exit, enter -1>
q
-- First -- 
You've entered garbage.
1
 -- input --1

You entered: 1
Please enter any integer. If you want to exit, enter -1>
aa
-- Second -- 
You've entered garbage.
 garbage1 -> 
-- Second -- 
You've entered garbage.
 garbage1 -> aa
ab
-- Second -- 
You've entered garbage.
 garbage1 -> ab

为了好玩，我做了一些'增强'实施。它应该正常工作。

package example;

    import java.util.Scanner;

    public class SafeInput {

        public static void main(String[] args) {
            System.out.println("Please enter any integer. If you want to exit, enter -1>");
            try (Scanner scan = new Scanner(System.in)) {
                while (true) {
                    int input = 0;
                    String garbageOutput = null;
                    if (scan.hasNextInt() && ((input = scan.nextInt()) != -1)) {
                        System.out.println("\nYou entered: " + input);
                        System.out.println("Please enter any integer. If you want to exit, enter -1>");
                    } else if (scan.hasNextInt() && ((input = scan.nextInt()) == -1)) {
                        System.out.println("-1 entered. Goodbye");
                        break;
                    } else if (!scan.hasNextInt() && (scan.hasNextLine())
                            && (!(garbageOutput = scan.nextLine()).isEmpty())) {
                        System.out.println(String.format("You've entered garbage - [%s]", garbageOutput));
                    }
                }

            }

        }

    }

使用嵌套的While循环确保类型安全输入

2 个答案: