java - Java 中正确的用户输入

Question

正在解决一个问题，基本代码如下所示，从用户处获取 4 个可能的输入，并根据输入的一个生成响应。不过，我需要添加一个测试来验证是否仅输入了 4 个可能答案中的 1 个。

我知道如何比较 4 个输入中的 2 个，但不能同时比较所有 4 个输入，有人能给我一个想法吗？我更愿意自己找出实际的代码，但指向正确方向的指针会很有帮助。

因此，为了澄清如何 - 如果输入是除“帐单、通函、明信片或信件”以外的任何内容，则产生错误消息 X

    System.out.println("What type of Letter has been received?");
    System.out.println("Bill, Circular, Postcard or Letter");
    String Letter = kybd.nextLine();

        {
            if (Letter.equalsIgnoreCase("Bill"))  
            {
                System.out.println("Bills must be paid");
            }
            else if (Letter.equalsIgnoreCase("Circular"))
            {
                System.out.println("Circulars are thrown away");
            } 
            else if (Letter.equalsIgnoreCase("Postcard"))
            {
                System.out.println("Postcards are put on the wall");
            } 
            else if (Letter.equalsIgnoreCase("Letter"))
            {
               System.out.println("Personal letters are read and have replies written for them");
            }
        }

Answer 1

            if (Letter.equalsIgnoreCase("Bill"))  
            {
                System.out.println("Bills must be paid");
            }
            else if (Letter.equalsIgnoreCase("Circular"))
            {
                System.out.println("Circulars are thrown away");
            } 
            else if (Letter.equalsIgnoreCase("Postcard"))
            {
                System.out.println("Postcards are put on the wall");
            } 
            else if (Letter.equalsIgnoreCase("Letter"))
            {
               System.out.println("Personal letters are read and have replies written for them");
            }else{
            System.out.println("ERROR");
            }