Java凯撒密码程序使用sc.nextLine()无法输入含空格字符串的问题

问题原因 #

使用sc.nextInt()读取旋转位数后，输入缓冲区中会残留一个换行符。后续调用sc.nextLine()时，会直接读取这个换行符，得到空字符串，导致加密/解密的内容为空。

解决方案 #

有两种可行的修复方式：

方式一：消耗遗留的换行符 #

在int r = sc.nextInt();之后，添加一行sc.nextLine();，专门读取并丢弃输入缓冲区里的换行符，这样后续的sc.nextLine()就能正常读取用户输入的消息了。

方式二：统一用nextLine()读取输入 #

先通过sc.nextLine()读取包含数字的整行内容，再将其转换为整数，从根源上避免换行符残留问题。

修改后的代码示例 #

方式一的修改版 #

import java.util.*;

public class CaesarCipher
{
    protected char[] encoder = new char[26];
    protected char[] decoder = new char[26];

    /** Constructor that initializes the encryption and decryption arrays */
    public CaesarCipher(int rotation)
    {
        for(int k=0;k<26;k++)
        {
            encoder[k] = (char)('A' + (k + rotation)%26);
            decoder[k] = (char)('A' + (k - rotation+26)%26);
        }
    }

    /** Returns String representing encrypted message. */
    public String encrypt(String message)
    {
        return transform(message,encoder);
    }

    public String decrypt(String Secret)
    {
        return transform(Secret, decoder);
    }

    public String transform(String Original,char[] code)
    {
        char msg[] = Original.toCharArray();

        for(int k=0;k<msg.length;k++)
        {
            if(Character.isUpperCase(msg[k]))
            {
                int j = msg[k] - 'A';
                msg[k] = code[j];
            }
        }
        return new String(msg);
    }

    public static void main(String args[])
    {
        Scanner sc = new Scanner(System.in);

        System.out.println("Enter any number between 1 and 26 : ");
        int r = sc.nextInt();
        sc.nextLine(); // 新增：消耗遗留的换行符
        System.out.println();

        CaesarCipher cipher = new CaesarCipher(r);

        System.out.println("Encryption code : " + new String(cipher.encoder));
        System.out.println("Decryption code : " + new String(cipher.decoder));
        System.out.println();

        System.out.println("Enter a Message to Encrypt : ");
        String message = sc.nextLine();

        String coded = cipher.encrypt(message.toUpperCase());
        System.out.println("Encrypted Message : " + coded);
        System.out.println();

        System.out.println("Decrypted Message : " + cipher.decrypt(coded));
        sc.close();
    }
}

方式二的修改版 #

import java.util.*;

public class CaesarCipher
{
    protected char[] encoder = new char[26];
    protected char[] decoder = new char[26];

    /** Constructor that initializes the encryption and decryption arrays */
    public CaesarCipher(int rotation)
    {
        for(int k=0;k<26;k++)
        {
            encoder[k] = (char)('A' + (k + rotation)%26);
            decoder[k] = (char)('A' + (k - rotation+26)%26);
        }
    }

    /** Returns String representing encrypted message. */
    public String encrypt(String message)
    {
        return transform(message,encoder);
    }

    public String decrypt(String Secret)
    {
        return transform(Secret, decoder);
    }

    public String transform(String Original,char[] code)
    {
        char msg[] = Original.toCharArray();

        for(int k=0;k<msg.length;k++)
        {
            if(Character.isUpperCase(msg[k]))
            {
                int j = msg[k] - 'A';
                msg[k] = code[j];
            }
        }
        return new String(msg);
    }

    public static void main(String args[])
    {
        Scanner sc = new Scanner(System.in);

        System.out.println("Enter any number between 1 and 26 : ");
        // 先读取整行再转整数
        int r = Integer.parseInt(sc.nextLine());
        System.out.println();

        CaesarCipher cipher = new CaesarCipher(r);

        System.out.println("Encryption code : " + new String(cipher.encoder));
        System.out.println("Decryption code : " + new String(cipher.decoder));
        System.out.println();

        System.out.println("Enter a Message to Encrypt : ");
        String message = sc.nextLine();

        String coded = cipher.encrypt(message.toUpperCase());
        System.out.println("Encrypted Message : " + coded);
        System.out.println();

        System.out.println("Decrypted Message : " + cipher.decrypt(coded));
        sc.close();
    }
}

内容的提问来源于stack exchange，提问作者Mikhil Angre