java实现替换法计算候选码

替换法计算候选码，可以通过Java语言实现。具体步骤如下：

1. 将待加密的明文按照密钥表进行替换，得到密文。

2. 将密文按照密钥表进行逆替换，得到候选码。

Java代码实现如下：

public class SubstitutionCipher {
    private static final String KEY = "abcdefghijklmnopqrstuvwxyz";
    private static final String CIPHER = "zyxwvutsrqponmlkjihgfedcba";

    public static String encrypt(String plainText) {
        StringBuilder cipherText = new StringBuilder();
        for (int i = 0; i < plainText.length(); i++) {
            char c = plainText.charAt(i);
            int index = KEY.indexOf(c);
            if (index >= 0) {
                cipherText.append(CIPHER.charAt(index));
            } else {
                cipherText.append(c);
            }
        }
        return cipherText.toString();
    }

    public static String decrypt(String cipherText) {
        StringBuilder plainText = new StringBuilder();
        for (int i = 0; i < cipherText.length(); i++) {
            char c = cipherText.charAt(i);
            int index = CIPHER.indexOf(c);
            if (index >= 0) {
                plainText.append(KEY.charAt(index));
            } else {
                plainText.append(c);
            }
        }
        return plainText.toString();
    }

    public static void main(String[] args) {
        String plainText = "hello world";
        String cipherText = encrypt(plainText);
        String candidateCode = decrypt(cipherText);
        System.out.println("Plain Text: " + plainText);
        System.out.println("Cipher Text: " + cipherText);
        System.out.println("Candidate Code: " + candidateCode);
    }
}

在上述代码中，KEY为密钥表，CIPHER为密文表。encrypt函数对明文进行加密，decrypt函数对密文进行解密，最终得到候选码。