MonoAlphabetic.java

packagecom.thealgorithms.ciphers;

publicfinalclassMonoAlphabetic {

privateMonoAlphabetic() {
thrownewUnsupportedOperationException("Utility class");
 }

// Encryption method
publicstaticStringencrypt(Stringdata, Stringkey) {
if (!data.matches("[A-Z]+")) {
thrownewIllegalArgumentException("Input data contains invalid characters. Only uppercase A-Z are allowed.");
 }
StringBuildersb = newStringBuilder();

// Encrypt each character
for (charc : data.toCharArray()) {
intidx = charToPos(c); // Get the index of the character
sb.append(key.charAt(idx)); // Map to the corresponding character in the key
 }
returnsb.toString();
 }

// Decryption method
publicstaticStringdecrypt(Stringdata, Stringkey) {
StringBuildersb = newStringBuilder();

// Decrypt each character
for (charc : data.toCharArray()) {
intidx = key.indexOf(c); // Find the index of the character in the key
if (idx == -1) {
thrownewIllegalArgumentException("Input data contains invalid characters.");
 }
sb.append(posToChar(idx)); // Convert the index back to the original character
 }
returnsb.toString();
 }

// Helper method: Convert a character to its position in the alphabet
privatestaticintcharToPos(charc) {
returnc - 'A'; // Subtract 'A' to get position (0 for A, 1 for B, etc.)
 }

// Helper method: Convert a position in the alphabet to a character
privatestaticcharposToChar(intpos) {
return (char) (pos + 'A'); // Add 'A' to convert position back to character
 }
}