Consider using java.util.HexFormat

[rntrp]

Java 17 introduced java.util.HexFormat, which provides a concise (and most likely hardware‑optimized) API for encoding and decoding hexadecimal data. Altcha has two methods which utilize custom hex conversion logic:

altcha-lib-java/src/main/java/org/altcha/altcha/v2/Altcha.java

Lines 852 to 859 in 5ba5379

	static byte[] hexToBytes(String hex) {
	if (hex.length() % 2 != 0) throw new IllegalArgumentException("Hex string must have even length: " + hex);
	var bytes = new byte[hex.length() / 2];
	for (var i = 0; i < bytes.length; i++) {
	bytes[i] = (byte) Integer.parseInt(hex, i * 2, i * 2 + 2, 16);
	}
	return bytes;
	}

altcha-lib-java/src/main/java/org/altcha/altcha/v2/Altcha.java

Lines 861 to 869 in 5ba5379

	public static String bytesToHex(byte[] bytes) {
	var sb = new StringBuilder(2 * bytes.length);
	for (var b : bytes) {
	var hex = Integer.toHexString(0xff & b);
	if (hex.length() == 1) sb.append('0');
	sb.append(hex);
	}
	return sb.toString();
	}

These method bodies can be replaced with HexFormat.of() one-liners:

• return HexFormat.of().parseHex(hex); (maybe retain the custom exception logic for API compatibility)
• return HexFormat.of().formatHex(bytes)

Please consider using HexFormat.of() for better readability and performance.