Signature

1. Assemble Parameters:
Assume all received data is represented as a set M. Exclude the `sign` field from set M, then sort all remaining fields actually sent by their names in ascending ASCII order (lexicographical order). Concatenate these parameters into a string str in URL key-value pair format as follows:
"key1=value1&key2=value2&key3=value3..."
Pay close attention to the following rules:
Parameter names must be sorted in ascending ASCII order (lexicographical order).
Parameter names are case-sensitive.
(Recommendation) If a request field is a string, remove any leading and trailing spaces before sending the request. Do not URL-encode values or silently omit empty values.

2. Append the Merchant Key:
Append the merchant key to the end of str, and perform an MD5 hash operation on str with the merchant key to obtain the sign value.

function sign($data, $secret){
	unset($data['sign']);
	ksort($data);
	$param = [];
	foreach ($data as $k => $v) {
			array_push($param, $k . '=' . $v);
	}
	$queryString = implode('&', $param);
	return md5($queryString . $secret);
}

		public static String sign(Map<String, Object> data, String secret) {
				SortedMap<String, Object> sortedData = new TreeMap<>(data);
				sortedData.remove("sign");

				StringBuilder param = new StringBuilder();
				for (Map.Entry<String, Object> entry : sortedData.entrySet()) {
						String key = entry.getKey();
						Object value = entry.getValue();
						String valueStr = value == null ? "" : value.toString();
						if (param.length() > 0) {
								param.append("&");
						}
						param.append(key).append("=").append(valueStr);
				}

				String queryString = param.toString() + secret;
				return md5(queryString);
		}

		private static String md5(String input) {
				try {
						java.security.MessageDigest md = java.security.MessageDigest.getInstance("MD5");
						byte[] array = md.digest(input.getBytes("UTF-8"));
						StringBuilder sb = new StringBuilder();
						for (byte b : array) {
								sb.append(String.format("%02x", b & 0xff));
						}
						return sb.toString();
				} catch (Exception e) {
						throw new RuntimeException("MD5 calculation error", e);
				}
		}