Update RFC reference

Author: ethauvin

README.md

@@ -12,7 +12,7 @@
 [![GitHub CI](https://github.com/ethauvin/httpstatus/actions/workflows/bld.yml/badge.svg)](https://github.com/ethauvin/httpstatus/actions/workflows/bld.yml)
 [![CircleCI](https://circleci.com/gh/ethauvin/HttpStatus/tree/master.svg?style=shield)](https://circleci.com/gh/ethauvin/HttpStatus/tree/master)
 
-A simple library to search for and display information about [HTTP status codes](http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html).
+A simple library to search for and display information about [HTTP status codes](https://datatracker.ietf.org/doc/html/rfc7231#section-6.1).
 
 ## Table of Contents
 
@@ -70,7 +70,7 @@ or
 <%= Reasons.getReasonPhrase(pageContext.getErrorData().getStatusCode()) %>
 ```
 
-would display on a [501 status code](https://www.rfc-editor.org/rfc/rfc9110.html#name-501-not-implemented):
+would display on a [501 status code](https://datatracker.ietf.org/doc/html/rfc7231#section-6.6.2):
 
 ```console
 Not Implemented
@@ -230,7 +230,7 @@ These methods are also available as [static methods](https://ethauvin.github.io/
 
 ### Status Code Class
 
-The [StatusCodeClass](https://ethauvin.github.io/HttpStatus/net/thauvin/erik/httpstatus/StatusCodeClass.html) can be used to retrieve all status codes in a specific [class](https://www.rfc-editor.org/rfc/rfc9110.html#name-status-codes):
+The [StatusCodeClass](https://ethauvin.github.io/HttpStatus/net/thauvin/erik/httpstatus/StatusCodeClass.html) can be used to retrieve all status codes in a specific [class](https://datatracker.ietf.org/doc/html/rfc7231#section-6):
 
 ```java
 var reasons = Reasons.getReasonClass(StatusCodeClass.SERVER_ERROR); // 5xx
@@ -386,7 +386,7 @@ $ java -jar httpstatus-1.1.1.jar
 ...
 ```
 
-You can also print status codes by [classes](https://www.rfc-editor.org/rfc/rfc9110.html#name-status-codes):
+You can also print status codes by [classes](https://datatracker.ietf.org/doc/html/rfc7231#section-6.1):
 
 ```console
 $ java -jar httpstatus-1.1.1.jar 2xx

src/main/java/net/thauvin/erik/httpstatus/StatusCodeClass.java

@@ -55,14 +55,38 @@
  * @since 2.0.0
  */
 public enum StatusCodeClass {
-    INFORMATIONAL(1), SUCCESSFUL(2), REDIRECTION(3), CLIENT_ERROR(4),
+    /**
+     * Represents HTTP responses with a status code starting with {@code 1}.
+     */
+    INFORMATIONAL(1),
+    /**
+     * Represents HTTP responses with a status code starting with {@code 2}.
+     */
+    SUCCESSFUL(2),
+    /**
+     * Represents HTTP responses with a status code starting with {@code 3}.
+     */
+    REDIRECTION(3),
+    /**
+     * Represents HTTP responses with a status code starting with {@code 4}.
+     */
+    CLIENT_ERROR(4),
+    /**
+     * Represents HTTP responses with a status code starting with {@code 5}.
+     */
     SERVER_ERROR(5);
 
     // Static map for O(1) lookup - initialized once
     private static final Map<Integer, StatusCodeClass> LOOKUP_MAP =
             Arrays.stream(values()).collect(Collectors.toMap(StatusCodeClass::getFirstDigit, Function.identity()));
     private final int firstDigit;
 
+    /**
+     * Constructs a new instance of {@link StatusCodeClass} with the specified first digit.
+     *
+     * @param firstDigit The first digit of the HTTP status code class. This value determines the classification of the
+     *                   HTTP status codes (e.g., {@code 1} for informational, {@code 2} for successful, etc.)
+     */
     StatusCodeClass(int firstDigit) {
         this.firstDigit = firstDigit;
     }
@@ -80,6 +104,11 @@ public static Optional<StatusCodeClass> fromFirstDigit(int firstDigit) {
         return Optional.ofNullable(LOOKUP_MAP.get(firstDigit));
     }
 
+    /**
+     * Retrieves the first digit representing the classification of HTTP status codes.
+     *
+     * @return The first digit of the HTTP status code class.
+     */
     public int getFirstDigit() {
         return firstDigit;
     }