fix: additional fields mutable

.github/CODEOWNERS

@@ -1 +1 @@
-* @ferhatelmas @miagilepner @driver-devel
+* @ferhatelmas @driver-devel @peterdeme

src/main/java/io/getstream/chat/java/models/Channel.java

@@ -239,6 +239,25 @@ public void setAdditionalField(String name, Object value) {
     public static ChannelRequestObject buildFrom(@Nullable Channel channel) {
       return RequestObjectBuilder.build(ChannelRequestObject.class, channel);
     }
+
+    private ChannelRequestObject(
+        @Nullable UserRequestObject createdBy,
+        @Nullable String team,
+        @Nullable Boolean autoTranslationEnabled,
+        @Nullable Language autoTranslationLanguage,
+        @Nullable Boolean frozen,
+        @Nullable List<ChannelMemberRequestObject> members,
+        @Nullable ConfigOverridesRequestObject configOverrides,
+        Map<String, Object> additionalFields) {
+      this.createdBy = createdBy;
+      this.team = team;
+      this.autoTranslationEnabled = autoTranslationEnabled;
+      this.autoTranslationLanguage = autoTranslationLanguage;
+      this.frozen = frozen;
+      this.members = members;
+      this.configOverrides = configOverrides;
+      this.additionalFields = new HashMap<String, Object>(additionalFields);
+    }
   }
 
   @Builder

src/test/java/io/getstream/chat/java/ChannelTest.java

@@ -32,6 +32,47 @@ void whenRetrievingChannel_thenNoException() {
                 .request());
   }
 
+  @DisplayName("Can set custom field on channel")
+  @Test
+  void whenSettingCustomField_thenNoException() {
+    var channelReq =
+        ChannelRequestObject.builder()
+            .createdBy(testUserRequestObject)
+            .members(buildChannelMembersList())
+            .build();
+    channelReq.setAdditionalField("fieldkey", "fieldvalue");
+    var channel =
+        Assertions.assertDoesNotThrow(
+            () ->
+                Channel.getOrCreate(testChannel.getType(), RandomStringUtils.randomAlphabetic(12))
+                    .data(channelReq)
+                    .request());
+    Assertions.assertEquals(
+        "fieldvalue", channel.getChannel().getAdditionalFields().get("fieldkey"));
+  }
+
+  @DisplayName("Can set custom field on channel after fields have already been set in builder")
+  @Test
+  void whenSettingCustomFieldInBuilderAndAfer_thenNoException() {
+    var channelReq =
+        ChannelRequestObject.builder()
+            .createdBy(testUserRequestObject)
+            .members(buildChannelMembersList())
+            .additionalField("fieldkey1", "fieldvalue1")
+            .build();
+    channelReq.setAdditionalField("fieldkey2", "fieldvalue2");
+    var channel =
+        Assertions.assertDoesNotThrow(
+            () ->
+                Channel.getOrCreate(testChannel.getType(), RandomStringUtils.randomAlphabetic(12))
+                    .data(channelReq)
+                    .request());
+    Assertions.assertEquals(
+        "fieldvalue1", channel.getChannel().getAdditionalFields().get("fieldkey1"));
+    Assertions.assertEquals(
+        "fieldvalue2", channel.getChannel().getAdditionalFields().get("fieldkey2"));
+  }
+
   @DisplayName("Can add a moderator to a channel (update)")
   @Test
   void whenAddingModerator_thenHasModerator() {