Addressed comments: two values set is now type() == null instead of type() == UNKNOWN_TO_SDK_VERSION.

Author: millems

codegen/src/main/java/software/amazon/awssdk/codegen/model/intermediate/ShapeModel.java

@@ -535,8 +535,8 @@ public String getUnionTypeGetterDocumentation() {
                + "When this class is returned in a service response, this will be {@link Type#UNKNOWN_TO_SDK_VERSION} if the "
                + "service returned a member that is only known to a newer SDK version."
                + LF + LF
-               + "When this class is created directly in your code, this will be {@link Type#UNKNOWN_TO_SDK_VERSION} if zero or "
-               + "more than one member is set.";
+               + "When this class is created directly in your code, this will be {@link Type#UNKNOWN_TO_SDK_VERSION} if zero "
+               + "members are set, and {@code null} if more than one member is set.";
     }
 
     @Override

codegen/src/main/java/software/amazon/awssdk/codegen/poet/model/ModelBuilderSpecs.java

@@ -349,8 +349,10 @@ private MethodSpec handleUnionValueChangeMethod() {
                      .endControlFlow()
                      .beginControlFlow("if (setTypes.size() == 1)")
                      .addStatement("this.type = setTypes.iterator().next()")
-                     .nextControlFlow("else")
+                     .nextControlFlow("else if (setTypes.isEmpty())")
                      .addStatement("this.type = Type.UNKNOWN_TO_SDK_VERSION")
+                     .nextControlFlow("else")
+                     .addStatement("this.type = null")
                      .endControlFlow()
                      .build();
 

codegen/src/test/resources/software/amazon/awssdk/codegen/poet/model/alltypesunionstructure.java

@@ -2234,8 +2234,8 @@ public static AllTypesUnionStructure fromAllTypesUnionStructure(Consumer<Builder
      * When this class is returned in a service response, this will be {@link Type#UNKNOWN_TO_SDK_VERSION} if the
      * service returned a member that is only known to a newer SDK version.
      *
-     * When this class is created directly in your code, this will be {@link Type#UNKNOWN_TO_SDK_VERSION} if zero or
-     * more than one member is set.
+     * When this class is created directly in your code, this will be {@link Type#UNKNOWN_TO_SDK_VERSION} if zero
+     * members are set, and {@code null} if more than one member is set.
      */
     public Type type() {
         return type;
@@ -3866,8 +3866,10 @@ private final void handleUnionValueChange(Type type, Object oldValue, Object new
             }
             if (setTypes.size() == 1) {
                 this.type = setTypes.iterator().next();
-            } else {
+            } else if (setTypes.isEmpty()) {
                 this.type = Type.UNKNOWN_TO_SDK_VERSION;
+            } else {
+                this.type = null;
             }
         }
     }

test/codegen-generated-classes-test/src/test/java/software/amazon/awssdk/services/UnionTypeTest.java

@@ -209,7 +209,7 @@ public void noMembersIsUnknownType() {
 
     @ParameterizedTest
     @MethodSource("testCases")
-    public <T> void twoMembersIsUnknownType(TestCase<T> testCase) {
+    public <T> void twoMembersIsNull(TestCase<T> testCase) {
         AllTypesUnionStructure.Builder builder = AllTypesUnionStructure.builder();
 
         testCase.setter.apply(builder, testCase.value);
@@ -219,7 +219,7 @@ public <T> void twoMembersIsUnknownType(TestCase<T> testCase) {
             builder.stringMember("foo");
         }
 
-        assertThat(builder.build().type()).isEqualTo(Type.UNKNOWN_TO_SDK_VERSION);
+        assertThat(builder.build().type()).isNull();
     }
 
     @ParameterizedTest