[Java] Fix generation of POJOs with sets of enums (fixes OpenAPITools#19957) (OpenAPITools#19960)

Author: AndreasHae

modules/openapi-generator/src/main/resources/Java/pojo.mustache

@@ -395,7 +395,7 @@ public class {{classname}} {{#parent}}extends {{{.}}} {{/parent}}{{#vendorExtens
     {{#uniqueItems}}
     if ({{getter}}() != null) {
       int i = 0;
-      for ({{{items.dataType}}} _item : {{getter}}()) {
+      for ({{{items.datatypeWithEnum}}} _item : {{getter}}()) {
         try {
           joiner.add(String.format("%s{{baseName}}%s%s=%s", prefix, suffix,
               "".equals(suffix) ? "" : String.format("%s%d%s", containerPrefix, i, containerSuffix),

modules/openapi-generator/src/test/java/org/openapitools/codegen/java/apachehttpclient/ApacheHttpClientCodegenTest.java

@@ -90,4 +90,28 @@ public void testApacheHttpClientExplodedQueryParamWithArrayProperty() throws IOE
             "localVarQueryParams.addAll(apiClient.parameterToPairs(\"multi\", \"values\", queryObject.getValues()))"
         );
     }
+
+    @Test
+    void testApacheHttpClientQueryParamHandlingUniqueItemsStringEnum() throws IOException {
+        // Arrange
+        var output = Files.createTempDirectory("test").toFile();
+        output.deleteOnExit();
+
+        var configurator = new CodegenConfigurator()
+                .setGeneratorName("java")
+                .setLibrary(JavaClientCodegen.APACHE)
+                .setInputSpec("src/test/resources/3_0/unique-items-string-enum.yaml")
+                .setOutputDir(output.getAbsolutePath().replace("\\", "/"));
+
+        final ClientOptInput clientOptInput = configurator.toClientOptInput();
+        DefaultGenerator generator = new DefaultGenerator();
+
+        // Act
+        var files = generator.opts(clientOptInput).generate();
+
+        // Assert
+        TestUtils.assertFileContains(Paths.get(output + "/src/main/java/org/openapitools/client/model/GetUsersRequestV1.java"),
+                "for (RolesEnum _item : getRoles()) {"
+        );
+    }
 }

modules/openapi-generator/src/test/resources/3_0/unique-items-string-enum.yaml

@@ -0,0 +1,20 @@
+openapi: 3.0.1
+info:
+  title: Sample API to test generation of schemas with uniqueItems arrays of enums
+  description: API description in Markdown.
+  version: 1.0.0
+servers: []
+paths: {}
+components:
+  schemas:
+    GetUsersRequestV1:
+      type: object
+      properties:
+        roles:
+          uniqueItems: true
+          type: array
+          items:
+            type: string
+            enum:
+              - ROLE1
+              - ROLE2