diff --git a/modules/openapi-generator/src/test/java/org/openapitools/codegen/java/JavaClientCodegenTest.java b/modules/openapi-generator/src/test/java/org/openapitools/codegen/java/JavaClientCodegenTest.java
index c07c74b8f1c9b..53910b3be6b08 100644
--- a/modules/openapi-generator/src/test/java/org/openapitools/codegen/java/JavaClientCodegenTest.java
+++ b/modules/openapi-generator/src/test/java/org/openapitools/codegen/java/JavaClientCodegenTest.java
@@ -3178,4 +3178,32 @@ public void testQueryParamsExploded_whenQueryParamIsNull() throws IOException {
         Path petApi = Paths.get(output + "/src/main/java/xyz/abcdef/api/DepartmentApi.java");
         TestUtils.assertFileContains(petApi, "if (filter != null) {");
     }
+
+    @Test
+    public void testRequiredAndNullableAreBothTrue() throws IOException {
+        File output = Files.createTempDirectory("test").toFile();
+        output.deleteOnExit();
+
+        final CodegenConfigurator configurator = new CodegenConfigurator()
+                .setGeneratorName("java")
+                .setLibrary(JavaClientCodegen.OKHTTP_GSON)
+                .setInputSpec("src/test/resources/bugs/issue_18516.yaml")
+                .setOutputDir(output.getAbsolutePath().replace("\\", "/"));
+
+
+        DefaultGenerator generator = new DefaultGenerator();
+        List<File> files = generator.opts(configurator.toClientOptInput()).generate();
+        files.forEach(File::deleteOnExit);
+
+        validateJavaSourceFiles(files);
+
+        Path modelFile = Paths.get(output + "/src/main/java/org/openapitools/client/model/SomeObject.java");
+        TestUtils.assertFileContains(
+                modelFile,
+                "} else if (!jsonObj.get(\"ids\").isJsonArray()  && !jsonObj.get(\"ids\").isJsonNull()) {",
+                "if (!jsonObj.get(\"users\").isJsonArray() && !jsonObj.get(\"users\").isJsonNull()) {",
+                "if (jsonObj.get(\"user\") != null && !jsonObj.get(\"user\").isJsonNull()) {",
+                "if (jsonObj.get(\"role\") != null && !jsonObj.get(\"role\").isJsonNull()) {",
+                "if (jsonObj.get(\"custom\") != null && !jsonObj.get(\"custom\").isJsonNull()) {");
+    }
 }
diff --git a/modules/openapi-generator/src/test/resources/bugs/issue_18516.yaml b/modules/openapi-generator/src/test/resources/bugs/issue_18516.yaml
new file mode 100644
index 0000000000000..fcefeb6b521f1
--- /dev/null
+++ b/modules/openapi-generator/src/test/resources/bugs/issue_18516.yaml
@@ -0,0 +1,61 @@
+openapi: 3.0.3
+info:
+  title: test
+  description: Test API
+  version: 1.0.1
+
+paths:
+  /test:
+    get:
+      responses:
+        200:
+          description: Valid response
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/SomeObject"
+
+components:
+  schemas:
+    SomeObject:
+      type: object
+      required: 
+        - ids
+        - users
+        - user
+        - role
+        - custom
+      properties:
+        ids:
+          type: array
+          nullable: true
+          items:
+            type: integer
+        users:
+          type: array
+          nullable: true
+          items:
+            type: object
+            properties:
+              id:
+                type: string
+        user:
+          type: object
+          nullable: true
+          properties:
+            id:
+              type: string
+        role:
+          type: string
+          nullable: true
+          enum:
+            - admin
+            - tenant
+        custom:
+          $ref: "#/components/schemas/customEnum"
+    customEnum:
+      type: string
+      nullable: true
+      enum:
+        - custom
+        
\ No newline at end of file
diff --git a/samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/model/PetWithRequiredNullableCases1.java b/samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/model/PetWithRequiredNullableCases1.java
index d2199df3eea77..ed950f185cbb6 100644
--- a/samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/model/PetWithRequiredNullableCases1.java
+++ b/samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/model/PetWithRequiredNullableCases1.java
@@ -427,7 +427,7 @@ public static void validateJsonElement(JsonElement jsonElement) throws IOExcepti
       // ensure the required json array is present
       if (jsonObj.get("photoUrls") == null) {
         throw new IllegalArgumentException("Expected the field `linkedContent` to be an array in the JSON string but got `null`");
-      } else if (!jsonObj.get("photoUrls").isJsonArray()) {
+      } else if (!jsonObj.get("photoUrls").isJsonArray() && !jsonObj.get("photoUrls").isJsonNull()) {
         throw new IllegalArgumentException(String.format("Expected the field `photoUrls` to be an array in the JSON string but got `%s`", jsonObj.get("photoUrls").toString()));
       }
       if (jsonObj.get("tags") != null && !jsonObj.get("tags").isJsonNull()) {