[kotlin-spring] fix kotlin map nullable (fix #16501) (#23074)

* [kotlin-spring] fix kotlin map nullable

* [kotlin-spring] fix kotlin map nullable add test

Author: pavelstackoverflow

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractKotlinCodegen.java

@@ -394,7 +394,7 @@ public String getTypeDeclaration(Schema p) {
                 inner = new StringSchema().description("TODO default missing map inner type to string");
                 p.setAdditionalProperties(inner);
             }
-            return getSchemaType(target) + "<kotlin.String, " + getTypeDeclaration(inner) + ">";
+            return getSchemaType(target) + "<kotlin.String, " + getItemsTypeDeclaration(inner) + ">";
         }
         return super.getTypeDeclaration(target);
     }

modules/openapi-generator/src/test/java/org/openapitools/codegen/kotlin/AbstractKotlinCodegenTest.java

@@ -383,6 +383,24 @@ public void handleInheritanceWithObjectTypeShouldNotBeAMap() {
         Assert.assertTrue(mapSchemaModel.isMap);
     }
 
+    @Test(description = "Issue #16501")
+    public void testNullableMap() {
+        final OpenAPI openAPI = TestUtils.parseFlattenSpec("src/test/resources/3_0/kotlin/issue16501-nullable-map.yaml");
+
+        Schema test1 = openAPI.getComponents().getSchemas().get("NullMapNotNullMap");
+        CodegenModel cm1 = codegen.fromModel("NullMapNotNullMap", test1);
+
+        codegen.postProcessModels(createCodegenModelWrapper(cm1));
+
+        // Assert the dataType properly generated
+        CodegenProperty nullableMap = cm1.vars.get(0);
+        CodegenProperty notNullableMap = cm1.vars.get(1);
+        CodegenProperty defaultMap = cm1.vars.get(2);
+        Assert.assertEquals(nullableMap.getDataType(), "kotlin.collections.Map<kotlin.String, kotlin.String?>");
+        Assert.assertEquals(notNullableMap.getDataType(), "kotlin.collections.Map<kotlin.String, kotlin.String>");
+        Assert.assertEquals(defaultMap.getDataType(), "kotlin.collections.Map<kotlin.String, kotlin.String>");
+    }
+
     @Test
     public void handleUseJakartaEeTrue() {
         codegen.additionalProperties().put("useJakartaEe", true);

modules/openapi-generator/src/test/resources/3_0/kotlin/issue16501-nullable-map.yaml

@@ -0,0 +1,22 @@
+openapi: 3.0.0
+info:
+  title: 'Issue 16501 Nullable map'
+  version: latest
+components:
+  schemas:
+    NullMapNotNullMap:
+      properties:
+        nullableMap:
+          type: object
+          additionalProperties:
+            type: string
+            nullable: true
+        notNullableMap:
+          type: object
+          additionalProperties:
+            type: string
+            nullable: false
+        defaultMap:
+          type: object
+          additionalProperties:
+            type: string