[Java] Fix uncompilable default value for alias-as-model array/map properties (#23988)

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

@@ -1380,6 +1380,22 @@ public String toArrayDefaultValue(CodegenProperty cp, Schema schema) {
 
     @Override
     public String toDefaultValue(CodegenProperty cp, Schema schema) {
+        // When generateAliasAsModel is enabled, a property that $refs an array/map alias is typed
+        // as the generated alias model (e.g. "ItemArray extends ArrayList<Item>"), not the inlined
+        // collection. Dereferencing below would otherwise yield a collection default such as
+        // "new ArrayList<>()", which is not assignable to the alias type and does not compile.
+        // Use the alias model's own default instead.
+        // See https://github.com/OpenAPITools/openapi-generator/issues/23988
+        if (schema.get$ref() != null && !cp.isArray && !cp.isMap) {
+            Schema referencedSchema = ModelUtils.getReferencedSchema(this.openAPI, schema);
+            if (ModelUtils.isArraySchema(referencedSchema)
+                    || (ModelUtils.isMapSchema(referencedSchema) && !ModelUtils.isComposedSchema(referencedSchema))) {
+                if (cp.isNullable || containerDefaultToNull) {
+                    return null;
+                }
+                return "new " + cp.datatypeWithEnum + "()";
+            }
+        }
         schema = ModelUtils.getReferencedSchema(this.openAPI, schema);
         if (ModelUtils.isArraySchema(schema)) {
             if (defaultToEmptyContainer) {

modules/openapi-generator/src/test/java/org/openapitools/codegen/java/spring/SpringCodegenTest.java

@@ -6582,6 +6582,39 @@ public void shouldAddNullableImportForArrayTypeModels() throws IOException {
                 .hasImports("org.springframework.lang.Nullable");
     }
 
+    @Test
+    public void shouldUseAliasModelDefaultValueForAliasAsModelProperties_issue23988() throws IOException {
+        File output = Files.createTempDirectory("test").toFile().getCanonicalFile();
+        output.deleteOnExit();
+
+        final OpenAPI openAPI = TestUtils.parseFlattenSpec("src/test/resources/3_0/spring/issue_23988.yaml");
+        final SpringCodegen codegen = new SpringCodegen();
+        codegen.setOpenAPI(openAPI);
+        codegen.setOutputDir(output.getAbsolutePath());
+        codegen.additionalProperties().put(INTERFACE_ONLY, "true");
+        codegen.additionalProperties().put(CodegenConstants.GENERATE_ALIAS_AS_MODEL, "true");
+
+        ClientOptInput input = new ClientOptInput();
+        input.openAPI(openAPI);
+        input.config(codegen);
+
+        DefaultGenerator generator = new DefaultGenerator();
+        generator.setGenerateMetadata(false);
+        generator.setGeneratorPropertyDefault(CodegenConstants.MODELS, "true");
+        generator.setGeneratorPropertyDefault(CodegenConstants.MODEL_TESTS, "false");
+        generator.setGeneratorPropertyDefault(CodegenConstants.MODEL_DOCS, "false");
+
+        Map<String, File> files = generator.opts(input).generate().stream()
+                .collect(Collectors.toMap(File::getName, Function.identity()));
+
+        // A property that $refs an array/map alias is typed as the generated alias model, so its
+        // default value must instantiate the alias model rather than the inlined collection type
+        // (which is not assignable to the alias and does not compile).
+        JavaFileAssert.assertThat(files.get("MyObject.java"))
+                .fileContains("private ItemArray itemArray = new ItemArray();")
+                .fileContains("private ItemMap itemMap = new ItemMap();");
+    }
+
     @Test
     public void testClientRegistrationIdAnnotation() throws IOException {
         final SpringCodegen codegen = new SpringCodegen();

modules/openapi-generator/src/test/resources/3_0/spring/issue_23988.yaml

@@ -0,0 +1,27 @@
+openapi: 3.0.1
+info:
+  title: Test
+  version: 0.0.1
+paths: {}
+components:
+  schemas:
+    Item:
+      type: object
+      properties:
+        property:
+          type: string
+    ItemArray:
+      type: array
+      items:
+        $ref: '#/components/schemas/Item'
+    ItemMap:
+      type: object
+      additionalProperties:
+        $ref: '#/components/schemas/Item'
+    MyObject:
+      type: object
+      properties:
+        itemArray:
+          $ref: '#/components/schemas/ItemArray'
+        itemMap:
+          $ref: '#/components/schemas/ItemMap'