fix(java): render enum default on allOf + $ref composed property (#24384)

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

@@ -1516,7 +1516,12 @@ public String toDefaultValue(CodegenProperty cp, Schema schema) {
                 if (!propertySchemas.isEmpty()) {
                     return toObjectDefaultValue(cp, schema.getDefault(), propertySchemas);
                 }
-                return null;
+                // No object properties resolved: the composition wraps a non-object, e.g. an `allOf`
+                // to an enum or scalar (`allOf: [{$ref: '#/.../CurrencyCode'}]` + sibling `default`).
+                // There is nothing to build via toObjectDefaultValue, so defer to the base behavior,
+                // which emits the raw default for later enum var-name / scalar conversion. Returning
+                // null here dropped the default and regressed enum defaults (see #24384).
+                return super.toDefaultValue(schema);
             }
             return null;
         }

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

@@ -650,6 +650,26 @@ public void toDefaultValueForComposedObjectWithDefaultTest() {
         Assert.assertEquals(rendered, "new " + cp.datatypeWithEnum + "().one(\"one\").two(\"two\")");
     }
 
+    @Test
+    public void toDefaultValueForComposedEnumWithDefaultTest() {
+        // A `$ref` to an enum schema combined with a sibling `default` is parsed as a composed (allOf)
+        // schema that wraps a non-object, so getComposedSchemaProperties resolves no properties. The raw
+        // enum default must still be preserved (for later enum var-name conversion, e.g. `CurrencyCode.EUR`)
+        // rather than dropped to null, which regressed the standard `allOf` + sibling-`default` idiom (see #24384).
+        codegen.setDateLibrary("java8");
+        codegen.setOpenAPI(new OpenAPI().components(new Components()
+                .addSchemas("CurrencyCode", new StringSchema()._enum(Arrays.asList("EUR", "USD")))));
+
+        Schema<?> composed = new ComposedSchema()
+                .addAllOfItem(new Schema<>().$ref("#/components/schemas/CurrencyCode"));
+        composed.setDefault("EUR");
+
+        CodegenProperty cp = codegen.fromProperty("currency", composed);
+        String rendered = codegen.toDefaultValue(cp, composed);
+
+        Assert.assertEquals(rendered, "EUR");
+    }
+
     @Test
     public void dateDefaultValueIsIsoDate() {
         final OpenAPI openAPI = FLATTENED_SPEC.get("3_0/spring/date-time-parameter-types-for-testing");