fix(java/restclient): avoid IndexOutOfBoundsException for empty multipart list params (#23153)

Author: JohnBryte

modules/openapi-generator/src/main/resources/Java/libraries/restclient/ApiClient.mustache

@@ -845,15 +845,20 @@ public class ApiClient{{#jsr310}} extends JavaTimeFormatter{{/jsr310}} {
         addCookiesToRequest(defaultCookies, requestBuilder);
 
         if (MediaType.MULTIPART_FORM_DATA.isCompatibleWith(contentType)) {
-            formParams.forEach(
-                    (k, v) -> {
-                        if (v instanceof java.util.ArrayList) {
-                            Object o = v.get(0);
-                            if (o != null && o.getClass().getEnumConstants() != null) {
-                                v.set(0, o.toString());
+            formParams.forEach((k, v) -> {
+                if (v instanceof List<?> && !((List<?>) v).isEmpty()) {
+                    List<Object> list = (List<Object>) v;
+                    Object first = list.get(0);
+                    if (first != null && first.getClass().isEnum()) {
+                        for (int i = 0; i < list.size(); i++) {
+                            Object item = list.get(i);
+                            if (item != null) {
+                                ((List) list).set(i, item.toString());
                             }
                         }
-                    });
+                    }
+                }
+            });
         }
 
         var selectedBody = selectBody(body, formParams, contentType);

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

@@ -4211,4 +4211,36 @@ public void testFeignHc5CustomTemplateDirIsPreserved() {
                 "feign-hc5 must preserve a user-provided templateDir and not overwrite it with 'feign'");
     }
 
+    @Test(description = "Regression test for multipart/form-data list handling in restclient ApiClient to avoid IndexOutOfBoundsException on empty lists")
+    public void testRestClientMultipartFormParamsGuardAgainstEmptyLists() {
+        final Path output = newTempFolder();
+        final CodegenConfigurator configurator = new CodegenConfigurator()
+                .setGeneratorName(JAVA_GENERATOR)
+                .setLibrary(JavaClientCodegen.RESTCLIENT)
+                .setAdditionalProperties(Map.of(CodegenConstants.API_PACKAGE, "xyz.abcdef.api"))
+                .setInputSpec("src/test/resources/3_0/form-multipart-binary-array.yaml")
+                .setOutputDir(output.toString().replace("\\", "/"));
+
+        List<File> files = new DefaultGenerator().opts(configurator.toClientOptInput()).generate();
+
+        validateJavaSourceFiles(files);
+
+        assertFileContains(
+                output.resolve("src/main/java/xyz/abcdef/ApiClient.java"),
+                "if (v instanceof List<?> && !((List<?>) v).isEmpty()) {",
+                "List<Object> list = (List<Object>) v;",
+                "Object first = list.get(0);",
+                "if (first != null && first.getClass().isEnum()) {",
+                "for (int i = 0; i < list.size(); i++) {",
+                "Object item = list.get(i);",
+                "if (item != null) {",
+                "((List) list).set(i, item.toString());"
+        );
+
+        TestUtils.assertFileNotContains(
+                output.resolve("src/main/java/xyz/abcdef/ApiClient.java"),
+                "if (v instanceof java.util.ArrayList) {",
+                "o.getClass().getEnumConstants() != null"
+        );
+    }
 }