[BUG][JAVA] Handling default values leads to wrong defaults or not compilable code fixed (#15836)

* (fix): default values for array types will be handled a bit more appropriate

* (fix): remove commented out block

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

@@ -974,6 +974,12 @@ public abstract class AbstractJavaCodegen extends DefaultCodegen implements Code
                     _default.elements().forEachRemaining((element) -> {
                         final_values.add(element.asText());
                     });
+                } else if (schema.getDefault() instanceof Collection) {
+                    var _default = (Collection<String>) schema.getDefault();
+                    List<String> final_values = _values;
+                    _default.forEach((element) -> {
+                        final_values.add(element);
+                    });
                 } else { // single value
                     _values = java.util.Collections.singletonList(String.valueOf(schema.getDefault()));
                 }
@@ -986,7 +992,7 @@ public abstract class AbstractJavaCodegen extends DefaultCodegen implements Code
                         defaultValues.add(cp.items.datatypeWithEnum + "." + toEnumVarName(_value, cp.items.dataType));
                     }
                     defaultValue = StringUtils.join(defaultValues, ", ");
-                } else {
+                } else if (_values.size() > 0) {
                     if (cp.items.isString) { // array item is string
                         defaultValue = String.format(Locale.ROOT, "\"%s\"", StringUtils.join(_values, "\", \""));
                     } else if (cp.items.isNumeric) {
@@ -1006,7 +1012,10 @@ public abstract class AbstractJavaCodegen extends DefaultCodegen implements Code
                     } else { // array item is non-string, e.g. integer
                         defaultValue = StringUtils.join(_values, ", ");
                     }
+                } else {
+                    return "new ArrayList<>()";
                 }
+
                 return String.format(Locale.ROOT, "new ArrayList<>(Arrays.asList(%s))", defaultValue);
             } else if (cp.isArray && cp.getUniqueItems()) { // set
                 // TODO

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

@@ -1617,6 +1617,37 @@ public class JavaClientCodegenTest {
                 .containsWithNameAndAttributes("JsonbProperty", ImmutableMap.of("value", "\"c\""));
     }
 
+    @Test
+    public void testJavaClientDefaultValues_issueNoNumber() throws Exception {
+        Map<String, Object> properties = new HashMap<>();
+        properties.put(JavaClientCodegen.MICROPROFILE_REST_CLIENT_VERSION, "3.0");
+
+        File output = Files.createTempDirectory("test").toFile();
+        output.deleteOnExit();
+
+        final CodegenConfigurator configurator = new CodegenConfigurator()
+                .setAdditionalProperties(properties)
+                .setGeneratorName("java")
+                .setLibrary(JavaClientCodegen.WEBCLIENT)
+                .setInputSpec("src/test/resources/bugs/java-codegen-empty-array-as-default-value/issue_wrong-default.yaml")
+                .setOutputDir(output.getAbsolutePath().replace("\\", "/"));
+
+        final ClientOptInput clientOptInput = configurator.toClientOptInput();
+        DefaultGenerator generator = new DefaultGenerator();
+        Map<String, File> files = generator.opts(clientOptInput).generate().stream()
+                .collect(Collectors.toMap(File::getName, Function.identity()));
+
+        JavaFileAssert.assertThat(files.get("DefaultValuesType.java"))
+                .hasProperty("stringDefault")
+                .asString().endsWith("= new ArrayList<>();");
+        JavaFileAssert.assertThat(files.get("DefaultValuesType.java"))
+                .hasProperty("stringDefault2")
+                .asString().endsWith("= new ArrayList<>(Arrays.asList(\"Hallo\", \"Huhu\"));");
+        JavaFileAssert.assertThat(files.get("DefaultValuesType.java"))
+                .hasProperty("objectDefault")
+                .asString().endsWith("= new ArrayList<>();");
+    }
+
     @Test
     public void testWebClientJsonCreatorWithNullable_issue12790() throws Exception {
         Map<String, Object> properties = new HashMap<>();

modules/openapi-generator/src/test/resources/bugs/java-codegen-empty-array-as-default-value/baseType.yaml

@@ -0,0 +1,10 @@
+$schema: https://json-schema.org/draft/2020-12/schema
+title: BaseType
+type: object
+required:
+ - property
+properties:
+  property1:
+    type: array
+    items:
+      $ref: 'defaultValuesType.yaml'

modules/openapi-generator/src/test/resources/bugs/java-codegen-empty-array-as-default-value/defaultValuesType.yaml

@@ -0,0 +1,23 @@
+$schema: https://json-schema.org/draft/2020-12/schema
+title: DefaultValuesType
+type: object
+properties:
+  stringDefault:
+    type: array
+    default: []
+    items:
+      type: string
+  stringDefault2:
+    type: array
+    default:
+      - "Hallo"
+      - "Huhu"
+    items:
+      type: string
+  objectDefault:
+    type: array
+    default: []
+    items:
+      $ref: 'objectType.yaml'
+
+

modules/openapi-generator/src/test/resources/bugs/java-codegen-empty-array-as-default-value/issue_wrong-default.yaml

@@ -0,0 +1,45 @@
+openapi: 3.0.3
+info:
+  version: 1.0.0
+  title: Subfile-Defaults
+
+paths:
+  "/dummy":
+    post:
+      operationId: doSomething
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              $ref: '#/components/schemas/BaseRequest'
+      responses:
+        "200":
+          description: Successful Operation.
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/BaseResponse'
+
+
+components:
+
+  schemas:
+    BaseRequest:
+      type: object
+      required:
+        - data
+      properties:
+        data:
+          $ref: './baseType.yaml'
+    BaseResponse:
+      type: object
+      properties:
+        status:
+          $ref: '#/components/schemas/Status'
+
+    Status:
+      type: string
+      enum:
+        - GOOD
+        - BAD

modules/openapi-generator/src/test/resources/bugs/java-codegen-empty-array-as-default-value/objectType.yaml

@@ -0,0 +1,6 @@
+$schema: https://json-schema.org/draft/2020-12/schema
+title: ObjectType
+type: object
+properties:
+  someInt:
+    type: integer