fix (JAVA CLIENTS): don't generate oneOf (not working) example code when using useOneOfInterfaces option

Author: Nicklas2751

modules/openapi-generator/src/main/resources/Java/libraries/jersey2/model_oneof_doc.mustache

@@ -13,7 +13,7 @@
 NOTE: this class is nullable.
 
 {{/isNullable}}
-## Example
+{{^vendorExtensions.x-is-one-of-interface}}## Example
 ```java
 // Import classes:
 import {{{package}}}.{{{classname}}};
@@ -35,4 +35,4 @@ public class Example {
         {{/oneOf}}
     }
 }
-```
+```{{/vendorExtensions.x-is-one-of-interface}}

modules/openapi-generator/src/main/resources/Java/libraries/jersey3/model_oneof_doc.mustache

@@ -13,7 +13,7 @@
 NOTE: this class is nullable.
 
 {{/isNullable}}
-## Example
+{{^vendorExtensions.x-is-one-of-interface}}## Example
 ```java
 // Import classes:
 import {{{package}}}.{{{classname}}};
@@ -35,4 +35,4 @@ public class Example {
         {{/oneOf}}
     }
 }
-```
+```{{/vendorExtensions.x-is-one-of-interface}}

modules/openapi-generator/src/main/resources/Java/libraries/native/model_oneof_doc.mustache

@@ -13,7 +13,7 @@
 NOTE: this class is nullable.
 
 {{/isNullable}}
-## Example
+{{^vendorExtensions.x-is-one-of-interface}}## Example
 ```java
 // Import classes:
 import {{{package}}}.{{{classname}}};
@@ -35,4 +35,4 @@ public class Example {
         {{/oneOf}}
     }
 }
-```
+```{{/vendorExtensions.x-is-one-of-interface}}

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

@@ -4421,4 +4421,35 @@ public void testClientWithUseOneOfInterfaceWithoutDiscriminator_issue_17419(Stri
 
     }
 
+    @Test(dataProvider = "allJavaClients")
+    public void testClientWithUseOneOfInterfaceShouldntGenerateOneOfExample_issue_17419(String client) {
+        // given
+        final Path output = newTempFolder();
+        final CodegenConfigurator configurator = new CodegenConfigurator()
+                .setGeneratorName("java")
+                .setLibrary(client)
+                .setAdditionalProperties(Map.of("useOneOfInterfaces", "true"))
+                .setInputSpec("src/test/resources/3_0/typescript-fetch/oneOf.yaml")
+                .setOutputDir(output.toString().replace("\\", "/"));
+
+        final ClientOptInput input = configurator.toClientOptInput();
+
+        // when
+        List<File> files = new DefaultGenerator().opts(input).generate();
+
+        // then
+        validateJavaSourceFiles(files);
+
+        // must only check if the library generates the doc
+        if(Files.exists(output.resolve("docs/TestResponse.md")))
+        {
+            TestUtils.assertFileNotContains(
+                    output.resolve("docs/TestResponse.md"),
+                    "## Example",
+                    "exampleTestResponse.setActualInstance(exampleTestA);"
+            );
+        }
+
+    }
+
 }