[BUG] [JAVA] [JAX-RS] Incorrect implementation of add/remove methods for JsonNullable<List<T>> fields in generated models

[TheNevim]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?

• Have you validated the input using an OpenAPI validator (example)?
  I assume OpenAPI Generator's own samples are valid.

• Have you tested with the latest master to confirm the issue still exists?

• Have you searched for related issues/PRs?

• What's the actual output vs expected output?

• [Optional] Sponsorship to speed up the bug fix or feature request ([Rust][Bounty] Issues with client code generation. #6178)

Description

Generated models contain incorrect implementations of add and remove helper methods for JsonNullable<List> fields.
The generated code fails to compile because these methods treat the field as an ArrayList, while its actual type is JsonNullable<List>.

openapi-generator version

7.20.0

OpenAPI declaration file content or url

paths:
  /hello-world:
    get:
      tags:
        - hello-world
      operationId: hello-world
      responses:
        200:
          description: Hello world
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/BugResponse'

components:
  schemas:
    BugResponse:
      type: object
      properties:
        nullableField:
          type: string
          nullable: true
          x-is-jackson-optional-nullable: true
        nullableList:
          type: array
          items:
            type: string
          nullable: true
          x-is-jackson-optional-nullable: true

Command line used for generation

      <plugin>
        <groupId>org.openapitools</groupId>
        <artifactId>openapi-generator-maven-plugin</artifactId>
        <version>7.20.0</version>
        <executions>
          <execution>
            <id>bug-test</id>
            <goals>
              <goal>generate</goal>
            </goals>
            <phase>generate-resources</phase>
            <configuration>
              <inputSpec>${project.basedir}/src/main/resources/api/test.yml</inputSpec>
              <generatorName>jaxrs-spec</generatorName>
              <generateSupportingFiles>false</generateSupportingFiles>
              <configOptions>
                <interfaceOnly>true</interfaceOnly>
                <useJakartaEe>true</useJakartaEe>
                <serializableModel>true</serializableModel>
                <useTags>true</useTags>
                <openapiNullable>true</openapiNullable>
              </configOptions>
            </configuration>
          </execution>
        </executions>
      </plugin>

Steps to reproduce

1. Copy the OpenAPI specification file to src/main/resources/api/test.yml.
2. Run:
   mvn generate-resources
3. Open the generated file:
   target/generated-sources/openapi/src/gen/java/org/openapitools/model/BugResponse.java
4. Observe that the generated add / remove methods reference the field as ArrayList, while the actual type is JsonNullable<List<String>>, causing compilation errors.

Actual output

  public BugResponse addNullableListItem(String nullableListItem) {
    if (this.nullableList == null) {
      this.nullableList = new ArrayList<>();
    }

    this.nullableList.add(nullableListItem);
    return this;
  }

  public BugResponse removeNullableListItem(String nullableListItem) {
    if (nullableListItem != null && this.nullableList != null) {
      this.nullableList.remove(nullableListItem);
    }

    return this;
  }

Expected output

  public BugResponse addNullableListItem(String nullableListItem) {
    if (this.nullableList == null || !this.nullableList.isPresent()) {
      this.nullableList = JsonNullable.<List<String>>of(new ArrayList<>());
    }
    try {
      this.nullableList.get().add(nullableListItem);
    } catch (java.util.NoSuchElementException e) {
      // this can never happen, as we make sure above that the value is present
    }
    return this;
  }

  public BugResponse removeNullableListItem(String nullableListItem) {
    if (nullableListItem != null && this.nullableList != null && this.nullableList.isPresent()) {
      try {
        this.nullableList.get().remove(nullableListItem);
      } catch (java.util.NoSuchElementException e) {
        // this can never happen, as we make sure above that the value is present
      }
    }

    return this;
  }

Suggest a fix

Replace the generated implementation with the following:

openapi-generator/modules/openapi-generator/src/main/resources/JavaJaxRS/spec/pojo.mustache

Lines 174 to 191 in bd7fc7f

	{{#isArray}}
	public {{classname}} add{{nameInPascalCase}}Item({{{items.datatypeWithEnum}}} {{name}}Item) {
	if (this.{{name}} == null) {
	this.{{name}} = {{{defaultValue}}}{{^defaultValue}}new {{#uniqueItems}}LinkedHashSet{{/uniqueItems}}{{^uniqueItems}}ArrayList{{/uniqueItems}}<>(){{/defaultValue}};
	}
	this.{{name}}.add({{name}}Item);
	return this;
	}
	public {{classname}} remove{{nameInPascalCase}}Item({{{items.datatypeWithEnum}}} {{name}}Item) {
	if ({{name}}Item != null && this.{{name}} != null) {
	this.{{name}}.remove({{name}}Item);
	}
	return this;
	}
	{{/isArray}}

  {{#isArray}}
  public {{classname}} add{{nameInPascalCase}}Item({{{items.datatypeWithEnum}}} {{name}}Item) {
    {{#vendorExtensions.x-is-jackson-optional-nullable}}
    if (this.{{name}} == null || !this.{{name}}.isPresent()) {
      this.{{name}} = JsonNullable.<{{{datatypeWithEnum}}}>of({{{defaultValue}}}{{^defaultValue}}new {{#uniqueItems}}LinkedHashSet{{/uniqueItems}}{{^uniqueItems}}ArrayList{{/uniqueItems}}<>(){{/defaultValue}});
    }
    try {
      this.{{name}}.get().add({{name}}Item);
    } catch (java.util.NoSuchElementException e) {
      // this can never happen, as we make sure above that the value is present
    }
    return this;
    {{/vendorExtensions.x-is-jackson-optional-nullable}}
    {{^vendorExtensions.x-is-jackson-optional-nullable}}
    if (this.{{name}} == null) {
      this.{{name}} = {{{defaultValue}}}{{^defaultValue}}new {{#uniqueItems}}LinkedHashSet{{/uniqueItems}}{{^uniqueItems}}ArrayList{{/uniqueItems}}<>(){{/defaultValue}};
    }

    this.{{name}}.add({{name}}Item);
    return this;
    {{/vendorExtensions.x-is-jackson-optional-nullable}}
  }

  public {{classname}} remove{{nameInPascalCase}}Item({{{items.datatypeWithEnum}}} {{name}}Item) {
    {{#vendorExtensions.x-is-jackson-optional-nullable}}
    if ({{name}}Item != null && this.{{name}} != null && this.{{name}}.isPresent()) {
      try {
        this.{{name}}.get().remove({{name}}Item);
      } catch (java.util.NoSuchElementException e) {
        // this can never happen, as we make sure above that the value is present
      }
    }
    {{/vendorExtensions.x-is-jackson-optional-nullable}}
    {{^vendorExtensions.x-is-jackson-optional-nullable}}
    if ({{name}}Item != null && this.{{name}} != null) {
      this.{{name}}.remove({{name}}Item);
    }
    {{/vendorExtensions.x-is-jackson-optional-nullable}}

    return this;
  }
  {{/isArray}}

and

openapi-generator/modules/openapi-generator/src/main/resources/JavaJaxRS/spec/pojo.mustache

Lines 192 to 209 in bd7fc7f

	{{#isMap}}
	public {{classname}} put{{nameInPascalCase}}Item(String key, {{{items.datatypeWithEnum}}} {{name}}Item) {
	if (this.{{name}} == null) {
	this.{{name}} = {{{defaultValue}}}{{^defaultValue}}new HashMap<>(){{/defaultValue}};
	}
	this.{{name}}.put(key, {{name}}Item);
	return this;
	}
	public {{classname}} remove{{nameInPascalCase}}Item(String key) {
	if (this.{{name}} != null) {
	this.{{name}}.remove(key);
	}
	return this;
	}
	{{/isMap}}

{{#isMap}}
  public {{classname}} put{{nameInPascalCase}}Item(String key, {{{items.datatypeWithEnum}}} {{name}}Item) {
    {{#vendorExtensions.x-is-jackson-optional-nullable}}
    if (this.{{name}} == null || !this.{{name}}.isPresent()) {
      this.{{name}} = JsonNullable.<{{{datatypeWithEnum}}}>of({{{defaultValue}}}{{^defaultValue}}new HashMap<>(){{/defaultValue}});
    }
    try {
      this.{{name}}.get().put(key, {{name}}Item);
    } catch (java.util.NoSuchElementException e) {
      // this can never happen, as we make sure above that the value is present
    }
    return this;
    {{/vendorExtensions.x-is-jackson-optional-nullable}}
    {{^vendorExtensions.x-is-jackson-optional-nullable}}
    if (this.{{name}} == null) {
      this.{{name}} = {{{defaultValue}}}{{^defaultValue}}new HashMap<>(){{/defaultValue}};
    }

    this.{{name}}.put(key, {{name}}Item);
    return this;
    {{/vendorExtensions.x-is-jackson-optional-nullable}}
  }

  public {{classname}} remove{{nameInPascalCase}}Item(String key) {
    {{#vendorExtensions.x-is-jackson-optional-nullable}}
    if (this.{{name}} != null && this.{{name}}.isPresent()) {
      try {
        this.{{name}}.get().remove(key);
      } catch (java.util.NoSuchElementException e) {
        // this can never happen, as we make sure above that the value is present
      }
    }
    {{/vendorExtensions.x-is-jackson-optional-nullable}}
    {{^vendorExtensions.x-is-jackson-optional-nullable}}
    if (this.{{name}} != null) {
      this.{{name}}.remove(key);
    }
    {{/vendorExtensions.x-is-jackson-optional-nullable}}

    return this;
  }
  {{/isMap}}