[BUG][JAVA] Numeric String enums cause duplicate enum keys

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used? 4.3.1
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

An openapi spec containing a string enum with numeric values has the possibility to generate duplicate enum keys, because the generator gives the strings "1" and "-1" the same key (_1). The declaration below will produce;

public class Foo {
  /**
   * Gets or Sets bar
   */
  @JsonAdapter(BarEnum.Adapter.class)
  public enum BarEnum {
    _1("1"),
    _1("-1");
  
  //...
  }
}

openapi-generator version

4.3.1

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  description: Example spec with numeric string enums
  version: 1.0.0
  title: OpenAPI Petstore
  license:
    name: Apache-2.0
    url: 'https://www.apache.org/licenses/LICENSE-2.0.html'
paths:
  /foo:
    get:
      responses:
        default:
          description: response
          content:
            application/json:
              schema:
                type: object
                properties:
                  string:
                    $ref: '#/components/schemas/Foo'
components:
  schemas:
    Foo:
      type: object
      properties:
        bar:
          type: string
          enum:
            - 1
            - -1

Generation Details

Applies to java generator, see AbstractJavaCodegen.

Steps to reproduce

Use yaml above openapi-generator -g java -i yaml.

Related issues/PRs

Could not find any through searching open issues / prs

Suggest a fix

Detecting numeric strings here and handling them as if they were a numeric type for enum var naming would fix the issue by producing NUMBER_1("1"), NUMBER_MINUS_1("-1"); or similar but this would be a large BC break for anyone with numeric string enums.

Adding a special case to the string handling for negative numeric strings to provide _1("1"), _MINUS_1("-1") reduces BC break (though still breaks in case of negative numeric strings) but is less consistent.

Switching behaviour on a config setting may be possible but is probably also undesireable.

It may also be possible be able to detect duplicate enum keys at generation time.

Start by reproducing the issue with the YAML specification and openapi-generator -g java -i yaml. Then inspect the enum variable naming logic in modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractJavaCodegen.java around the referenced lines. Done means the generated Java enum has unique keys for numeric string values such as "1" and "-1".