[BUG][Java] the client generation in java instantiate Set as ArrayList

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• 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?

Description

When generating a java client the Collections are generated as Sets but assigned to a new ArrayList if null.

For example, I have an ActivityRequest class with a list of languages. The generated client will be :

public class ActivityRequest {

  private Set<String> languages = new LinkedHashSet<>();


  public ActivityRequest addLanguagesItem(String languagesItem) {
    if (this.languages == null) {
      this.languages = new ArrayList<>();
    }
    this.languages.add(languagesItem);
    return this;
  }

}

And to add more to the issue, the ArrayList type is not even imported.

Therefore, when trying to compile the client there are failures.

Expected

I would have expected the same type as its initialization.

 public ActivityRequest addLanguagesItem(String languagesItem) {
    if (this.languages == null) {
      this.languages = new LinkedHashSet<>();
    }
    this.languages.add(languagesItem);
    return this;
  }

openapi-generator version

This is appearing in 6.3.0.
The same json input was working previously.

OpenAPI declaration file content or url

example ActivityRequest entity

Generation Details

command used to generate : openapi-generator-cli generate -i openapi.json -g java -o clients/java --additional-properties=groupId=com.company,modelPackage=com.company.client.model,invokerPackage=com.company.client,apiPackage=com.company.client.api,artifactId=api-client,artifactVersion=0.0.1-SNAPHOT,snapshotVersion=true,library=native,serializationLibrary=jackson

Suggest a fix

The problem might be related to the following PR : https://github.com/OpenAPITools/openapi-generator/pull/14130

Reproduce the issue with the provided ActivityRequest example and the openapi-generator-cli command using generator version 6.3.0. Compare the Java collection field initialization with the generated addLanguagesItem method, and review PR #14130 for related context. Done means the generated client uses a consistent collection type and compiles without the missing import failure.