[JAVA] using `additionalProperties` does not generate a Map

Description

When I generate a schemas model with additionalProperties in addition to other defined properties, there is no ability to set the additional properties in the generated client code - i.e. there's no Map.

In previous versions, I understand that the generated model object would extend HashMap.

The code that's generated is:

public class StorageNode {

  @SerializedName("lib")
  private String lib = null;

  @SerializedName("path")
  private String path = null;

  @SerializedName("fields")
  private StorageNodeFields fields = null;
  ...
}

public class StorageNodeFields {

  @SerializedName("NAME")
  private String NAME = null;

  @SerializedName("DESC")
  private String DESC = null;

  @SerializedName("COMPLEX")
  private StorageNodeFieldsCOMPLEX COMPLEX = null;
  ...
}

Swagger-codegen version

swagger-codegen-cli 3.0.5

Swagger declaration file content or url

openapi: 3.0.1
info:
  title: Map-test
  description: 'fdsa'
  version: 0.0.0
components:
  schemas:
    StorageNode:
      type: object
      properties:
        lib:
          type: string
        path:
          type: string
        fields:
          type: object
          properties:
            NAME: 
              type: string
            DESC: 
              type: string
            COMPLEX:
              type: object
              properties:
                x: 
                  type: integer
                y: 
                  type: integer
                size: 
                  type: integer
                desc: 
                  type: string
          additionalProperties: 
            type: object

(Note that an NPE may be thrown because there are no paths defined - however the schema should still generate. With paths defined the problem is still exhibited - I just wanted to cut this down to the minimum reproducible case).

Command line used for generation

I used gradle generateSwaggerCodeMapTest using https://github.com/int128/gradle-swagger-generator-plugin

with:

swaggerSources {
	maptest {
		inputFile = file('src/main/resources/map-test.yaml')
		code {
			language = 'java'
		}
	}
}

Steps to reproduce

gradle generateSwaggerCodeMapTest generates the code the same every time.

Related issues/PRs

There appear to be many questions regarding Maps generation, but they change with different versions. It looks like in previous versions StorageNodeFields would extend HashMap.

Suggest a fix/enhancement

Either this is achieved via inheritance (as before, it seems) or composition (include a Map inside StorageNodeFields and add getters/setters to write to it).

Run gradle generateSwaggerCodeMapTest with the provided map-test.yaml and inspect the generated StorageNodeFields model. Trace how the Java generator handles additionalProperties; done means the generated model exposes those properties through a usable Map while retaining its defined fields.