[JAVA] Definition of HasMap<String, List<Element>>

Description

This issue is about domain class definition. To map some third libraries data, I need to define an object representing a HashMap<String, List>. The generated class Only extends HashMap<String, List> witch breaks the serialization.

Swagger-codegen version

3.0.41

Swagger declaration file content

---
swagger: "2.0"
info
        description: "File to reproduce the issue"
        version: "1.0.0"
        title: "Issue file"
definition:
        MainHashMapObject:
                type: object
                additionalProperties:
                        type: array
                        items:
                                type: object
                                $ref: '#/definitions/HashMapElement'

        HashMapElement:
                type: object
                properties:
                        name:
                                type: string

Command used for generation

io.swagger.codegen.v3.swagger-codegen-maven-plugin

Steps to reproduce

Init a maven project using the swagger gen plugin.

With this set, and the given yaml file, generate java files.

You will get a MainHashMapObject.java defining a class wich extends HashMap<String, List>.

The problem is : for deserialization, this class will generate HashMap<String, List> as it doesn't know about the type of the list's objects.

Suggest a fix/enhancement

Is it possible to always add type paramters to parameterized type in any definitions (fields / extends / implements / function / ...) ?
Eg : List<HashMapElement> insteed of List

Reproduce the issue with the Swagger 2.0 YAML and the io.swagger.codegen.v3.swagger-codegen-maven-plugin at version 3.0.41. Inspect the generation path for MainHashMapObject.java and its parameterized collection types. Done means the generated class preserves HashMap<String, List> so deserialization retains the list element type.