Creating Map without fields/properties

components:
    schemas:
        example-file:
            type: object
            additionalProperties:
                type: object

I am trying to create a Map, the documentation says that I can create a Map using something like this but it does not work.

Something like this but without properties:

public class ExampleFile extends HashMap<String, Object> {

I could create a Map like this but with properties, something like this:

example-file:
    properties:
        object_key:
            type: string
    additionalProperties:
        type: object

I need it without fields/properties.