Json – Creatina new structure with _family object in FHIR

How can get in Java this structure about FHIR?

{
  "resourceType": "Patient",
  "name": [
    {
      "family": "Family1-Family2",
      "_family": {
        "extension": [
          {
            "url": "http://hl7.org/fhir/StructureDefinition/humanname-mothers-family",
            "valueString": "Family2"
          }
        ]
      }
      "given": ["Names"],
    }
  ]
}

I try with

HumanName name = HumanName();
name.setFamilyElement(_family).addExtension(ext);

But this instruction rename mi obeject name.Family to _family and not generate extensión into _family.

I tried with

HumanName name = HumanName();
Extensión ext = new Extensión() ......
......
name.setFamilyElement(_family).addExtension(ext);

But this instruction rename my object name.Family to _family and not generate extensión into _family.

I try with name.addChild(_family), but dosent work.

Some idea?