public Field getField(String name) {
        if (name.contains(".")) {
            String superFieldName = name.substring(0,name.indexOf("."));
            String subFieldName = name.substring(name.indexOf(".")+1);
            Field f = docType.getField(superFieldName);
            if (f != null) {
                if (f instanceof SDField superField) {
                    return superField.getStructField(subFieldName);
                } else {
                    throw new IllegalArgumentException("Field " + f.getName() + " is not an SDField");
                }
            }
        }
        Field f = docType.getField(name);
        if (f == null) {
            for(SDDocumentType parent : inheritedTypes.values()) {
                f = parent.getField(name);
                if (f != null) return f;
            }
        }
        return f;
    }
Override getField, as it may need to ask inherited types that isn't registered in document type.
config-model/src/main/java/com/yahoo/schema/document/SDDocumentType.java