setButtonValue

Set a button field (/Btn) to a named export value, the mechanism behind checkboxes and radio groups. exportValue is the on-state name of a widget, or, for a field with /Opt, one of PdfFormField.options, which selects the kid at its index. The field's /V becomes that on-state name; every widget's /AS is set to it when the widget defines it as an appearance state, or to /Off otherwise (so sibling radios in the group are cleared). Pass "Off" to clear the field. A value that names no state throws IllegalArgumentException.

A widget that ships no appearance for the state gets one drawn for it, from its own /MK colours, so a ticked box actually looks ticked.