How to extract the object id from a form field?

[Roberto Nibali]

Hi

I'd like to print out the corresponding object id given a specific form
field. How would I do that with PDFBox programmatically?

Let's for the sake of the argument, assume that the form field is
represented by the following obj:

obj 218 0
  <<
    /DA <2B94B0298F2FD7F81F32C6E22043>
    /F 4
    /FT /Tx
    /Ff 4194304
    /MK
    /P 28 0 R
    /Parent 46 0 R
    /Rect [159.781 764.53 347.142 777.195]
    /Subtype /Widget
    /T <5EB6B730886188AB3D3194B9654C18094C>
    /Type /Annot
    /V <45BBBA249C618BBD3974A4BE61501E57181D>
    /AP 666 0 R
  >>

If I am going over all PDField entries of a PDF, how would I get to the
underlying obj number (in the above case 218) from a PDField object?

Best regards
Roberto