Accessing private variable inside a library class

I am trying to get an array from inside a library class, but the field is set as private :((

I used a java decompiler website to see this source of code:

public class MultiMarker
 extends NyARPsgBaseClass {
 protected PImageSensor _ss;
 protected NyARMarkerSystem _ms;
 public static final double DEFAULT_CF_THRESHOLD = 0.51;
 public static final int DEFAULT_LOST_DELAY = 10;
 public static final int THLESHOLD_AUTO = -1;
 private ArrayList<Integer> _id_map = new ArrayList();  //<<------- I WANT TO ACCESS THIS
 private final PMatrix3D _ps_projection = new PMatrix3D();
 private boolean _is_in_begin_end_session = false;

So this is what I had tried following some examples using Java as reference but still variable is NOT VISIBLE. maybe some casting problem...

import java.lang.reflect.*;
import jp.nyatla.nyar4psg.*;

MultiMarker nya;
void setup() {
  size(640,360);
  nya=new MultiMarker(this, 640, 360, "camera_para.dat", NyAR4PsgConfig.CONFIG_PSG);
  nya.addARMarker("patt.hiro", 80);//id=0
  nya.addARMarker("patt.kanji", 80);//id=1

  try {
      MultiMarker.class.getDeclaredField("_id_map").setAccessible(true);
      //or
      //Field idmap = MultiMarker.class.getDeclaredField("_id_map");
      //idmap.setAccessible(true);
      // ArrayList<Integer> castTest = idmap.get( ArrayList<Integer>() ); //--error
      // ArrayList<Integer> castTest = (ArrayList<Integer>)idmap.get(idmap); //--error

  }
    catch (ReflectiveOperationException e) {
    throw new RuntimeException(e);
  }

  println(nya._id_map.size()); //field is not visible
}