/**
* Compare
*
* @param o1 object 1
* @param o2 object 2
* @return -1,0, 1
*/
@Override
public int compare(PO o1, PO o2) {
String s1 = o1.toString();
if (s1 == null) s1 = "";
String s2 = o2.toString();
if (s2 == null) s2 = "";
return s1.compareTo(s2);
} // compare
