code: public void put(PdfName key, PdfObject object) {
          if (object == null || object.isNull()) {
              remove(key);
          }
          else {
              hashMap.put(key, object);
          }
      }
docs: Associates the specified <CODE>PdfObject</CODE> as <VAR>value</VAR> with
  the specified <CODE>PdfName</CODE> as <VAR>key</VAR> in this map.
  
  If the map previously contained a mapping for this <VAR>key</VAR>, the
  old <VAR>value</VAR> is replaced. If the <VAR>value</VAR> is
  <CODE>null</CODE> or <CODE>PdfNull</CODE> the key is deleted.
  
  @param key a <CODE>PdfName</CODE>
  @param object the <CODE>PdfObject</CODE> to be associated with the
  <VAR>key</VAR>