Complicated Inheritance Problems

Question

Ok, some of you may be getting familiar with these classes :P
Basically;
public class CentredDictionaryWindow extends JFrame
public class welcomeScreenFrame extends CentredDictionaryWindow
public class searchWindowFrame extends CentredDictionaryWindow
public class resultsFrame extends CentredDictionaryWindowNow, searchWindowFrame searches a set of words in a dictionary based on your input, which it then passes to resultsFrame to display.
I have a JMenuBar that I created in CentredDictionaryWindow as well as a JPopupMenu, so that all the screens have it.
One of the options is save. To do this, CentredDictionaryWindow needs the results.
In searchWindowFrame and welcomeScreenFrame I've used
super(null)So they don't need to pass anything in.
resultsFrame uses
super(results)and passes in a TreeSet containing all the results. This seems logically sound, and I've put
if (results != null)
     resultList.addAll(results);which again, should make sense.
However, when I press 'Search' on searchWindowFrame, the end of the code for which is here: (the beginning simply performs the search, that's all fine)
if (operatorSearchFour.size() > 0 && !failedQuery)
     JFrame loading = new loadingWindow();
     setVisible(false);
     loading.show();
     TreeSet searchResults = new TreeSet();
     searchResults.addAll(operatorSearchFour);
     resultsFrame resultFrame = new resultsFrame(searchResults, this, parentWindow);  //(results, parent, home)
     loading.setVisible(false);
     resultFrame.show();
     if (operatorSearchFour.size() == 0)
          JOptionPane.showMessageDialog(new JFrame(), "The Query you entered returned no results." + "\n" + "Please try again.", "Zero Results", JOptionPane.INFORMATION_MESSAGE);I get a null pointer exception on the second line of
if (results != null)
     resultList.addAll(results);Is there something I'm doing wrong? (don't know how more code examples could help none of the rest really has anything to do with this but if you want it, ask) or is there an easier way of doing this? (While still being able to leave the menu in CentredDictionaryWindow - hopefully - because I know I'll lose marks for repeated code otherwise). I can't figure it out at all, especially since the set of results resultsFrame sends in definately isn't null.

Answer 1

if (results != null)
     resultList.addAll(results);To be getting a null pointer here I'm pretty sure resultList == null. Are you initialising this beforehand?