[rules-users] problem with drools

hi, im new to drools..im trying to create a project as follows. i have a class called Monitor which monitors a folder and creates two lists called fileOld( which is the list of filenames in that folder at time t1) and fileNew(which is the list of filenames in that folder at time t2). i have another class called FileData which contains two members fileOld and fileNew (list of strings) with getters,setters and constructor. fileOld and fileNew from Monitor Class are passed to FileData class. now i have to compare these two lists in a rule file and if they are not equal i have to print the lists of files added/ deleted. how to do this in drools. here is my rule file: rule "files are equal" when FileData(fileOld == fileNew) then System.out.println("files are equal"); end rule "files not equal" when not FileData(fileOld == fileNew) then System.out.println("files are not equal"); difference(FileData.getFileOld(),FileData.getFileNew()); end function void difference(List fileOld, List fileNew) { ArrayList<String> add = new ArrayList<String>(fileNew); add.removeAll(fileOld); System.out.println("files Added: " + add); ArrayList<String> remove = new ArrayList<String>(fileOld); remove.removeAll(fileNew); System.out.println("files Removed: " + remove); } but this displays the following error... Rule Compilation error : [Rule name='files not equal'] Cannot make a static reference to the non-static method getFileOld() from the type FileData Cannot make a static reference to the non-static method getFileNew() from the type FileData what can be done??