glazedlists

You need to change the Worker class so that it accepts List<? extends Orderable> public class Worker { private List<? extends Orderable> workingList; public void setList(List<? extends Orderable> value) {this.workingList=value;} public void changePlaces(Orderable o1,Orderable o2) { // implementation that make o1.order=o2.order and vice verca } }