A container fires a container event whenever a component is added or removed. 
    // Create a listen for container events
    ContainerListener listener = new ContainerAdapter() {
        public void componentAdded(ContainerEvent evt) {
            // Get component that was added
            Component c = evt.getChild();
        }
        public void componentRemoved(ContainerEvent evt) {
            // Get component that was removed
            Component c = evt.getChild();
        }
    };
    
    // Register the listener with the container
    container.addContainerListener(listener);