@Override
	protected final JScrollPane initCenter(
			final Map<Integer, DragObject<JPanel, JPanel, JPanel>> trackList) {
		final JPanel panel = new JPanel();
		final TreeSet<DropTarget<JPanel, JPanel, JPanel>> set = new TreeSet<>();
		for (final DragObject<JPanel, JPanel, JPanel> o : trackList.values()) {
			for (final DropTarget<JPanel, JPanel, JPanel> t : o) {
				if (t != this.state.emptyTarget) {
					set.add(t);
				}
			}
		}
		panel.setLayout(new GridLayout(0, 1));
		for (final DropTarget<JPanel, JPanel, JPanel> t : set) {
			initTarget(t);
			panel.add(t.getDisplayableComponent());

		}
		this.center = new JScrollPane(panel);
		return this.center;
	}