### Question:
FileSystemModel implements TreeModel { @Override public void addTreeModelListener(final TreeModelListener listener) { listeners.add(listener); } FileSystemModel(final File rootDirectory); @Override Object getRoot(); @Override Object getChild(final Object parent, final int index); @Override int getChildCount(final Object parent); @Override boolean isLeaf(final Object node); @Override int getIndexOfChild(final Object parent, final Object child); @Override void valueForPathChanged(final TreePath path, final Object value); @Override void addTreeModelListener(final TreeModelListener listener); @Override void removeTreeModelListener(final TreeModelListener listener);  }### Answer:
@Test public void testAddTreeModelListener() { System.out.println("addTreeModelListener"); TreeModelListener listener = null; FileSystemModel instance = null; }