Explorer views are generic Swing components, not subclasses of TopComponent, the Swing panel class that is used for top level components (tabs) in the main window. So an explorer view component is added to a TopComponent, using the TopComponent as a Swing container for the view.

A little bit of plumbing is needed to wire up an explorer view to the global Node selection so that code that is sensitive to selection such as context sensitive actions. Basically you want the TopComponent to expose the selection in your Explorer View so that when your view has focus, the global selection that affects everything will be whatever the user selects in your view.

In olden times, there was a convenient class called ExplorerPanel (now in org.openide.compat) which would do this for you; due to a tragedy of being in the wrong package, it is now deprecated, but the required plumbing is not hard:

 public class MyView extends TopComponent implements ExplorerManager.Provider {
    private final ExplorerManager manager = new ExplorerManager();
    private final JComponent view = new BeanTreeView();
    public MyView() {
        setLayout (new BorderLayout());
        add(view, BorderLayout.CENTER);
        manager.setRootContext(someNode);

        // Probably boilerplate (depends on what you are doing):
        ActionMap map = getActionMap();
        map.put(DefaultEditorKit.copyAction, ExplorerUtils.actionCopy(manager));
        map.put(DefaultEditorKit.cutAction, ExplorerUtils.actionCut(manager));
        map.put(DefaultEditorKit.pasteAction, ExplorerUtils.actionPaste(manager));
        // This one is sometimes changed to say "false":
        map.put("delete", ExplorerUtils.actionDelete(manager, true));
        // Boilerplate:
        associateLookup(ExplorerUtils.createLookup(manager, map));
    }
    // This is optional:
    public boolean requestFocusInWindow() {
        super.requestFocusInWindow();
        // You will need to pick a view to focus:
        return view.requestFocusInWindow();
    }
    // The rest is boilerplate.
    public ExplorerManager getExplorerManager() {
        return manager;
    }
    protected void componentActivated() {
        ExplorerUtils.activateActions(manager, true);
    }
    protected void componentDeactivated() {
        ExplorerUtils.activateActions(manager, false);
    }
}

The primary difference between the above code and ExplorerPanel is that ExplorerPanel automagically persisted paths from the selected nodes to the root, so that it could be deserialized on restart with the same selection it had before shutdown (assuming that selection still existed - this was never terribly robust).

Source: NetBeans FAQ