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= DevFaqWindowsTopComponent
:jbake-type: wiki
:jbake-tags: wiki, devfaq, needsreview
:jbake-status: published
:keywords: Apache NetBeans wiki DevFaqWindowsTopComponent
:description: Apache NetBeans wiki DevFaqWindowsTopComponent
:toc: left
:toc-title:
:syntax: true

=== What is a TopComponent in the Window System?

[link:http://bits.netbeans.org/dev/javadoc/org-openide-windows/org/openide/windows/TopComponent.html[http://bits.netbeans.org/dev/javadoc/org-openide-windows/org/openide/windows/TopComponent.html] `org.openide.windows.TopComponent`] is a `JComponent` subclass which knows how to work with
the NetBeans window system.  If you want to add components to the main window, typically you will subclass TopComponent, using it the same way you would a `JPanel`.

TopComponents live inside link:DevFaqWindowsMode.asciidoc[Mode]s - docking containers.

TopComponents can have various states:

* *open/closed* - When a TopComponent is _open_, a tab exists somewhere in the main window or UI for it
* *visible/invisible* - When a TopComponent is _visible_, it is the selected tab in its link:DevFaqWindowsMode.asciidoc[Mode]
* *active/inactive* - When a TopComponent is _active_, it or a child component of it has keyboard focus, and it is currently the global link:DevFaqTrackGlobalSelection.asciidoc[selection context]

Each `TopComponent` has a `link:DevFaqLookup.asciidoc[Lookup]` and one or more `link:DevFaqWhatIsANode.asciidoc[activated Node]`s.  These define the link:DevFaqTrackGlobalSelection.asciidoc[selection context] for the window system, which determines what actions (menu items, toolbar buttons, etc.) are enabled, and in some cases, what they will do or operate on if invoked.

`TopComponent`s are part of the link:http://bits.netbeans.org/dev/javadoc/org-openide-windows/org/openide/windows/doc-files/api.html[Windows API].

TopComponents that were already open may be deserialized and reopened on restart.  The template that NetBeans 5.0 provides includes code for this - it is actually using saved using the Externalizable interface.  Whether or not it is saved is determined by what you return from `getPersistenceType()`.

For information on how and when these are reconstructed on startup, see the link:DevFaqWindowsInternals.asciidoc[gory details of the window system on startup].

=== Apache Migration Information

The content in this page was kindly donated by Oracle Corp. to the
Apache Software Foundation.

This page was exported from link:http://wiki.netbeans.org/DevFaqWindowsTopComponent[http://wiki.netbeans.org/DevFaqWindowsTopComponent] , 
that was last modified by NetBeans user Tboudreau 
on 2010-02-25T16:18:58Z.


*NOTE:* This document was automatically converted to the AsciiDoc format on 2018-02-07, and needs to be reviewed.