I want to show my own component(s) in the main window - where do I start?

Use File -> New File wizard, Module Development category and Window Component item. It will generate all necessary background code for you and open GUI Builder to design UI of your own component.

But what is behind the scenes, created by wizard?

• Subclass of TopComponent is created. Design it just as you would a JPanel.
• Simple subclass of javax.swing.AbstractAction in created and added to main menu. It's implemented it to open your component like follows:

public class MyAction extends AbstractAction {
   public MyAction() {
      putValue (Action.NAME, "Open My Component");
   }

   public void actionPerformed(ActionEvent ae) {
      new MyComponent().open();
   }
}

• Declarative XML descriptions of component are created following declarative XML API principles.

TopComponents are part of the Windows API.