Several hundred classes in the java.desktop module reply on default constructors as part of their public API.
This is for javax.swing.plaf APIs
Default ctor on javax.swing.plaf.ButtonUI
Default ctor on javax.swing.plaf.ColorChooserUI
Default ctor on javax.swing.plaf.ComboBoxUI
Default ctor on javax.swing.plaf.DesktopIconUI
Default ctor on javax.swing.plaf.DesktopPaneUI
Default ctor on javax.swing.plaf.FileChooserUI
Default ctor on javax.swing.plaf.InternalFrameUI
Default ctor on javax.swing.plaf.LabelUI
Default ctor on javax.swing.plaf.LayerUI
Default ctor on javax.swing.plaf.ListUI
Default ctor on javax.swing.plaf.MenuBarUI
Default ctor on javax.swing.plaf.MenuItemUI
Default ctor on javax.swing.plaf.OptionPaneUI
Default ctor on javax.swing.plaf.PanelUI
Default ctor on javax.swing.plaf.PopupMenuUI
Default ctor on javax.swing.plaf.ProgressBarUI
Default ctor on javax.swing.plaf.RootPaneUI
Default ctor on javax.swing.plaf.ScrollBarUI
Default ctor on javax.swing.plaf.ScrollPaneUI
Default ctor on javax.swing.plaf.SeparatorUI
Default ctor on javax.swing.plaf.SliderUI
Default ctor on javax.swing.plaf.SpinnerUI
Default ctor on javax.swing.plaf.SplitPaneUI
Default ctor on javax.swing.plaf.TabbedPaneUI
Default ctor on javax.swing.plaf.TableHeaderUI
Default ctor on javax.swing.plaf.TableUI
Default ctor on javax.swing.plaf.TextUI
Default ctor on javax.swing.plaf.ToolBarUI
Default ctor on javax.swing.plaf.ToolTipUI
Default ctor on javax.swing.plaf.TreeUI
Default ctor on javax.swing.plaf.ViewportUI