Several hundred classes in the java.desktop module reply on default constructors as part of their public API.
This is for javax.swing.plaf.basic APIs

Default ctor on javax.swing.plaf.basic.BasicLabelUI
Default ctor on javax.swing.plaf.basic.BasicListUI
Default ctor on javax.swing.plaf.basic.BasicListUI.FocusHandler
Default ctor on javax.swing.plaf.basic.BasicListUI.ListDataHandler
Default ctor on javax.swing.plaf.basic.BasicListUI.ListSelectionHandler
Default ctor on javax.swing.plaf.basic.BasicListUI.MouseInputHandler
Default ctor on javax.swing.plaf.basic.BasicListUI.PropertyChangeHandler
Default ctor on javax.swing.plaf.basic.BasicLookAndFeel
Default ctor on javax.swing.plaf.basic.BasicMenuBarUI
Default ctor on javax.swing.plaf.basic.BasicMenuItemUI
Default ctor on javax.swing.plaf.basic.BasicMenuUI
Default ctor on javax.swing.plaf.basic.BasicOptionPaneUI
Default ctor on javax.swing.plaf.basic.BasicOptionPaneUI.PropertyChangeHandler
Default ctor on javax.swing.plaf.basic.BasicPanelUI
Default ctor on javax.swing.plaf.basic.BasicPasswordFieldUI
Default ctor on javax.swing.plaf.basic.BasicPopupMenuSeparatorUI
Default ctor on javax.swing.plaf.basic.BasicProgressBarUI
Default ctor on javax.swing.plaf.basic.BasicProgressBarUI.ChangeHandler
Default ctor on javax.swing.plaf.basic.BasicRadioButtonMenuItemUI
Default ctor on javax.swing.plaf.basic.BasicRadioButtonUI
Default ctor on javax.swing.plaf.basic.BasicRootPaneUI
Default ctor on javax.swing.plaf.basic.BasicScrollBarUI
Default ctor on javax.swing.plaf.basic.BasicScrollBarUI.PropertyChangeHandler
Default ctor on javax.swing.plaf.basic.BasicScrollPaneUI
Default ctor on javax.swing.plaf.basic.BasicScrollPaneUI.HSBChangeListener
Default ctor on javax.swing.plaf.basic.BasicScrollPaneUI.PropertyChangeHandler
Default ctor on javax.swing.plaf.basic.BasicScrollPaneUI.VSBChangeListener
Default ctor on javax.swing.plaf.basic.BasicScrollPaneUI.ViewportChangeHandler
Default ctor on javax.swing.plaf.basic.BasicSeparatorUI
Default ctor on javax.swing.plaf.basic.BasicSliderUI.ChangeHandler
Default ctor on javax.swing.plaf.basic.BasicSliderUI.ComponentHandler
Default ctor on javax.swing.plaf.basic.BasicSliderUI.FocusHandler
Default ctor on javax.swing.plaf.basic.BasicSliderUI.PropertyChangeHandler
Default ctor on javax.swing.plaf.basic.BasicSliderUI.TrackListener
Default ctor on javax.swing.plaf.basic.BasicSpinnerUI
Default ctor on javax.swing.plaf.basic.BasicSplitPaneUI
Default ctor on javax.swing.plaf.basic.BasicSplitPaneUI.FocusHandler
Default ctor on javax.swing.plaf.basic.BasicSplitPaneUI.KeyboardDownRightHandler
Default ctor on javax.swing.plaf.basic.BasicSplitPaneUI.KeyboardEndHandler
Default ctor on javax.swing.plaf.basic.BasicSplitPaneUI.KeyboardHomeHandler