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

Default ctor on javax.swing.plaf.multi.MultiButtonUI
Default ctor on javax.swing.plaf.multi.MultiColorChooserUI
Default ctor on javax.swing.plaf.multi.MultiComboBoxUI
Default ctor on javax.swing.plaf.multi.MultiDesktopIconUI
Default ctor on javax.swing.plaf.multi.MultiDesktopPaneUI
Default ctor on javax.swing.plaf.multi.MultiFileChooserUI
Default ctor on javax.swing.plaf.multi.MultiInternalFrameUI
Default ctor on javax.swing.plaf.multi.MultiLabelUI
Default ctor on javax.swing.plaf.multi.MultiListUI
Default ctor on javax.swing.plaf.multi.MultiLookAndFeel
Default ctor on javax.swing.plaf.multi.MultiMenuBarUI
Default ctor on javax.swing.plaf.multi.MultiMenuItemUI
Default ctor on javax.swing.plaf.multi.MultiOptionPaneUI
Default ctor on javax.swing.plaf.multi.MultiPanelUI
Default ctor on javax.swing.plaf.multi.MultiPopupMenuUI
Default ctor on javax.swing.plaf.multi.MultiProgressBarUI
Default ctor on javax.swing.plaf.multi.MultiRootPaneUI
Default ctor on javax.swing.plaf.multi.MultiScrollBarUI
Default ctor on javax.swing.plaf.multi.MultiScrollPaneUI
Default ctor on javax.swing.plaf.multi.MultiSeparatorUI
Default ctor on javax.swing.plaf.multi.MultiSliderUI
Default ctor on javax.swing.plaf.multi.MultiSpinnerUI
Default ctor on javax.swing.plaf.multi.MultiSplitPaneUI
Default ctor on javax.swing.plaf.multi.MultiTabbedPaneUI
Default ctor on javax.swing.plaf.multi.MultiTableHeaderUI
Default ctor on javax.swing.plaf.multi.MultiTableUI
Default ctor on javax.swing.plaf.multi.MultiTextUI
Default ctor on javax.swing.plaf.multi.MultiToolBarUI
Default ctor on javax.swing.plaf.multi.MultiToolTipUI
Default ctor on javax.swing.plaf.multi.MultiTreeUI
Default ctor on javax.swing.plaf.multi.MultiViewportUI