Several hundred classes in the java.desktop module reply on default constructors as part of their public API. 

This is for the Swing APIs that do not fall under the plaf package.
Default ctor on javax.swing.AbstractButton 
Default ctor on javax.swing.AbstractCellEditor 
Default ctor on javax.swing.AbstractListModel 
Default ctor on javax.swing.AbstractSpinnerModel 
Default ctor on javax.swing.DefaultDesktopManager 
Default ctor on javax.swing.DefaultListCellRenderer.UIResource 
Default ctor on javax.swing.DefaultListModel 
Default ctor on javax.swing.DefaultListSelectionModel 
Default ctor on javax.swing.DefaultSingleSelectionModel 
Default ctor on javax.swing.FocusManager 
Default ctor on javax.swing.InputVerifier 
Default ctor on javax.swing.InternalFrameFocusTraversalPolicy 
Default ctor on javax.swing.JFormattedTextField.AbstractFormatter 
Default ctor on javax.swing.JFormattedTextField.AbstractFormatterFactory 
Default ctor on javax.swing.LookAndFeel 
Default ctor on javax.swing.MenuSelectionManager 
Default ctor on javax.swing.PopupFactory 
Default ctor on javax.swing.RowFilter 
Default ctor on javax.swing.ScrollPaneLayout 
Default ctor on javax.swing.ScrollPaneLayout.UIResource 
Default ctor on javax.swing.UIManager 
Default ctor on javax.swing.ViewportLayout 
Default ctor on javax.swing.border.AbstractBorder 
Default ctor on javax.swing.colorchooser.AbstractColorChooserPanel 
Default ctor on javax.swing.event.EventListenerList 
Default ctor on javax.swing.event.InternalFrameAdapter 
Default ctor on javax.swing.event.MouseInputAdapter 
Default ctor on javax.swing.filechooser.FileFilter 
Default ctor on javax.swing.filechooser.FileView 
Default ctor on javax.swing.table.AbstractTableModel 
Default ctor on javax.swing.table.DefaultTableCellRenderer.UIResource 
Default ctor on javax.swing.table.TableStringConverter 
Default ctor on javax.swing.text.DefaultTextUI 
Default ctor on javax.swing.text.DocumentFilter 
Default ctor on javax.swing.text.DocumentFilter.FilterBypass 
Default ctor on javax.swing.text.FlowView.FlowStrategy 
Default ctor on javax.swing.text.GlyphView.GlyphPainter 
Default ctor on javax.swing.text.LayeredHighlighter 
Default ctor on javax.swing.text.LayeredHighlighter.LayerPainter 
Default ctor on javax.swing.text.NavigationFilter 
Default ctor on javax.swing.text.NavigationFilter.FilterBypass 
Default ctor on javax.swing.text.Utilities 
Default ctor on javax.swing.text.html.HTML 
Default ctor on javax.swing.text.html.HTMLDocument.HTMLReader.BlockAction 
Default ctor on javax.swing.text.html.HTMLDocument.HTMLReader.CharacterAction 
Default ctor on javax.swing.text.html.HTMLDocument.HTMLReader.FormAction 
Default ctor on javax.swing.text.html.HTMLDocument.HTMLReader.HiddenAction 
Default ctor on javax.swing.text.html.HTMLDocument.HTMLReader.IsindexAction 
Default ctor on javax.swing.text.html.HTMLDocument.HTMLReader.ParagraphAction 
Default ctor on javax.swing.text.html.HTMLDocument.HTMLReader.PreAction 
Default ctor on javax.swing.text.html.HTMLDocument.HTMLReader.SpecialAction 
Default ctor on javax.swing.text.html.HTMLDocument.HTMLReader.TagAction 
Default ctor on javax.swing.text.html.HTMLDocument.Iterator 
Default ctor on javax.swing.text.html.HTMLEditorKit.HTMLFactory 
Default ctor on javax.swing.text.html.HTMLEditorKit.LinkController 
Default ctor on javax.swing.text.html.HTMLEditorKit.Parser 
Default ctor on javax.swing.text.html.HTMLEditorKit.ParserCallback 
Default ctor on javax.swing.tree.AbstractLayoutCache 
Default ctor on javax.swing.tree.AbstractLayoutCache.NodeDimensions 
Default ctor on javax.swing.undo.CannotRedoException 
Default ctor on javax.swing.undo.CannotUndoException 
Default ctor on jdk.swing.interop.SwingInterOpUtils