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