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


Default ctor on javax.swing.plaf.synth.SynthButtonUI
Default ctor on javax.swing.plaf.synth.SynthCheckBoxMenuItemUI
Default ctor on javax.swing.plaf.synth.SynthCheckBoxUI
Default ctor on javax.swing.plaf.synth.SynthColorChooserUI
Default ctor on javax.swing.plaf.synth.SynthComboBoxUI
Default ctor on javax.swing.plaf.synth.SynthDesktopIconUI
Default ctor on javax.swing.plaf.synth.SynthDesktopPaneUI
Default ctor on javax.swing.plaf.synth.SynthEditorPaneUI
Default ctor on javax.swing.plaf.synth.SynthFormattedTextFieldUI
Default ctor on javax.swing.plaf.synth.SynthLabelUI
Default ctor on javax.swing.plaf.synth.SynthListUI
Default ctor on javax.swing.plaf.synth.SynthMenuBarUI
Default ctor on javax.swing.plaf.synth.SynthMenuItemUI
Default ctor on javax.swing.plaf.synth.SynthMenuUI
Default ctor on javax.swing.plaf.synth.SynthOptionPaneUI
Default ctor on javax.swing.plaf.synth.SynthPainter
Default ctor on javax.swing.plaf.synth.SynthPanelUI
Default ctor on javax.swing.plaf.synth.SynthPasswordFieldUI
Default ctor on javax.swing.plaf.synth.SynthPopupMenuUI
Default ctor on javax.swing.plaf.synth.SynthProgressBarUI
Default ctor on javax.swing.plaf.synth.SynthRadioButtonMenuItemUI
Default ctor on javax.swing.plaf.synth.SynthRadioButtonUI
Default ctor on javax.swing.plaf.synth.SynthRootPaneUI
Default ctor on javax.swing.plaf.synth.SynthScrollBarUI
Default ctor on javax.swing.plaf.synth.SynthScrollPaneUI
Default ctor on javax.swing.plaf.synth.SynthSeparatorUI
Default ctor on javax.swing.plaf.synth.SynthSpinnerUI
Default ctor on javax.swing.plaf.synth.SynthSplitPaneUI
Default ctor on javax.swing.plaf.synth.SynthTabbedPaneUI
Default ctor on javax.swing.plaf.synth.SynthTableHeaderUI
Default ctor on javax.swing.plaf.synth.SynthTableUI
Default ctor on javax.swing.plaf.synth.SynthTextAreaUI
Default ctor on javax.swing.plaf.synth.SynthTextFieldUI
Default ctor on javax.swing.plaf.synth.SynthTextPaneUI
Default ctor on javax.swing.plaf.synth.SynthToggleButtonUI
Default ctor on javax.swing.plaf.synth.SynthToolBarUI
Default ctor on javax.swing.plaf.synth.SynthToolTipUI
Default ctor on javax.swing.plaf.synth.SynthTreeUI
Default ctor on javax.swing.plaf.synth.SynthViewportUI