As you maybe know Swing allows you to specify a default button which can be selected by a special key - even if the button isn't owner of the focus. The related button is passed to the JRootPane by calling JRootPane#setDefaultButton(JButton b) just like below.
JButton button = new JButton("My Default Buttton");
frame.setDefaultButton(button);
Synthetica acts just like Metal LaF which means that the key for pressing a regular button is the 'space'-key and the default button is pressed with the 'enter'-key. In Synthetica you can also specify that the default button follows the focus - so each button can be pressed with Enter/Return:
UIManager.setLookAndFeel(...)
UIManager.put("Button.defaultButtonFollowsFocus", true);
Another way to support pressing buttons by Enter or any other key, is to specify an input map for each supported key, just like in the example below.
UIManager.setLookAndFeel(...);
UIManager.put("Button.focusInputMap", new UIDefaults.LazyInputMap(new Object[]
{
"SPACE", "pressed",
"released SPACE", "released",
"ENTER", "pressed",
"released ENTER", "released"
}));
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