TextField field = ...;
Border b = field.getUnselectedStyle().getBorder();
field.getAllStyles().setBorder(null);
Button clearButton.= ...;
clearButton.addActionListener(e -> {
field.stopEditing();
field.setText("");
});
Container clearable = BorderLayout.centerEastWest(field, clearButton, null);
clearable.getUnselectedStyle().setBorder(b);