Question

How can I update the state (enable/disable) of my button when the user changes the selected item in a combo box.?

The button has a reference to the combo box, but the combo box does not know anything about the button.

Answer 1

If the button has a reference to the combo box, then the button can register an action listener at the combo box in which you can change the state of your button.

final JButton button = new JButton();
final JComboBox comboBox = new JComboBox();

comboBox.addActionListener( new ActionListener() {
    @Override
    public void actionPerformed( final ActionEvent event ) {
        // Your logic to determine when to enable/disable:
        final boolean enabled = comboBox.getSelectedIndex() == 0;
        button.setEnabled( enabled );
    }
} );