How does JSpinner stateChanged works?

Why does the code enters twice in the change event of the JSpinner?

private javax.swing.JSpinner spinner = new javax.swing.JSpinner()
spinner.setModel(new javax.swing.SpinnerDateModel());
    spinner.addChangeListener(new javax.swing.event.ChangeListener() {
        @Override
        public void stateChanged(javax.swing.event.ChangeEvent evt) {
            System.out.println("Just a test");
        }
    });

The code above shows the message twice when u click only one time.

2 events are generated: one for the value being deselected and another for the new value being selected in the component. As @camickr notes in his comment this behavior occurs in SpinnerDateModel but not in the default SpinnerNumberModel

As a workaround you could use

spinner.addChangeListener(new ChangeListener() {

    Object lastValue;

    @Override
    public void stateChanged(ChangeEvent evt) {

        if (lastValue != null && !spinner.getValue().equals(lastValue)) {
           // expensive code calls here!
        }
        lastValue = spinner.getValue();
    }
});

This wont prevent the listener being called twice but will prevent any expensive code being invoked unnecessarily