How does JSpinner stateChanged works?

Question

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.

Answer 1

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

Answer 2

Just bumped into the same problem and found a different workaround, as the one in https://stackoverflow.com/a/19166589/5326620 caused it to miss the event when first editing the date directly on the text field.

In my case, I'm using a SpinnerDateModel for Calendar.DAY_OF_MONTH (same as Calendar.DATE). If the SpinnerDateModel is initialized with a value precisely at midnight, the event is no longer fired twice.

Calendar now = Calendar.getInstance();
now.set(Calendar.HOUR_OF_DAY, 0);
now.set(Calendar.MINUTE, 0);
now.set(Calendar.SECOND, 0);
now.set(Calendar.MILLISECOND, 0);
Date value = now.getTime();
JSpinner dateSpn = new JSpinner(new SpinnerDateModel(value, null, null, Calendar.DAY_OF_MONTH));

This is probably because the commitEdit of the JFormattedTextField tests the old and new value by equality, and Date equality is on the millisecond.