public class ClockDisplay
{
    private NumberDisplay hours;
    private NumberDisplay minutes;
    private String displayString; // simulates the actual display

    /**
     * Constructor for ClockDisplay objects. This constructor
     * creates a new clock set at 00:00.
     */
    public ClockDisplay() {
        hours = new NumberDisplay(24);
        minutes = new NumberDisplay(60);
        updateDisplay();
    }

    /**
     * Constructor for ClockDisplay objects. This constructor
     * creates a new clock set at the time specified by the
     * parameters.
     */
    public ClockDisplay(int hour, int minute) {
        hours = new NumberDisplay(24);
        minutes = new NumberDisplay(60);
        setTime(hour, minute);
    }

    /**
     * This method should get called once every minute - it
     * makes the clock display go one minute forward.
     */
    public void timeTick() {
        minutes.increment();
        if(minutes.getValue() == 0) { // it just rolled over!
            hours.increment();
        }
        updateDisplay();
    }

    /**
     * Set the time of the display to the specified hour and
     * minute.
     */
    public void setTime(int hour, int minute) {
        hours.setValue(hour);
        minutes.setValue(minute);
        updateDisplay();
    }

    /**
     * Return the current time of this display in the format
     * HH:MM.
     */
    public String getTime() {
        return displayString;
    }

    /**
     * Update the internal string that represents the
     * display.
     */
    private void updateDisplay() {
        int hour = hours.getValue();
        String suffix;

        if (hour >= 12) {
            suffix = " pm";
            if (hour > 12) hours.setValue(hour-12);
        } else {
            suffix = " am";
        }

        displayString = hours.getDisplayValue() + ":" +
            minutes.getDisplayValue() + suffix;
    }
}