Buenas, estoy leyendo el libro de "Programación orientada a objeto con java" y proponen ejercicios pero no dan la solución a muchos de ellos.

Uno de estos ejercicios es convertir un clase preparada para un reloj de 24 horas para que funcione como un reloj de 12 horas... y soy incapza.... Me podríais echar una mano?

son dos clases:

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

public class NumberDisplay
{
private int limit;
private int value;

/**
* Constructor for objects of class NumberDisplay.
* Set the limit at which the display rolls over.
*/
public NumberDisplay(int rollOverLimit)
{
limit = rollOverLimit;
value = 0;
}

/**
* Return the current value.
*/
public int getValue()
{
return value;
}

/**
* Return the display value (that is, the current value as a two-digit
* String. If the value is less than ten, it will be padded with a leading
* zero).
*/
public String getDisplayValue()
{
if(value < 10) {
return "0" + value;
}
else {
return "" + value;
}
}

/**
* Set the value of the display to the new specified value. If the new
* value is less than zero or over the limit, do nothing.
*/
public void setValue(int replacementValue)
{
if((replacementValue >= 0) && (replacementValue < limit)) {
value = replacementValue;
}
}

/**
* Increment the display value by one, rolling over to zero if the
* limit is reached.
*/
public void increment()
{
value = (value + 1) % limit;
}
}

Muchísimas gracias

Mu has gracias por tu respuesta