Few books explain that it is desirable for a class to provide a cloning operation.

Date tBirthDate = new DateImpl(2000, 1, 24);
Person tSomePerson = new PersonImpl("Joe", tBirthDate);

Do we share?

public class PersonImpl
{
   private String iName;
   private Date iDateOfBirth;
   public PersonImpl(final String pName,
                     final Date pDateOfBirth)
   {
      iName = pName;
      iDateOfBirth = pDateOfBirth;                // share
      ...

Or do we clone?

      iDateOfBirth = new DateImpl(pDateOfBirth);  // clone

Note that you need not do this copying if the objects of the class (in this case, the class DateImpl) are immutable.

It is best to provide a method called clone as this can be used when inheritance is involved. However, getting the code of a clone method completely right is difficult. See the article by Bill Venners at http://www.artima.com/innerjava/webuscript/cloning.html

Because it is hard to get right, it is also difficult to teach. Instead, I cheat by just providing a constructor that can be used for cloning:

public DateImpl(final Date pDate)
{
   iYear = pDate.getYear();
   iMonth = pDate.getMonth();
   iDay = pDate.getDay(); 
}