The need to provide a cloning operation

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

When creating a Person, we might let a client supply a Date that is the person's date-of-birth:

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

where Person is as follows:

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

Here the Person object is sharing the Date supplied by the client.
If Date provides mutable objects, this may be undesirable.

Instead of sharing the Date with the client, the Person object may prefer to have its own copy.
Java's Core APIs use two ways of producing a copy of an object:
- a class sometimes provides a method called clone;
- a class sometimes provides a suitable constructor.
So, if Date provided clone, the Person constructor could use:
```
      iDateOfBirth = (Date)pDateOfBirth.clone();  // clone
```
Or, if Date provided a cloning constructor, the Person constructor could use:
```
      iDateOfBirth = new Date(pDateOfBirth);      // clone
```
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.