• The class java.lang.Object provides a method called clone.
• The header of this method is:

  public Object clone();
  

• When it is used on an object, it returns a new instance which contains a copy of all the fields of the object.
• So Date's clone just has to return the result of calling Object's clone:

  public class Date
  {
     ...
     public Object clone()
     {
        return super.clone();
     }
  } 
  

• This won't compile: need to write it as:

  public class Date implements Cloneable
  {
     ...
     public Object clone()
     {
        try
        {
           return super.clone();
        }
        catch(final CloneNotSupportedException pCloneNotSupportedException)
        {
           throw new InternalError();
        }
     }
  } 
  

Object's clone method only produces a shallow copy. So, if a class has one or more fields that are of a reference type, we may want to provide a deep copy by cloning these fields. For example, for the Person class, we could provide:

public class Person implements Cloneable
{
   ...
   public Object clone()
   {
      try
      {
         final Person tPerson = (Person)super.clone();
         if (iDateOfBirth!=null)
         {
            tPerson.iDateOfBirth = (Date)iDateOfBirth.clone();
         }
         return tPerson;
      }
      catch(final CloneNotSupportedException pCloneNotSupportedException)
      {
         throw new InternalError();
      }
   }
}