Defining Classes II - Absolute Java - page 315

This version, which we did use, makes the instance variable this.born an independent
Date object that represents the same date as original.born . So if you change a date
in the Person object created by the copy constructor, you will not change that date in
the original Person object.
Note that if a class, such as Person , has instance variables of a class type, such as the
instance variables born and died , then to define a correct copy constructor for the class
Person , you must already have copy constructors for the class Date of the instance
variables. The easiest way to ensure this for all your classes is to always include a copy
constructor in every class you define.

Copy Constructor

A copy constructor is a constructor with one parameter of the same type as the class.
A copy constructor should be designed so the object it creates is intuitively an exact copy of
its parameter, but a completely independent copy. See Displays 5.19 and 5.20 for examples
of copy constructors.

clone

The Java documentation says to use a method named clone instead of a copy
constructor, and, as you will see later in this topic, there are situations where the copy
constructor will not work as desired and you need the clone method. However, we
do not yet have enough background to delve into this method.(It is discussed later
in this topic in Chapters 8 and 13 .) Despite the Java documentation, many excellent
programmers prefer to sometimes use copy constructors. In this topic, we will use both
copy constructors and the clone method.

PITFALL: Privacy Leaks

Consider the accessor method getBirthDate for the class Person ( Display 5.19 ),
which we reproduce in what follows: leaking

accessor
methods

public Date getBirthDate()
{
return new Date(born);
}
Do not make the mistake of defining the accessor method as follows:

public Date getBirthDate() //Unsafe
{
return born; //Not good
}

(continued)