Defining Classes II - Java - page 303

This version, which we did use, makes the instance variable this.born an indepen-
dent 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 exam-
ples of copy constructors.

The Java documentation says to use a method named clone instead of a copy con-
structor, and, as you will see later in this topic, there are situations where the copy con-
structor will not work as desired and you need the clone method. However, we do not
yet have enough background to discuss the clone method. The clone method is dis-
cussed later in this topic (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.

clone

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)