Defining Classes II - Absolute Java - page 318

PITFALL: (continued)

Because dateName contains the same reference as the private instance variable born
of the object personObject , changing the year part of dateName changes the year
part of the private instance variable born of personObject . Not only does this bypass
the consistency checks in the mutator method setBirthDate , but it also is a likely
source of an inadvertent change to the born instance variable.
If we defi ne setBirthDate as we did in Display 5.19 and as shown in the follow-
ing, this problem does not happen. (If you do not see this, go through the code step
by step and trace what happens.)

public void setBirthDate(Date newDate)
{
if (consistent(newDate, died))
born = new Date(newDate);
. . .

One final word of warning: Using copy constructors in this manner is not the
officially sanctioned way to make copies of an object in Java. The authorized way to is
to define a method named clone . We will discuss clone methods in Chapters 8 and 13 .
In Chapter 8, we show you that, in some situations, there are advantages to using a
clone method instead of a copy constructor. In Chapter 13, we describe the official way
to define the clone method. For what we will be doing until then, a copy constructor
will be a very adequate way of creating copies of an object.

clone

Self Test Exercises

38. What is a copy constructor?
39. What output is produced by the following code?

Date date1 = new Date("January", 1, 2006);
Date date2;
date2 = date1;
date2.setDate("July", 4, 1776);
System.out.println(date1);
What output is produced by the following code? Only the third line is different
from the previous case.

Date date1 = new Date("January", 1, 2006);
Date date2;
date2 = new Date(date1);
date2.setDate("July", 4, 1776);
System.out.println(date1);