Defining Classes II - Absolute Java - page 317

PITFALL: (continued)

This return statement does not return the reference in the private instance variable born .
Instead, it uses the copy constructor to return a reference to a new object that is an exact
copy of the object named by born . If the copy is changed, that has no effect on the date
whose reference is in the instance variable born . Thus, a privacy leak is avoided.
Note that returning a reference is not the only possible source of privacy leaks.
A privacy leak can also arise from an incorrectly defi ned constructor or mutator
method. Notice the defi nition for the method setBirthDate in Display 5.19 and
reproduced as follows:

leaking
mutator
methods

public void setBirthDate(Date newDate)
{
if (consistent(newDate, died))
born = new Date(newDate);
else
{
System.out.println("Inconsistent dates. Aborting.");
System.exit(0);
}
}

Note that the instance variable born is set to a copy of the parameter newDate .
Suppose that instead of

born = new Date(newDate);
we simply use

born = newDate;

And suppose we use the following code in some program:

Person personObject = new Person(
"Josephine", new Date("January", 1, 2000), null );
Date dateName = new Date("February", 2, 2002);
personObject.setBirthDate(dateName);

where personObject names an object of the class Person . The following will change
the year part of the Date object named by the born instance variable of the object
personObject and will do so without going through the checks in the mutator
methods for Person :

dateName.setYear(1000);

(continued)