Handling Exceptions and Debugging - Beginning Java Programming: The Object-Oriented Approach - page 184

Now when you try to run this code, you will receive a null pointer exception , as shown in
Figure 6-8.
figure 6-8
This looks very similar to the ArithmeticException you saw in the retirement fund examples.
However, here, because you have more than just the main method, there is some further information
about the location where the exception occurred. You can see from the first line the type of excep-
tion and that it occurred during the execution of the main method. After that, it's easier to read
from the bottom up to try to find the exact cause of the error. During the execution, there was an
error at line 7, when the printPerson() method was called. The execution then jumps to the body
of that method and the exception occurred during line 13 of the printPerson() method. In that
line, the only reference was to myPerson.job.JobName , so you know that the job or JobName was
never initialized for the Person object referenced by myPerson . myPerson references employee from
line 7.
One way to avoid this kind of situation is by requiring initialization as part of the constructor.
Previously, you saw empty constructors for Person and JobType . This means that when you create a
Person object, you do not specify a name, age, or job type. Depending on the system you are creating,
there may be reasons to leave some of these empty when you create a new person. For example, if you
create a Person object as soon as you start a job search, you may only know the job type. On the other
hand, if you create a Person object as soon as someone applies to work at your company, you may only
know their name and age, but not which job you will hire them for (if you hire them at all). However,
if you create a Person object precisely when you hire a person for a specific job, you'll have all three
pieces of information at creation. This last situation is implemented here, where all the information
is known and can be initialized in the constructor. In real applications, you may need more than one
constructor to handle different cases, but if you leave some fields null, you will need to handle null
pointer exceptions in other ways. To avoid this, you might decide to create a JobType for every
case, including Interviewee for people who have not yet been hired or NewHire for new employees
who have not been given a specific JobType . Alternatively, you could have a Boolean method hasJob-
Type() check whether the Person already has a JobType assigned and handle these cases as needed.
public class ExceptionExamples {
public static void main(String[] args) {
JobType manager = new JobType("Manager", 6);
Person employee = new Person("Bob Little", 47, manager);