Identifiers, Keywords, and Types
Objectives
Upon completion of this lab, you should be able to:
- Explore reference variable assignment
- Use a reference variable to encode an object association
Exercise 1: Investigating Reference Assignment
In this exercise, you will investigate reference variables, object
creation, and reference variable assignment.
Figure 3-1 shows a class diagram for the MyPoint class that
is provided in the exercise directory. Notice that the instance
variables, x
and y,
are both public so you can access these data members in your test
program directly. Also, the toString
method is used when you print the object using the System.out.println
method.
| MyPoint |
+ x : int
+ y : int |
| +
toString() : String |
Figure 3-1
UML Class Diagram for the MyPoint
Class
Your task is to create a test program that explores object references.
Preparation
Before
you begin, make sure that you have changed directories to exercises/mod03_types/exercise1/
using the cd
command in the terminal window.
cd
~/exercises/mod03_types/exercise1/
Task
1 - Creating the TestMyPoint
Program
Using
a text editor, create a TestMyPoint
program that does the following:
- Declare
two variables of type MyPoint
and called start
and end.
Assign both of these variables a new MyPoint
object.
- Set
the x
and y
values of start
to 10. Set the x
value of end
to 20 and the y
value to 30
- Print
out both point variables. Use code similar to:
System.out.println("Start
point is" +start);
To
make sure that you are using the MyPoint
class correctly, you might want to compile and run TestMyPoint
now (see
"Task 2 - Compiling the TestMyPoint
Program" and
"Task 3 - Running the TestMyPoint
Program"). If
you do so, the output will look something like:
Start
point is [10,10]
End
point is [20,30]
- Declare
a new variable of type MyPoint
and call it stray.
Assign stray
the
reference value of the existing variable end.
- Print
out stray
and end.
- Assign
new values to the x (such
as 47) and y
(such as 50) members of the variable stray.
- Print
out stray, end
and start.
Task
2 - Compiling the TestMyPoint
Program
On
the command line, use the <tt>javac</tt>
command to compile the test program.
Task
2 - Running the TestMyPoint
Program
On
the command line, use the java
command to run the test program.
The
output should look similar to the following:
Start
point is [10,10]
End
point is [20,30]
Stray
point is [20,30]
End
point is [20,30]
Stray
point is [47,50]
End
point is [47,50]
Start
point is [10,10]
The
values reported by end
reflect the change
made in stray,
indicating that both variables
refer to the same MyPoint
object. However, start
has not changed, which indicates that it is
independent of the other two variables.
Exercise 2: Creating Customer Accounts (Level
1,
Level 2, Level
3)