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The following code presents example involving three classes Flea
, Dog
and DogOwner
to represent the idea that a dog has a flea and a
dog-owner has a dog. The class
DogTest
is the driver class
. The key
concept of this tutorial is that classes can have references of
objects of another class in order to set up a relationship between the
two classes.
Question 4.11.1: Study the following code and find the two bugs in it. Fix the bugs and then compile and run it to verify that it prints out "p=I am a flea called Pop".
class
Fleabegin
property
String name;constructor
Flea(String aName)begin
aName = name;end
public
method
String toString()begin
return "I am a flea called " + name;end
end
class
Dogbegin
property
String name;property
int age; // Age in yearsproperty
Flea dogsFlea;constructor
Turtle(String aName, int anAge, Flea aFlea)begin
name = aName; age = anAge; dogsFlea = aFlea;end
end
class
DogTestbegin
beginMain
Flea p = new Flea("Pop"); Flea s = new Flea("Squeak"); Flea z = new Flea("Zip"); System.out.println("p=" + p);endMain
end
Question 4.11.2: In the main
function
of the DogTest class, write code
to call the toString
method
for the fleas referenced by s and z.
Question 4.11.3: In the main
function
of the DogTest class, write code
to construct three dogs called "Fido", "Jimbo" and "Rex". For the
purposes of the rest of these questions, let the name of the
references for Fido, Jimbo and Rex be f
, j
and r
. Note
that the third parameter to the Dog class is of type Flea. Therefore
you will need to supply a Flea reference for each dog. Make it so that
Fido has a flea called Pop, Jimbo has a flea called Squeak, and Rex
has a flea called Zip.
HINT: If the flea called Pop is referenced by the variable name
p
, then this reference should appear as the third argument in one
of the calls to the Dog constructor.
Question 4.11.4: Write a toString
method in the Dog
class
that
works like the toString
method in the Flea
class
. Then call
this method
from the main
function
to print out the full statistics of
the three dogs that you have just created in Question 11.3.
Question 4.11.5: By copying the pattern of the Flea
and Dog
classes, write a class DogOwner
that has three non-private
properties: name, salary and ownersDog. Also write a three-parameter
constructor for the DogOwner
class
that sets these properties.
Question 4.11.6: Add some code into the main
function
to construct three
dog owners called Angus, Brian and Charles. Make it so that Angus has
a dog called Rex, Brian has a dog called Jimbo, and Charles has a dog
called Fido. For the purposes of the rest of these questions, let the
name of the references for Angus, Brian and Charles be (respectively)
a
, b
and c
. Use the Dog
references that you created in
Question 11.3 to achieve this. Make it so that Angus, Brian and
Charles have initial salaries of 10,000, 20,000 and 30,000.
Question 4.11.7: Without changing the call to the DogOwner
constructor, change the value of the salary property of object
referenced by a
to 1,000,000. Note that since the salary property
of the DogOwner
class is non-private you should be able to set the
value of the salary property from the main
function
of DogTest.
Question 4.11.8: Write a toString
method
for the class
DogOwner
and add some code to the main
function
to call it for
Angus, Brian and Charles.
Question 4.11.9: What is the value of:
a.ownersDog.dogsFlea.toString()
? Add some code to the main
function
to find out if it does what you think it should do.
Next: Tutorial 12 Overloading methods, Previous: Tutorial 10 Object arrays, Up: J.T.W. Tutorials [Contents][Index]