Next: Tutorial 12 Overloading methods, Previous: Tutorial 10 Object arrays, Up: J.T.W. Tutorials [Contents][Index]
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".
classFleabeginpropertyString name;constructorFlea(String aName)beginaName = name;endpublicmethodString toString()beginreturn "I am a flea called " + name;endendclassDogbeginpropertyString name;propertyint age; // Age in yearspropertyFlea dogsFlea;constructorTurtle(String aName, int anAge, Flea aFlea)beginname = aName; age = anAge; dogsFlea = aFlea;endendclassDogTestbeginbeginMainFlea p = new Flea("Pop"); Flea s = new Flea("Squeak"); Flea z = new Flea("Zip"); System.out.println("p=" + p);endMainend
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]