This is a lab assignment and so you will not be submitting it. However, the concepts and practice will help you on both the homework and exams so I encourage you to make a serious effort on it during class and consider finishing it outside of class.

Goal

Get practice writing code that uses inheritance and see how the finer details of inheritance work.

Setup

Exercise 1

We’ll first make some classes, remember to compile and run your code after each step to catch any errors early!

  1. Create an Animal class that has the instance variables happiness and age and getters/setters for these variables. (Make them Integers for convinience later.)

  2. Choose a species of animal and create a subclass of Animal in its own file. Remember that you create a subclass with extends. To compile and test your subclass, you’ll need to also compile Animal. An easy way to do this is to type javac *.java, which will compile all the Java files in the directory.

  3. Within your subclass, make another instance variable called happiness. Make a getter and setter with different names from the Animal class getter and setter (so if you had setHappiness in Animal, you could do setHappinessDog in a Dog subclass).

  4. In main of your subclass create an instance of your subclass to make sure everything is working.

Exercise 2

We’ll now explore what is going on with those happiness variables.

  1. In main, use your setters to set the happiness of your instance to two different values (meaning use the Animal setter to set to 5 and the subclass setter to set to 10, for example)

  2. Add println statements that demonstrate if the setters are accessing the same variable or different variables and write your answer on the worksheet.

Exercise 3

We’ll now explore how methods in subclasses interact with methods in superclasses.

  1. Create a method printStatus in your subclass that prints something to the terminal.

  2. In main create two instances of your subclass but declare one as type Animal and one as your subclass type:

Animal test = new Dog();
Dog test2 = new Dog();

  1. Try to call printStatus on both objects. One will report an error. Record which one and the error on the worksheet.

  2. Create a method printStatus in Animal that prints something different than the method in your subclass.

  3. Try again to call printStatus on the objects you created in (b). Record on your worksheet which method is accessed for each object.

  4. On your worksheet, reflect on why you think you got the errors that you did and what that tells you about what Java is doing when it tries to compile and when it runs a program. (Don’t worry about being right, we’ll talk about it!)

Exercise 4

Let’s think some more about overriding methods in a subclass.

  1. In your subclass method printStatus, use super to call the printStatus method of Animal.

  2. In your subclass, add a new method pet using the @Override annotation:

@Override
public void pet() {
    System.out.println("Petting the animal!");
}

  1. Report the error that you see on your worksheet.

  2. Correct the error by adding pet to the Animal class as well and verify the error is resolved.

  3. Reflect on your worksheet on whether you find the @Override annotation useful and if you plan to use it (there is no right or wrong answer, I just am curious what you think!).

Exercise 5

If you didn’t already, go back and add JavaDocs method documentation for each of your methods.