Building Java Programs

Lab: Inheritance

Except where otherwise noted, the contents of this document are Copyright 2019 Stuart Reges and Marty Stepp.

Some extensions by T W Bennet

Lab goals

Goals for this problem set:

• get more practice with classes and object-oriented programming
• use inheritance to create hierarchies of related classes
• extend behavior and override existing behavior

Inheritance (syntax)

public class ClassName extends SuperClass {
    ...
}
		

• a subclass inherits all of the superclass's behavior and can override methods.
• To call an overridden method from the superclass, use the super keyword:

super.methodName(parameters);
		

Exercise : Car and Truck

public class Car {
   public void m1() {
      System.out.println("car 1");
   }

   public void m2() {
      System.out.println("car 2");
   }

   public String toString() {
      return "vroom";
   }
}
			    

public class Truck extends Car {
   public void m1() {
      System.out.println("truck 1");
   }
}

• What is the output from the following code?

Truck mycar = new Truck();
System.out.println(mycar);    // vroom
mycar.m1();                   // truck 1
mycar.m2();                   // car 2

Exercise : Car and Truck revisited

public class Car {
   public void m1() {
      System.out.println("car 1");
   }

   public void m2() {
      System.out.println("car 2");
   }

   public String toString() {
      return "vroom";
   }
}

public class Truck extends Car {
   public void m1() {
      System.out.println("truck 1");
   }
    
   public void m2() {
       super.m1();
   }
				    
   public String toString() {
      return super.toString() + super.toString();
   }
}

• Suppose the Truck code changes as shown above. What is the output now?

Truck mycar = new Truck();
System.out.println(mycar);    // vroomvroom
mycar.m1();                   // truck 1
mycar.m2();                   // car 1

Exercise : MonsterTruck

• Download the following files from the previous exercise and open them in your editor:
  • Car.java, Truck.java, AutoMain.java
• Write a class MonsterTruck that has the behavior below. Test by running AutoMain.
  • Some methods produce 2 lines of output; the split between lines is indicated by a /.
  • Don't just print/return the output; if possible, use inheritance to reuse behavior from the superclass.

MonsterTruck bigfoot = new MonsterTruck();
bigfoot.m1();                  // monster 1
bigfoot.m2();                  // truck 1 / car 1
System.out.println(bigfoot);   // monster vroomvroom

Employee class hierarchy

• Employees classes have the hierarchy shown at right.
• Each type of employee extends a common base class Employee and has the methods getHours, getSalary, getVacationDays, getVacationForm, work.
• For the next several exercises you will add classes to this inheritance hierarchy.
• To do this, download each of these files to your machine: Employee, Secretary, Lawyer, and LegalSecretary.

Exercise : Marketer

• Write an employee class Marketer to accompany the other employees. Marketers make $50,000 ($10,000 more than general employees), and they have an additional method named advertise that prints "Act now, while supplies last!"
• Use the super keyword to interact with the Employee superclass as appropriate.
• Make sure to create a work method which calls advertise. See the existing classes for examples.

Exercise : Janitor

• Write an employee class Janitor to accompany the other employees. Janitors work twice as many hours (80 hours/week), they make $30,000 ($10,000 less than others), they get half as much vacation (only 5 days), and they have an additional method named clean that prints "Workin' for the man."
• Use the super keyword to interact with the Employee superclass as appropriate.
• Create a work method which calls clean.

Exercise : HarvardLawyer

• Write an employee class HarvardLawyer to accompany the other employees. Harvard lawyers are like normal lawyers, but they make 20% more money than a normal lawyer, they get 3 days more vacation, and they have to fill out four of the lawyer's forms to go on vacation. That is, the getVacationForm method should return "pinkpinkpinkpink".
• (If the normal Lawyer's vacation form ever changed, the HarvardLawyer's should as well. For example, if Lawyer's vacation form changed to "red", the HarvardLawyer's should return "redredredred".)
• Use the super keyword to interact with the Employee superclass as appropriate.