Building Java Programs, 4th edition

Lab: Inheritance

Except where otherwise noted, the contents of this document are Copyright 2013 Stuart Reges and Marty Stepp.

lab document created by Marty Stepp, Stuart Reges and Whitaker Brand

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
• Where you see this icon, you can click it to check the problem in Practice-It!

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 jGRASP:
  • 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

• Recall the inheritance hierarchy of employee classes shown in lecture.
• Each type of employee extends a common base class Employee and has the methods getHours, getSalary, getVacationDays, and getVacationForm.
• For the next several exercises you will add classes to this inheritance hierarchy.
• To do this, either solve the problems in Practice-It!, or 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.

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.

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.