Building Java Programs

Lab 9: Inheritance

Except where otherwise noted, the contents of this document are Copyright 2010 Stuart Reges and Marty Stepp.

lab document created by Whitaker Brand and Marty Stepp

Today's lab

Goals for today:

• 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!

Recall: Inheritance (syntax)

public class class name 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.
  • solution - don't peek!

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

Exercise : Janitor

• Download the employee classes from lecture: Employee, Secretary, Lawyer, and LegalSecretary.
• Write a 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." ( solution - don't peek!)
• If you finish, modify it to use the super keyword to connect with the Employee superclass as appropriate. ( solution )

If you finish them all...

If you finish all the exercises, try out our Practice-It web tool. It lets you solve Java problems from our Building Java Programs textbook.

You can view an exercise, type a solution, and submit it to see if you have solved it correctly.

Choose some problems from Chapter 9 or Section 9 or the Practice Final Exams and try to solve them!