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AP Computer Science A Textbook

1. Primitive Types

2. Using Objects

3. Boolean Expressions and if Statements

4. Iteration

5. Writing Classes

6. Arrays

7. ArrayLists

8. 2D Arrays

9. Inheritance

9.1 Inheritance

Superclasses and Subclasses

The extends keyword

Person Superclass

Vehicle Superclass

High School Student

Check Your Understanding

Exercise: Computers

9.2 Writing Constructors for Subclasses

9.3 Overriding Methods

9.4 super Keyword

9.5 Creating References Using Inheritance Hierarchies

9.6 Polymorphism

9.7 Object Superclass

10. Recursion

Chapter 9

Inheritance

9.1 Inheritance

Large classes often rely on several different instance variables in order to successfully create comprehensive programming solutions. For example, a Person class might have several instance variables, like name or birthday. These instance variables are said to have a “Has-A” relationship to their class objects. A Person has a name, and has a birthday. Values that use a “has a” relationship represent instance variables in our projects.

Another type of relationship in object-oriented programming is the “Is-A” relationship. “Is-A” is used when one thing is a more specific version of another thing. As an example, think about the relationship of a Student to a Person. A Student does not have a Person, but rather, a Student is a Person. In this example, a Student is a more specific example of a Person. While it’s possible to say that every Student is a Person, it’s not possible to say that every Person is a Student. The Person class is a more general class and the Student class is a more specific class. Some other examples of is-a relationships are:

  • A Dog is an Animal.
  • A Square is a Rectangle.
  • San Francisco is a City.
  • Computer science is a Class.

Superclasses and Subclasses

Because many classes, like Person and Student have an is-a relationship, they share some common attributes. Just as a Person has a name, and a birthday, a Student has the same attributes. Since these two classes share many of the same attributes, there should be a way to consolidate these two classes so as to avoid writing the same attributes and instance variables in each class.

In Java, the is-a relationship is expressed in the form of superclasses and subclasses. A class hierarchy can be developed by putting common attributes and behaviors of related classes into a single class called a superclass. In the current example, the attributes name and birthday belong to both Person and Student, so they should be written in a superclass.

Subclasses inherit the public attributes and behaviors from the superclass. As a result, a subclass object can use any public attribute or behavior without having to repeat these in its code. A superclass object does not have access to the attributes and behaviors of the subclass class. In this case, a Student object will have access to all the attributes and behaviors of both the Student class, like GPA and grade level, and the Person class, but the Person object will only have access to the attributes and behaviors of the Person class.

The extends keyword

To create a superclass/subclass relationship, the keyword extends is added to a class header followed by the name of the superclass:

public class Student extends Person
{
   //Student class implementation

}
Plain text

This creates the subclass. Student, in this case, is a subclass of Person, which is the superclass. Subclasses are only capable of extending a single superclass. While a class can only extend one class, it can extend a class that has already been extended:

public class Person {
   // Person class implementation
}

public class Student extends Person {
   // Student class implementation
}

public class HighSchoolStudent extends Student {
   // HighSchoolStudent class implementation
}
Plain text

In this case, a HighSchoolStudent object will have access to all the attributes and behaviors of the HighSchoolStudent class, the Student class, and the Person class.

As programs become more complicated, oftentimes creating a hierarchy of classes allows for the reuse of common attributes and behaviors. Figuring out which variables and methods are shared across class implementations is the first step to developing a successful hierarchical structure.

Person Superclass

Vehicle Superclass

High School Student

Check Your Understanding

  1. Incorrect Correct No Answer was selected Invalid Answer

    Which of the following best describes a Subclass / Superclass relationship?

Exercise: Computers

For this exercise, you are going to design 3 classes:

  • Computer - Superclass
  • Laptop - Subclass
  • Desktop - Subclass

You will design these classes to optimize the superclass/subclass relationship by creating instance variables and getter/setter methods.

Include the following instance variables:

  • int screenSize - Inches of monitor space
  • int memory - GB of ram
  • double batteryLife - Hours of battery life
  • boolean monitor - Whether or not a monitor is included

Each class should have at least one variable in it.

Once completed, the Tester class should execute without error.