Inheritance Hierarchies and Design

• Inheritance- A way to create a relationship amongst classes.

• Inheritance Hierarchy- A way to determine how information travels amongst the classes. Ex: The subclasses inherit characteristics of the parent class.

• The parent class is referred to as the superclass, and the classes that inherit from it are referred to as subclasses of the superclass.

• The parent class is the most general form of the objects that will be instantiated in the program.

• All the other classes in the program will lie lower in the hierarchy, but they will be more specific compared to the parent class.

• Let’s think of inheritance using an example, it’s the easiest way to explain this concept.

  • Ex: Say you want to buy something, and you decide to buy a snack. Then you wonder which type of snack should I get. You narrow it down to potato chips and cookies. However, there are 2 types of potato chips(Classic, and BBQ), and 2 types of Cookies(Chocolate Chip, and Macadamia Nut.)

• In the example above each of the different snack items is divided up into their own specific categories, and is like a hierarchy because one thing follows the other.

• Note: On the AP Exam there is at least 1 free-response question and many multiple-choice questions that focus on the design and implementation of inheritance relationships.

• Important!!- Classes inherit variables and methods from their superclasses!

• In some instances, a class may not have more than one direct superclass.

• This is referred to as multiple inheritance which ISN’T allowed in Java.

• Overridden- When a new version of the code takes over from the old version.

  Polymorphism

• Polymorphism- When many classes are related to each other by inheritance. Inheritance lets us inherit attributes and methods from another class. Polymorphism uses those methods to perform different tasks.

  • Ex: Let’s look at a class that will be used to represent a rectangle with the concepts we just learned.

public class Rectangle
{

    private double width;

    private  double height;

    public Rectangle()

        {

            width = w;

            height = 0;

        }

       public Rectangle(double w, double h)

       {

            width = w;

            height = h;

       }

      public double getHeight()

      {

            return height;

       }

      public double getWidth()

      {

          return width;

      }

}

• Ex: What output will this line of code produce?

  class Animal
  
  {
  
     void someSound()
  
     {
  
       System.out.println(“Screech”);
  
     }
  
  }
  
  class Cat extends Animal
  
  {
  
     public Cat ()
  
    {
  
       System.out.print(“Meow”);
  
       super.someSound();
  
    }
  
  }
  
      class Garfield extends Cat
  
  {
  
       public Garfield()
  
       {
  
          System.out.print(“Lasagna”);
  
       }
  
  }
  
  public class Mainclass
  
  {
  
      public static void main(String [ ]args)
  
      {
  
         Garfield garfield = new Garfield();
  
      }
  
  }