INHERITANCE

This document was written by CS 290W TA Joshua Kay and was last modified

You know that Classes are generic definitions of Objects, used when you want to create a specific Object.

Now let's talk about Events, briefly.

Events are just what they sound like - happenings around the object which can stimulate the object in some way.

Most objects have built-in methods which can handle Events - In Java 1.1 these methods are called Listeners.

Due to limitations of Netscape 4.05, we will be learning Java 1.02 - which has slightly different Event Handling.

We will go into this in great detail later, so hold on for now.

We'll leave you with an example - you create a Button named myButton. Button is a predefined class of the java.awt package.

So we have to create an Event Handling method to control what happens when the user clicks on the myButton with his mouse.

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We now know what Objects are, how they are defined, how they respond to Events, and what methods they have.

So what's the big deal about OOP?

Let's take a closer look at Objects. What are they, exactly?

Objects are nothing but specific instances of classes, with unique properties and methods.

We can look at an object as a set of properties, with methods which can manipulate those properties.

Objects are somewhat alone, in that they support themselves. An example would be a die object:
  -a die object can roll itself, to get its value
  - in the real world we have to roll the die, but in programming we try to make things self-contained.

You can replace one object with another, without making huge changes in the program, but this might have consequences
  -Example - You build a house out of Legos - you take off one of the beams and replace it with a small one
  The house might fall, or there could be a draft and Lego rats can get inside.

More examples --

The Physics Building is a Building Object. The room we are in would be considered a Classroom object, full of People objects.

Students and Teachers would be considered two forms of People objects.

We are all wearing clothes - represented by Clothes objects.

This is broken down to Blue Jeans objects, T-shirt Objects, etc.

And remember all of the other objects in the room - Desks, Tables, Chairs, Doors.

So we need to look at one last thing, to really grasp OOP.

We have Objects, created from Classes, we know how they interact - but how are they related?

The relation between objects is known as inheritance.

For instance - we have Ball class - we'll call this the superclass.

Now we need a Basketball class - well, this inherits from the Ball class.

This is because Basketball, Football, Soccerball, Golf ball - they are all types of ball - hence they inherit certain things.

Let's look at this in a Family sort of way.

Josh Kay is his father's son. Josh has certain properties - Height, Eyecolor, Haircolor, etc. - that he inherited from his dad.

So, Josh is a subclass of the Kay object.

However, Josh has certain characteristics of his own as well. - He is not an exact copy.

Let's look at bodies of water. We have salt-water versus freshwater, first, right?

Now - we look at saltwater - we have oceans and seas, and the salt lake in Utah.

So, the Pacific Ocean is a subclass of oceans which is a subclass of saltwater which is a subclass of bodies of water.

Lake Michigan is one of the Great Lakes.

It might be considered a subclass of Lake which is a subclass of freshwater which is a subclass of bodies of water.

Java does NOT support multiple inheritance
-Ex: Salt Lake in Utah is a Lake, but it's also saltwater
- It would have to inherit from both Saltwater and Freshwater, because Lake is a subclass of Freshwater
- Java does not allow this - so you would have to program around it
- This does cause some limitations - however - it makes most programming in Java much easier