Java1600: objects and encapsulation  (Page 2/7)

You will learn about Java references and reference variables. You will also learn a little about memory allocation for objects and variables in Java.

Discussion and sample code

Purpose of the miniseries

As mentioned earlier, I will describe and discuss the necessary and most significant aspects of OOP using Java.

The three pillars

Most books on OOP will tell you that in order to understand OOP, you need to understand the following three concepts:

• Encapsulation
• Inheritance
• Polymorphism

I agree with that assessment.

(Some books will also add abstraction and/or late binding to the list. I tend to think of those two topics as being included in one or more of the three concepts listed above.)

Begin with encapsulation

Generally, speaking, these three concepts increase in difficulty going down the list from top to bottom. Therefore, I will begin with Encapsulation and work my way down the list in successive modules.

What is an Object-Oriented Program?

Many authors would answer this question something like the following:

An Object-Oriented Program consists of a group of cooperating objects, exchanging messages, for the purpose of achieving a common objective.

What is an object?

An object is a software construct that encapsulates data, along with the ability to use or modify that data, into a software entity.

What is encapsulation?

An interesting description of encapsulation was provided in an article by Rocky Lhotka regarding VB.NET. That description reads as follows:

"Encapsulation is the concept that an object should totally separate its interface from its implementation. All the data and implementation code for an object should be entirely hidden behind its interface.

The idea is that we can create an interface (Public methods in a class) and, as long as that interface remains consistent, the application can interact with our objects. This remains true even if we entirely rewrite the code within a given method thus the interface is independent of the implementation."

I like this description, so I won't try to improve on it. However, I will try to illustrate it in the paragraphs that follow.

A real-world analogy

Abstract concepts, such as the concept of an object or encapsulation, can often be best understood by comparing them to real-world analogies. One imperfect, but fairly good analogy to a software object is the radio in your car.

The ability to store data

Your car radio probably has the ability to store data, and to allow you to use and modify that data at will. (However, you can only use and modify that data through use of the human interface that is provided by the manufacturer of the radio.)

The data that can be stored in your car radio probably includes a list of five or more frequencies that correspond to your favorite radio stations.

Using the stored data

The radio provides a mechanism (human interface) that allows you to use the data stored therein.

When you press one of the frequency-selector buttons on the front of the radio, the radio automatically tunes itself to the frequency corresponding to that button. (In this case, you, the user, are sending a message to the radio object asking it to perform a particular action.)