This time I will focus on one of the most important basics of JAVA. Object and classes are the bricks and mortar used to build JAVA. Object-oriented programming is modeled on how, in the real world, objects are often made up of many kinds of smaller objects. This capability of combining objects, however, is only one very general aspect of object-oriented programming.

Object-oriented programming provides several other concepts and features to make creating and using objects easier and more flexible, and the most important of these features is that of classes.

A class is a template for multiple objects with similar features. Classes embody all the features of a particular set of objects. When you write a program in an object-oriented language, you don't define actual objects. You define classes of objects.

For example, you might have a Tree class that describes the features of all trees (has leaves and roots, grows, creates chlorophyll). The Tree class serves as an abstract model for the concept of a tree-to reach out and grab, or interact with, or cut down a tree you have to have a concrete instance of that tree. Of course, once you have a tree class, you can create lots of different instances of that tree, and each different tree instance can have different features (short, tall, bushy, drops leaves in Autumn), while still behaving like and being immediately recognizable as a tree.

An instance of a class is another word for an actual object. If class is the general representation of an object, an instance is its concrete representation.

So what, precisely, is the difference between an instance and an object? Nothing, really. Object is the more general term, but both instances and objects are the concrete representation of a class. In fact, the terms instance and object are often used interchangeably in OOP language. An instance of a tree and a tree object are both the same thing.

In an example closer to the sort of things you might want to do in Java programming, you might create a class for the user interface element called a button. The Button class defines the features of a button (its label, its size, its appearance) and how it behaves (does it need a single click or a double click to activate it, does it change color when it's clicked, what does it do when it's activated?). Once you define the Button class, you can then easily create instances of that button-that is, button objects-that all take on the basic features of the button as defined by the class, but may have different appearances and behavior based on what you want that particular button to do. By creating a Button class, you don't have to keep rewriting the code for each individual button you want to use in your program, and you can reuse the Button class to create different kinds of buttons as you need them in this program and in other programs.

Tip: If you're used to programming in C, you can think of a class as sort of creating a new composite data type by using struct and typedef. Classes, however, can provide much more than just a collection of data, as you'll discover in the rest of today's lesson.

When you write a Java program, you design and construct a set of classes. Then, when your program runs, instances of those classes are created and discarded as needed. Your task, as a Java programmer, is to create the right set of classes to accomplish what your program needs to accomplish.

Fortunately, you don't have to start from the very beginning: the Java environment comes with a library of classes that implement a lot of the basic behavior you need-not only for basic programming tasks (classes to provide basic math functions, arrays, strings, and so on), but also for graphics and networking behavior. In many cases, the Java class libraries may be enough so that all you have to do in your Java program is create a single class that uses the standard class libraries. For complicated Java programs, you may have to create a whole set of classes with defined interactions between them. Class library is a set of classes.

Behavior and Attributes

Every class you write in Java is generally made up of two components: attributes and behavior. In this section, you'll learn about each one as it applies to a theoretical class called car .

Attributes

Attributes are the individual things that differentiate one object from another and determine the appearance, state, or other qualities of that object. Let's create a theoretical class called car. The attributes of a car might include the following:

Color: red, green, silver, brown

Style: E-class, family purpose, utility vehicle

Make: Honda, BMW, HYUNDAI 

Attributes of an object can also include information about its state; for example, you could have features for engine condition (off or on) or current gear selected.

Attributes are defined by variables; in fact, you can consider them analogous to "global" variables for the entire object. Because each instance of a class can have different values for its variables, each variable is called an instance variable.

Instance variables define the attributes of an object. The class defines the type of the attribute, and each instance stores its own value for that attribute.

Each attribute, as the term is used here, has a single corresponding instance variable; changing the value of a variable changes the attribute of that object. Instance variables may be set when an object is created and stay constant throughout the life of the object, or they may be able to change at will as the program runs.

In addition to instance variables, there are also class variables, which apply to the class itself and to all its instances. Unlike instance variables, whose values are stored in the instance, class variables' values are stored in the class itself.

Behavior

A class's behavior determines what instances of that class do to change their internal state changes or when that instance is asked to do something by another class or object. Behavior is the only way objects can do anything to themselves or have anything done to them. For example, to go back to the theoretical car class, here are some behaviors that the car class might have: 

Start the engine Stop the engine Speed up Change gear Stall 

To define an object's behavior, you create methods, which look and behave just like functions in other languages, but are defined inside a class. Java does not have functions defined outside classes (as C++ does).

Methods are functions defined inside classes that operate on instances of those classes. Methods don't always affect only a single object; objects communicate with each other using methods as well. A class or object can call methods in another class or object to communicate changes in the environment or to ask that object to change its state.

Just as there are instance and class variables, there are also instance and class methods. Instance methods (which are so common they're usually just called methods) apply and operate on an instance of a class; class methods apply and operate on the class itself. You'll learn more about class methods later on in my forthcoming articles.     

Next:  First Java Application