Classes and Object Oriented Programming (OOP)

What is Object Oriented Programming

Briefly, Object Oriented Programming (OOP) is a way of organizing the code in a program. It is the process of defining classes that are used to create objects which are used to perform all of the actions of the program. For example, in the Kennel program we created classes called Kennel and Dog. Then we created PitBull, Cocker, and Chihuahua as sub-classes of Dog. When the program ran it created a Kennel object using our Kennel class as a blueprint. Next we created one object each of PitBull, Cocker, and Chihuahua and let each of these objects handle the action of "barking". In the GUI (Graphical User Interface) demonstration programs each widget (text box, button, list box, combo box, label, menubar, menu, menu item, etc.) was created as a separate object. All of these objects then worked together to implement the program. Java is a programming language that is entirely object oriented. Other programming languages, like C++, Python, etc., do not require you to use OOP, even though most programmers do.

Some important terms related to OOP

Class -- A class is a source code file which the programmer writes that provides the "blueprint" for creating an object. Examples of classes in the programs created in this course are Kennel.java, Dog.java, PitBull.java, Cocker.java, Chihuahua.java, JavaGuiDemo.java, GUIDemoMenuBar.java, etc. When compiled each of these source files becomes a .class (Kennel.class, Dog.class, etc.) file from which the program can create objects when running. There are also many classes included in the Java library. Examples are the GUI classes of JTextField, JLabel, JButton, etc.
Object -- What is created when the program is running using the classes as "blueprints" or guides.

Figure 1: Classes and Objects
Instantiate -- (verb) The process of creating an object from a class "blueprint" using the new operator. For example: Kennel kn = new Kennel(); This creates a reference variable called kn which can reference an object of type Kennel. Then, new Kennel() actually creates (instantiates) the Kennel object and sets kn referencing it.
Instance -- (noun) The object that is created using the new operator. In the example above of create a Kennel object we say that kn is an instance of the class Kennel.
Member (instance) variables -- These are variables that are created as part of a class which can be accessed and used by any method (function) in the class. The code defining member variables is usually placed between the opening brace ({) and the constructor method of a class file.
Member methods (functions) -- These are blocks of code in a class that define what a class does. Each consists of an access specifier (public, private, or protected), a return type, a method name, a list of arguments to the function enclosed in parentheses, and an opening ({) and closing (}) brace. All of the code for the function is placed between the opening and closing braces. For more details see below.

Figure 2: Layout of a member method (function)
Constructor -- This is a special method included in all class files. The method has the same name as the class, but has no return type. When you instantiate a class in a program using the new operator this function gets called automatically in the new object. It is here that you do all the set up and initialization needed to get this object ready to be used in your program.
Inheritance -- The ability to create a class that is a child class of another class. We say that the child class extends the parent class (Figure 3). The child class then inherits from its parent all member variables and member methods from the parent class which has an access specifier of public or protected, but not those that are private. For example, in the Kennel project PitBull, Cocker, and Chihuahua were sub-classes of Dog so they inherited the m_iSize, m_sBreed, and m_sName member variables from Dog as well as the member method bark(). Each of the sub-classes however, overrode the bark() method with their own version of bark().

Figure 3: Class PitBull is a sub-class of Dog, i.e. it extends Dog

How class files are organized

A class consists of the header (public class MyApplication) in the image below, an opening brace, a list of member variables, a constructor method (function), any number of other methods (functions), and a closing brace. There must be one and only one class in a program that also contains a main() function in the format shown below. All main() should really do is instantiate one instance of the class it is in. A Java program may consist of more than one class.

Figure 4: Layout of a class file

Creating instances of classes using the new operator

In code when you want to create an instance of a class you use the new operator followed by what looks like a call to the class constructor of the class with appropriate arguments. In figure 5 below the new operator is used to create three instances of the class Dog. Each instance is passed three arguments: an int specifying the size of the Dog, a String giving the "type" of Dog, and a String giving the name of the Dog.

Figure 5: Using new to create instances of classes

The Java Library

There are many, many classes built-in to Java that you can use in your programs. All of these are listed with complete descriptions of all the member variables and methods at the Java web site. Click here to visit the library.

Figure 6: A wealth of classes can be found in the Java Library