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6.1 Classes and Objects

Java is a true Object-Oriented Programming (OOP) language. Modeling real-world objects or concepts into the form of a computer program 'object' is the core of object-oriented programming.

1. Definition of Class and Object​

Class​

  • Definition: A blueprint or template that defines objects.
  • Purpose: Used to create objects. (e.g., A waffle maker tool, a TV blueprint)

Object / Instance​

  • Definition: Something that actually exists (the physical entity created in memory by a class).
  • Purpose: Used differently depending on the functionalities and properties it holds. (e.g., The cooked waffle, individual TV products being sold)

The process of creating a certain object from a class is called instantiation, and the resulting created object is referred to as an ** instance**of that class.

2. Components of an Object​

An object consists of multiple properties and functions, which are defined within the class. In Java, they are specifically termed as follows:

  • Property: Member variable, Field
  • Function: Method

Example of Writing a Class​

class Tv {
    // Properties (Member variables)
    String color;   // Color
    boolean power;  // Power state
    int channel;    // Channel number
    
    // Functions (Methods)
    void power() { power = !power; } // Turn power on/off
    void channelUp() { channel++; }  // Channel up
    void channelDown() { channel--; }// Channel down
}

3. Creation and Usage of Objects​

Merely declaring a class is insufficient; you must physically create an instance (object) to use its variables and methods effectively.

// 1. Declare a reference variable to handle the object
Tv t;
// 2. Create the actual object and store its memory address inside the references (Instantiation)
t = new Tv();

// Using the object
t.channel = 7;     // Set the channel member variable of the Tv instance to 7.
t.channelDown();   // Execute the channelDown() method explicitly targeting the Tv instance.
System.out.println("The current channel is " + t.channel); // Outputs 6

The greatest characteristic of OOP is that once a class definition logic exists, you can mass-produce numerous independent instances infinitely whenever necessary. If you dynamically generate numerous divergent instances, each firmly maintains entirely separate properties (like color, channels) inherently isolated.

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