OBJECT ORIENTED PROGRAMMING THROUGH JAVA : Unit - 2 : Topic - 1 : Classes Introduction

Unit - 2

Topic 1 : Introduction to Classes

In Java, a class is a user-defined data type that acts as a blueprint or template for creating objects. Once a class is defined, we can create multiple objects from it, each having its own data but sharing the same structure and behavior defined in the class.

Key Points:

• A class defines what an object will contain (variables) and what it can do (methods).

• An object is an instance of a class.

• Instance variables store data for each object.

• Methods define the operations that can be performed on the data.

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General Form of a Java Class

class ClassName {
    // Instance variables
    type variable1;
    type variable2;
    ...
    type variableN;

    // Methods
    returnType methodName1(parameter-list) {
        // method body
    }

    returnType methodName2(parameter-list) {
        // method body
    }
    ...
    returnType methodNameN(parameter-list) {
        // method body
    }
}

Components of a Class:

• Instance Variables: Data members of the class that hold values for each object.

• Methods: Functions defined inside the class that operate on instance variables.

• Members: Both instance variables and methods together are called members of a class.

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Example: A Simple Class

class Box {
    double width;   // instance variable
    double height;  // instance variable
    double depth;   // instance variable

    // Method to calculate and display volume
    void volume() {
        System.out.print("Volume is ");
        System.out.println(width * height * depth);
    }
}

In this example:

• The class Box has three instance variables: width, height, and depth (all of type double).

• It has one method, volume(), that calculates and prints the volume of the box using the instance variables.

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Creating and Using Objects

Once we have defined a class, we can create objects from it:

class BoxDemo {
    public static void main(String[] args) {
        Box myBox = new Box(); // create object

        // assign values to instance variables
        myBox.width = 10;
        myBox.height = 20;
        myBox.depth = 15;

        // call the method
        myBox.volume();
    }
}

Explanation:

1. Box myBox = new Box(); → Creates a new object of type Box.

2. myBox.width = 10; → Assigns values to the object's instance variables.

3. myBox.volume(); → Calls the volume() method to calculate and display the volume.

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Key Notes about Classes in Java

• A class can have multiple objects, each with independent values for its instance variables.

• Methods in a class can access and modify the instance variables.

• Classes help in achieving Object-Oriented Programming (OOP) concepts like encapsulation, inheritance, and polymorphism.

Example Program

Example Program on Java Classes

// Example of a simple Java class and its usage

// Class definition
class Box {
    double width;   // instance variable
    double height;  // instance variable
    double depth;   // instance variable

    // Method to calculate and display volume
    void volume() {
        double vol = width * height * depth;
        System.out.println("Volume is " + vol);
    }
}

// Main class to test the Box class
public class BoxDemo {
    public static void main(String[] args) {
        // Create first object of Box
        Box box1 = new Box();
        box1.width = 10;
        box1.height = 20;
        box1.depth = 15;

        // Create second object of Box
        Box box2 = new Box();
        box2.width = 3;
        box2.height = 6;
        box2.depth = 9;

        // Display volumes of both boxes
        box1.volume();
        box2.volume();
    }
}

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Explanation:

1. Class Definition:

   • The Box class has three instance variables: width, height, and depth.

   • It contains a method volume() to calculate and print the volume.

2. Creating Objects:

   • Box box1 = new Box(); creates an object box1 of type Box.

   • Box box2 = new Box(); creates another object box2.

3. Assigning Values:

   • Each object has its own set of instance variables.

   • We assign values to them separately.

4. Calling Methods:

   • box1.volume(); calls the method for the first object.

   • box2.volume(); calls the method for the second object.

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Sample Output:

Example Program on Java Classes
// Example of a simple Java class and its usage

// Class definition
class Box {
    double width;   // instance variable
    double height;  // instance variable
    double depth;   // instance variable

    // Method to calculate and display volume
    void volume() {
        double vol = width * height * depth;
        System.out.println("Volume is " + vol);
    }
}

// Main class to test the Box class
public class BoxDemo {
    public static void main(String[] args) {
        // Create first object of Box
        Box box1 = new Box();
        box1.width = 10;
        box1.height = 20;
        box1.depth = 15;

        // Create second object of Box
        Box box2 = new Box();
        box2.width = 3;
        box2.height = 6;
        box2.depth = 9;

        // Display volumes of both boxes
        box1.volume();
        box2.volume();
    }
}

Explanation:

Class Definition:
The Box class has three instance variables: width, height, and depth.
It contains a method volume() to calculate and print the volume.
Creating Objects:
Box box1 = new Box(); creates an object box1 of type Box.
Box box2 = new Box(); creates another object box2.
Assigning Values:
Each object has its own set of instance variables.
We assign values to them separately.
Calling Methods:
box1.volume(); calls the method for the first object.
box2.volume(); calls the method for the second object.
Sample Output:
Volume is 3000.0
Volume is 162.0