OBJECT ORIENTED PROGRAMMING THROUGH JAVA : Unit - 2 : Topic - 6 : Constructor Methods for Classes

 

Unit - 2

Topic - 6 : Constructor Methods for Classes

A constructor is a special method in Java that is used to initialize an object immediately upon creation.

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Key Features of a Constructor

• Has the same name as the class in which it is defined.

• Does not have a return type (not even void).

• Is called automatically when an object is created.

• Its purpose is to set up the object so it is ready to use immediately after creation.

• The implicit return type of a constructor is the class itself.

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Example: Default Constructor

class Box { 
    double width; 
    double height; 
    double depth;

    // Constructor for Box
    Box() {
        System.out.println("Constructing Box");
        width = 10;
        height = 10;
        depth = 10;
    }

    // Method to compute volume
    double volume() {
        return width * height * depth;
    }
}

class BoxDemo6 {
    public static void main(String[] args) {
        // Create Box objects
        Box mybox1 = new Box();
        Box mybox2 = new Box();

        double vol;

        // Get volume of first box
        vol = mybox1.volume();
        System.out.println("Volume is " + vol);

        // Get volume of second box
        vol = mybox2.volume();
        System.out.println("Volume is " + vol);
    }
}

Sample Output:

Constructing Box
Constructing Box
Volume is 1000.0
Volume is 1000.0

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Types of Constructors in Java

There are two main types:

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1. Default Constructor (No-Argument Constructor)

• A constructor with no parameters.

• Used to set default values for object properties.

Example:

class Bike1 {
    // Default constructor
    Bike1() {
        System.out.println("Bike is created");
    }

    public static void main(String[] args) {
        Bike1 b = new Bike1(); // Constructor is called here
    }
}

Output:

Bike is created

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2. Parameterized Constructor

• Accepts arguments to initialize object properties with specific values.

• Useful for assigning different values to different objects at creation time.

Example:

class Student4 {
    int id;
    String name;

    // Parameterized constructor
    Student4(int i, String n) {
        id = i;
        name = n;
    }

    void display() {
        System.out.println(id + " " + name);
    }

    public static void main(String[] args) {
        Student4 s1 = new Student4(111, "Karan");
        Student4 s2 = new Student4(222, "Aryan");

        s1.display();
        s2.display();
    }
}

Output:

111 Karan
222 Aryan

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Key Points to Remember

• If no constructor is defined, Java provides a default constructor automatically.

• If you define your own constructor (default or parameterized), the automatic default constructor is not provided.

• Constructors can be overloaded (multiple constructors with different parameter lists in the same class).

• The main job of a constructor is object initialization, not performing logic unrelated to setup.