Constructor

❔ What is a Constructor?

A constructor is a special method that’s automatically called when an object is created.

• Its purpose is to initialize objects.

• It has the same name as the class and no return type (not even void).

🏗️ Why Constructors Are Required in Java

🔹 Used for Object Initialization

Constructors are called automatically when an object is created to set initial values.

🔹 Same Name as Class

A constructor must have the same name as the class and has no return type.

🔹 Avoid Manual Setup

Without constructors, you would need to manually set fields using setter methods — constructors make this process easier.

🔹 Allows Overloading

You can create multiple constructors with different parameters to initialize objects in different ways (constructor overloading).

🔹 Ensures Valid State

Constructors can include logic to validate inputs, ensuring the object is always in a valid state when created.

🔹 Improves Readability

Object creation becomes more compact and readable when values are set directly through constructors.

🔹 Default Constructor

If you don't write any constructor, Java provides a default constructor with no parameters.

🔹 Supports Encapsulation

Helps in keeping fields private and still initializing them safely through constructor parameters.

🔹 Used in Inheritance

When a subclass object is created, parent class constructors are called first, helping with proper initialization.

🔸 Syntax:

class ClassName {
    ClassName() {
        // constructor code
    }
}

🛠️ Types of Constructors

🏗️ Default Constructor

If you don’t define any constructor, Java automatically creates a default constructor for you.

class Student {
    Student() {
        System.out.println("Default constructor called.");
    }
}

public class Main {
    public static void main(String[] args) {
        Student s1 = new Student();  // constructor auto-called
    }
}

Default constructor called.

➕ Parameterized Constructor

Allows values to be passed when creating an object.

class Student {
    String name;

    Student(String studentName) {
        name = studentName;
    }

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

public class Main {
    public static void main(String[] args) {
        Student s1 = new Student("Aanya");
        s1.display();
    }
}

Name: Aanya

Constructor vs Method

🧱 Constructor Overloading

Like methods, constructors can also be overloaded — same name but different parameters.

class Student {
    String name;
    int age;

    Student() {
        name = "Unknown";
        age = 0;
    }

    Student(String n, int a) {
        name = n;
        age = a;
    }

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

public class Main {
    public static void main(String[] args) {
        Student s1 = new Student();
        Student s2 = new Student("Riya", 20);
        s1.display();
        s2.display();
    }
}

Unknown - 0
Riya - 20

Recap