The correct place for easy learning....: Constructor in Java

A constructor in Java is a special method used to initialize objects. It is called automatically when an object is created using the new keyword.

Features of Constructors:

Has the same name as the class.
No return type (not even void).
Called automatically at object creation.
Can be parameterized or non-parameterized.

Types of Constructors:

Type	Description
Default Constructor	No parameters. Java provides it by default if none is defined.
Parameterized Constructor	Takes arguments to initialize fields.
Copy Constructor (Manual)	Used to copy data from one object to another (not built-in like C++).

Default Constructor

public class Student {

	Student() {
        System.out.println("Default constructor called");
    }
	public static void main(String[] args) {
		  Student s1 = new Student();  // Constructor is called

	}

}

Output:

Default constructor called

Parameterized Constructor

public class Student {

	String name;
    int age;

    // Constructor with parameters
    Student(String n, int a) {
        name = n;
        age = a;
    }

    void display() {
        System.out.println("Name: " + name + ", Age: " + age);
    }
    
	public static void main(String[] args) {
		  Student s1 = new Student("Mansi",24);  // Constructor is called
		  s1.display();

	}

}

Output

Name: Mansi, Age: 24

Copy Constructor

class Student {
    String name;
    int age;

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

    // Copy constructor
    Student(Student s) {
        name = s.name;
        age = s.age;
    }

    void display() {
        System.out.println("Name: " + name + ", Age: " + age);
    }
    
	public static void main(String[] args) {
		Student s1 = new Student("Ravi", 22);
        Student s2 = new Student(s1);  // Copy constructor called

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

	}

}

Output:

Name: Ravi, Age: 22
Name: Ravi, Age: 22

Constructor with user Input

package com.studyopedia;

import java.util.Scanner;
class Student {
    String name;
    int rollNo;

    // ✅ 1. Default Constructor
    Student() {
        name = "Not Assigned";
        rollNo = 0;
        System.out.println("Default constructor called");
    }

    // ✅ 2. Parameterized Constructor
    Student(String n, int r) {
        name = n;
        rollNo = r;
        System.out.println("Parameterized constructor called");
    }

    // ✅ 3. Copy Constructor
    Student(Student s) {
        name = s.name;
        rollNo = s.rollNo;
        System.out.println("Copy constructor called");
    }

    // Method to display data
    void display() {
        System.out.println("Name: " + name + ", Roll No: " + rollNo);
    }
    
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

        // ➤ Using default constructor
        Student s1 = new Student();
        s1.display();

        // ➤ Taking user input for parameterized constructor
        System.out.print("\nEnter student name: ");
        String name = sc.nextLine();

        System.out.print("Enter roll number: ");
        int roll = sc.nextInt();

        // ➤ Using parameterized constructor
        Student s2 = new Student(name, roll);
        s2.display();

        // ➤ Using copy constructor
        Student s3 = new Student(s2);
        s3.display();

        sc.close();
    }

}

Output:

Default constructor called
Name: Not Assigned, Roll No: 0

Enter student name: Prasanjeet
Enter roll number: 81901
Parameterized constructor called
Name: Prasanjeet, Roll No: 81901
Copy constructor called
Name: Prasanjeet, Roll No: 81901

Pages

Constructor in Java

Types of Constructors: