Learn what constructor overloading is in Java, why it’s useful, and how to implement it with real-world examples and simple explanations.

Ramesh Fadatare

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Apr 2, 2025

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📌 Definition

Constructor overloading in Java means creating multiple constructors in the same class, each with different parameter lists.

➡️ It allows you to create objects in multiple ways depending on what data is available.

🧱 Basic Syntax

public class Book {
    String title;
    String author;

    // Constructor 1
    public Book() {
        this.title = "Unknown";
        this.author = "Unknown";
    }

    // Constructor 2
    public Book(String title) {
        this.title = title;
        this.author = "Unknown";
    }

    // Constructor 3
    public Book(String title, String author) {
        this.title = title;
        this.author = author;
    }
}

🧪 Usage:

Book b1 = new Book();                     // uses constructor 1
Book b2 = new Book("Java in Action");     // uses constructor 2
Book b3 = new Book("Spring Boot", "John"); // uses constructor 3

✅ Each constructor initializes the object differently based on the provided parameters.

Why Use Constructor Overloading?

Real-World Example

Let’s say you’re building a User class for a login system:

public class User {
    private String username;
    private String email;
    private boolean isActive;

    public User(String username) {
        this.username = username;
        this.isActive = true;
    }

    public User(String username, String email) {
        this.username = username;
        this.email = email;
        this.isActive = true;
    }

    public User(String username, String email, boolean isActive) {
        this.username = username;
        this.email = email;
        this.isActive = isActive;
    }
}

This allows the API consumer to create a user with just a username, or a fully populated user object — all using different constructors.