Java Program - Using Constructors¶

Concept Explanation¶

What is it?¶

A constructor is a special block of code similar to a method that is called when an instance of an object is created. Its primary purpose is to initialize the object's state.

Implementations¶

Java (JDK 17)

class Book {
    String title;
    String author;
    double price;

    // 1. Default Constructor (No arguments)
    Book() {
        title = "Unknown Title";
        author = "Unknown Author";
        price = 0.0;
    }

    // 2. Parameterized Constructor
    Book(String t, String a, double p) {
        title = t;
        author = a;
        price = p;
    }

    void displayInfo() {
        System.out.println("Title:  " + title);
        System.out.println("Author: " + author);
        System.out.println("Price:  $" + price);
        System.out.println("-----------------");
    }
}

public class ConstructorDemo {
    public static void main(String[] args) {
        // Calls Default Constructor
        Book b1 = new Book();
        b1.displayInfo();

        // Calls Parameterized Constructor
        Book b2 = new Book("The Alchemist", "Paulo Coelho", 15.99);
        b2.displayInfo();
    }
}

Explanation¶

Name: The constructor name must match the class name exactly.
Return Type: Constructors have no return type, not even void.
Overloading: You can have multiple constructors with different parameter lists (Constructor Overloading).

Interview Tips¶

Explain the difference between this() (calling another constructor) and super() (calling parent constructor).
What happens if you don't define any constructor? (Java provides a default one).

"Construction is the art of making a whole out of many parts." - Anonymous