Java Constructors: Default, Parameterized & Constructor Chaining

A constructor is a specialized member block designed to initialize the state of a newly created object. It is automatically triggered whenever the new operator allocates memory on the Heap.

Key Rules of Java Constructors:

• Name Rule: The constructor name must identically match the class name.
• No Return Type: A constructor must have no return type—not even void. If you specify void MyClass(), the compiler treats it as a standard method, not a constructor!
• Automatic Trigger: Constructors cannot be invoked directly like regular methods; they are called exclusively by new or via constructor chaining (this(), super()).

1. The Default (No-Arg) Constructor

If a class defines zero constructors, the compiler automatically generates an invisible default constructor:

public class Student {
    int rollNumber;
    String name;

    // Explicit No-Argument Constructor
    public Student() {
        this.rollNumber = 100;
        this.name = "Unknown";
        System.out.println("Default constructor initialized!");
    }
}

2. Parameterized Constructor & Overloading

Allows creating objects with customized initial state at the moment of creation:

public class Student {
    int rollNumber;
    String name;
    double gpa;

    // Constructor 1: Default
    public Student() {
        this(0, "Unassigned", 0.0); // Chaining to Constructor 3!
    }

    // Constructor 2: Two parameters
    public Student(int rollNumber, String name) {
        this(rollNumber, name, 0.0); // Chaining to Constructor 3!
    }

    // Constructor 3: Three parameters (Master Constructor)
    public Student(int rollNumber, String name, double gpa) {
        this.rollNumber = rollNumber;
        this.name = name;
        this.gpa = gpa;
    }
}

Constructor Chaining using this()

Constructor Chaining prevents duplicate initialization code by allowing one constructor to delegate to another overloaded constructor in the same class using this(...):