Constructor

• A constructor is a special type of method used to initialize class variables.

• It is automatically called when an object is created.

Rules

• Constructor name must be same as class name

• No return type allowed

• Cannot call constructor explicitly like a method

• Executes automatically at object creation

• Can have parameters like methods

java

class A {

    A() {
        System.out.println("Constructor in A class");
    }

    void A() {
        System.out.println("This is a method, not constructor");
    }
}

class Main {
    public static void main(String[] args) {
        A a1 = new A();
    }
}

A() → constructor
void A() → normal method

Parameterized Constructor

java

class Student {
    int sid;
    String sname;
    String email;
    static String nationality;

    Student(int id, String name, String email, String nat) {
        sid = id;
        sname = name;
        this.email = email;
        nationality = nat;
    }

    void show() {
        System.out.println(sid + " " + sname + " " + email + " " + nationality);
    }
}

🔹 Default Constructor

• Created automatically by Java compiler if no constructor is defined

🔹 Important Points:

• Default constructor is non-parameterized

• If you create your own constructor → default constructor will NOT be created

🔹 Constructor Chaining

• Calling one constructor from another constructor

Using this()

plaintext

class A {
    A() {
        System.out.println("No-arg constructor");
    }

    A(int a) {
        this();
        System.out.println("1-arg constructor");
    }

    A(int a, int b) {
        this(10);
        System.out.println("2-arg constructor");
    }
}

class Main {
    public static void main(String[] args) {
        new A(10, 20);
    }
}

🔹 Rules of this():

• Must be first statement

• Only one constructor call allowed

• No cyclic calling

this Keyword

• this is a reference variable that refers to the current object

• Cannot be used in static context

• Used to:

  • Access instance variables

  • Call constructors (this())

java

class Student {
    int sid;
    String name;

    Student(int sid, String name) {
        this.sid = sid;
        this.name = name;
    }
}

this.sid → current object variable

HashCode Example (Understanding this)

java

class A {
    void m1() {
        System.out.println("this: " + this.hashCode());
    }
}

Shows current object reference

Variable Arguments (Var-Args)

• Allows passing multiple arguments dynamically

java

int... a

• Works like array internally

• Can pass any number of values

• Must be last parameter

• Only one var-arg allowed

java

class A {

    void m1(int... a) {
        for (int i = 0; i < a.length; i++) {
            System.out.println(a[i]);
        }
    }
}

class Main {
    public static void main(String[] args) {
        A obj = new A();
        obj.m1(10, 20, 30, 40);
    }
}

Mixed Parameters Example:

java

void m1(String s, int... a) {
    System.out.println(s);
    for (int i : a) {
        System.out.println(i);
    }
}