Java Constructors

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

1. Key Features of Constructors

Same name as the class
No return type (not even void)
Can have parameters
Used to initialize instance variables
Can be overloaded

2. Types of Constructors

Default Constructor – No parameters; provided automatically if no constructor is defined.
Parameterized Constructor – Takes parameters to initialize an object with specific values.
No-Arg Constructor – A user-defined constructor without parameters (similar to default).

3. Example 1: Default Constructor

class Car {
String brand;
int year;// Default constructor
Car() {
brand = "Unknown";
year = 0;
}

void display() {
System.out.println("Brand: " + brand + ", Year: " + year);
}
}

public class Main {
public static void main(String[] args) {
Car car1 = new Car(); // Default constructor called
car1.display();
}
}

class Car {

String brand;

int year;// Default constructor

Car() {

brand = "Unknown";

year = 0;

}

void display() {

System.out.println("Brand: " + brand + ", Year: " + year);

}

public class Main {

public static void main(String[] args) {

Car car1 = new Car(); // Default constructor called

car1.display();

}

📌 Output:

4. Example 2: Parameterized Constructor

class Car {
String brand;
int year;// Parameterized constructor
Car(String b, int y) {
brand = b;
year = y;
}

void display() {
System.out.println("Brand: " + brand + ", Year: " + year);
}
}

public class Main {
public static void main(String[] args) {
Car car1 = new Car("Toyota", 2022);
Car car2 = new Car("Honda", 2023);

car1.display();
car2.display();
}
}

class Car {

String brand;

int year;// Parameterized constructor

Car(String b, int y) {

brand = b;

year = y;

}

void display() {

System.out.println("Brand: " + brand + ", Year: " + year);

}

public class Main {

public static void main(String[] args) {

Car car1 = new Car("Toyota", 2022);

Car car2 = new Car("Honda", 2023);

car1.display();

car2.display();

}

📌 Output:

5. Example 3: Constructor Overloading

class Rectangle {
int length, width;// No-arg constructor
Rectangle() {
length = 0;
width = 0;
}

// Parameterized constructor
Rectangle(int l, int w) {
length = l;
width = w;
}

int area() {
return length * width;
}
}

public class Main {
public static void main(String[] args) {
Rectangle r1 = new Rectangle();
Rectangle r2 = new Rectangle(5, 10);

System.out.println("Area of r1 = " + r1.area());
System.out.println("Area of r2 = " + r2.area());
}
}

class Rectangle {

int length, width;// No-arg constructor

Rectangle() {

length = 0;

width = 0;

}

// Parameterized constructor

Rectangle(int l, int w) {

length = l;

width = w;

}

int area() {

return length * width;

}

public class Main {

public static void main(String[] args) {

Rectangle r1 = new Rectangle();

Rectangle r2 = new Rectangle(5, 10);

System.out.println("Area of r1 = " + r1.area());

System.out.println("Area of r2 = " + r2.area());

}

📌 Output:

6. Key Points About Constructors

Constructor name must match the class name.
No return type, not even void.
Can be overloaded to provide multiple ways of initializing objects.
Automatically called when an object is created.
this keyword can be used to refer to current object or call another constructor.

Java Constructors