What is Java class constructor?

A constructor initializes an object during object creation when using new operator.
Java allows objects to initialize themselves when they are created. This automatic initialization is performed through the use of a constructor.

It has the same name as the class. Constructors have no return type, not even void.

 class ClassName{
   ClassName(parameter list){ // constructor
In the following code the Rectangle class in the following uses a constructor to set the dimensions:
class Rectangle {
  double width;
  double height;
  Rectangle() {
    width = 10;
    height = 10;
  double area() {
    return width * height;
public class Main {
  public static void main(String args[]) {
    Rectangle mybox1 = new Rectangle();
    double area;
    area = mybox1.area();
    System.out.println("Area is " + area);