Using Object as a parameter

class Demo{
    double base;
    double height;
    Demo(){
        System.out.println("Constructor with no parameter");
    }
    Demo(Demo dm){
        base = dm.base;
        height = dm.height;
    }
    Demo(double height, double base){
        this.height = height;
        this.base = base;
    }
    Demo(double len){
        this.height = len;
        this.base = len;
    }
    double area(){
        return 0.5*base*height;
    }
}
public class Main{
    public static void main(String args[]){
        Demo dm = new Demo(10.0, 5.0);
        Demo dm2 = new Demo(5.0,3.0);
        Demo dm3 = new Demo(dm);
        System.out.println(dm.area());
        System.out.println(dm2.area());
        System.out.println(dm3.area());
    }
}