//Square.java
public class Square extends Polygon {
private  double length;
//constructor
public Square (double length )
{   super(4);
sides[0] = length;
sides[1] = length;
sides[2] = length;
sides[3] = length;
calArea();
}
public void calArea() {
area = sides[0] * sides[0];
}
public void setlength(double length ) {
this.length = length;
}
public double getlength() {
    return this.length;
}
}