class Square
{
int leght;
Square(int len)
{
leght=len;
}

void getPer()
{
System.out.println(":"+(4*leght));
}
}

class Rectang extends Square
{
int width;
Rectang(int len,int wid)
{
super(len);
width=wid;
}

void getPer()
{
System.out.println(":"+(2*(leght+width)));
}
}

public class Calculate
{
public static void main (String[] args)
{
Square sq=new Square(15);
sq.getPer();
Rectang re=new Rectang(8,19);
//Square re=new Rectang(8,19);
re.getPer();
}
}

看上面的红色代码部分,Rectang是继承自Square的子类,在创建Rectang对象实例时,以上两端红色代码的效果有啥不同?
我看过结果都一样,"Square re"与"Rectang re"有啥区别?在继承中象这种前面是父类名,后面是子类名创建对象的方法,能说明一下么?不太明白.