/*這是子類——圓類*/

package?com.imooc;
import?java.util.Scanner;
public?class?Circle?extends?Shape?{
Scanner?in?=?new?Scanner(System.in);
public?void?call(){
System.out.println("現在是計算圓的面積和周長");
System.out.println("請輸入圓的半徑");
double?r=in.nextDouble();
double?Pi=3.14;
System.out.println("圓的面積："+Square(r,Pi));
System.out.println("圓的周長："+Leng(r,Pi));
}
public?double?Square(double?a,?double?b)?{
//?TODO?Auto-generated?method?stub
double?S;
S=b*a*a;
return?S;
}
public?double?Leng(double?a,?double?b)?{
//?TODO?Auto-generated?method?stub
double?l;
l=b*2*a;
return?l;
}
}

/*這是子類——矩形類*/

package?com.imooc;
import?java.util.Scanner;
public?class?Rectangle?extends?Shape?{
Scanner?in?=?new?Scanner(System.in);
public?void?call()?{
System.out.println("計算矩形的周長和面積");
System.out.println("請輸入兩條邊長");
double?a?=?in.nextDouble();
double?b?=?in.nextDouble();
System.out.println("矩形的面積是："+Square(a,b));
System.out.println("矩形的周長是："+Leng(a,b));
}
@Override
public?double?Square(double?a,?double?b)?{
//?TODO?Auto-generated?method?stub
double?S;
S=a*b;
return?S;
}
@Override
public?double?Leng(double?a,?double?b)?{
//?TODO?Auto-generated?method?stub
double?l;
l=2*(a+b);
return?l;
}
}

/*這是父類*/

package?com.imooc;
public?abstract?class?Shape?{
public?abstract?double?Square(double?a,double?b);
public?abstract?double?Leng(double?a,double?b);
}

/*這是測試類*/

package?com.imooc;
public?class?Initial?{
public?static?void?main(String[]?args)?{
/*Rectangle?rec?=?new?Rectangle();
rec.call();
Circle?cir?=?new?Circle();
cir.call();*/
//以上方法是可行的
Shape?rec?=?new?Rectangle();
((Rectangle)?rec).call();
Shape?cir?=?new?Circle();
((Circle)?cir).call();
}//這個方法也是可行的
}

我主要是想知道為什么以下內容會報錯，必須改成上面那兩種才可以

Shape?rec?=?new?Rectangle();
rec.call();
Shape?cir?=?new?Circle();
cir.call()