// 抽象方法和抽象類.
abstract class Figure {
  double dim1;
  double dim2; 
  Figure(double a, double b) {
    dim1 = a;
    dim2 = b;
  }
  // 方法area()現在是一個抽象方法 
  abstract double area();
}
class Rectangle extends Figure {
  Rectangle(double a, double b) {
    super(a, b);
  }
  // 類Rectangle重寫方法area()
  double area() {
    System.out.println("Inside Area for Rectangle.");
    return dim1 * dim2;
  }
}
class Triangle extends Figure {
  Triangle(double a, double b) {
    super(a, b);
  }
  // 正三角形重寫方法area()
  double area() {
    System.out.println("Inside Area for Triangle.");
    return dim1 * dim2 / 2;
  }
}
class AbstractAreas {
  public static void main(String args[]) {
  // Figure f = new Figure(10, 10); // illegal now
    Rectangle r = new Rectangle(9, 5);
    Triangle t = new Triangle(10, 8);
    Figure figref; // this is OK, no object is created
    figref = r;
    System.out.println("Area is " + figref.area());
    figref = t;
    System.out.println("Area is " + figref.area());
  }
}