Java 中的抽象介绍

abstract class Shape {
 String color;

 abstract double area();
 public abstract String toString();

 public Shape(String color){
  System.out.println("Shape constructor called");
  this.color = color;
 }
 public String getColor() { return color; }
}
class Circle extends Shape {
 double radius;

 public Circle(String color, double radius){
  super(color);
  System.out.println("Circle constructor called");
  this.radius = radius;
 }

 @Override double area(){
  return Math.PI * Math.pow(radius, 2);
 }

 @Override public String toString(){
  return "Circle color is " + super.getColor()
   + "and area is : " + area();
 }
}
class Rectangle extends Shape {

 double length;
 double width;

 public Rectangle(String color, double length,double width){
  super(color);
  System.out.println("Rectangle constructor called");
  this.length = length;
  this.width = width;
 }

 @Override double area() { return length * width; }

 @Override public String toString(){
  return "Rectangle color is " + super.getColor()
   + "and area is : " + area();
 }
}
public class Test {
 public static void main(String[] args){
  Shape s1 = new Circle("Red", 2.2);
  Shape s2 = new Rectangle("Yellow", 2, 4);
  System.out.println(s1.toString());
  System.out.println(s2.toString());
 }
}