实训java第九课 类的继承

package com.tarena.day09.shape;public class Point {public int x;public int y;public Point() {super();}public Point(int x, int y) {super();this.x = x;this.y = y;}} package com.tarena.day09.shape;public class ShapeDemp {/** * @param args */public static void main(String[] args) {Circle c = new Circle(3,4,5);System.out.println(c.area());Shape s = c;//圆的实例也是图形的实例System.out.println(s.area());//s.area() 是在Shape 上声明的，但是被Circle覆盖了//java在执行期间确定最终的方法。由实际的对象决定最终的方法（方法的动态绑定）。s = new Rectangle(2,3,4,5);System.out.println(s.area());Shape[] ss = {new Circle(1,2,4),new Rectangle(1,2,3,4)};//print(ss[0]);//print(ss[1]);Shape x =c;check(x);}public static void print(Shape s){System.out.println(s.area());}public static void check(Shape s){if(s instanceof Circle){Circle c = (Circle)s;System.out.println("是圆形"+c.getR());}}}

 

package com.tarena.day09;public class ConstructorsDemo {public static void main(String[] args) {Koo koo = new Koo();//koo先System.out.println(koo.a);}}class Goo{int a;public Goo(int a){this.a = a;}}//class Koo extends Goo{}//错，子类默认调用父类无参数构造器class Koo extends Goo{int b;public Koo(){//super() 必须写在第一行，与this()互斥super(1);//super()调用复类构造器//子类一定调用父类构造器}}

 

package com.tarena.day09.shape;public class Shape {public double area(){return 0;}}

package com.tarena.day09.shape;public class Rectangle extends Shape {private Point p;private int width;private int height;public Rectangle() {super();}public Rectangle(int x,int y,int width, int height) {super();p = new Point(x,y);this.width = width;this.height = height;}@Overridepublic double area() {return width * height;}}

package com.tarena.day09.shape;public class Circle extends Shape{private Point center;private int r;public Circle(){}public Circle(int x,int y,int r){this.center = new Point(x,y);this.r = r;}public Point getCenter() {return center;}public void setCenter(Point center) {this.center = center;}public int getR() {return r;}public void setR(int r) {this.r = r;}public double area(){return Math.PI *r*r;}}