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abstract类中的abstract方法只允许声明，比允许实现，要实现抽象方法必须将abstract类的对象转变为abstract类的子类的上转型对象。
实例分析：
Geometry抽象类：
package zjl.java; public abstract class Geometry { public String name; public abstract double getArea(); }
Circle子类：
package zjl.java; public class Circle extends Geometry{ double r; Circle(double r,String s){ this.r = r; this.name = s; } public double getArea(){ return 3.14*r*r; } }
Rectanger子类：
package zjl.java; public class Rectanger extends Geometry{ double a,b; public Rectanger(double a,double b,String s) { this.a = a; this.b = b; this.name = s; } public double getArea(){ return a*b; } }
Pillar类：
package zjl.java; public class Pillar { Geometry bottom; double height; Pillar(Geometry botton,double height){ this.bottom = botton; this.height = height; } public double getVolume(){ return bottom.getArea()*height; } }
主类：
package zjl.java; public class TestAbstractProgram { public static void main(String[] args) { Pillar pillar; Circle circle = new Circle(2.3,"circle"); Rectanger rectanger = new Rectanger(1.2, 2.9,"rectanger"); Geometry bottom; //botton = new Geometry(); //abstract 类不能用new 创建对象 bottom = circle; pillar = new Pillar(bottom, 10); System.out.println(bottom.name + "‘s volume is "+ pillar.getVolume()); bottom = rectanger; pillar = new Pillar(bottom, 10); System.out.println(bottom.name + "‘s volume is "+pillar.getVolume()); } }
运行结果：
circle‘s volume is 166.106
rectanger‘s volume is 34.8
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原文地址：http://www.cnblogs.com/zhangjialu2015/p/5358695.html
