java 多态 向上转型 后期绑定一例

switch中的 default 语句不管放在哪个位置，都是在所有case都不满足的情况下最后执行。 foreach语句 for(元素类型 元素变量；遍历对像) 变量元素.draw()即 遍历对象.draw（）；foreach即for每一个。 for (Shap shp: s) shp.draw(); 个人理解，当 s[0]=gen.next();时 调用RandomShapeGenerator 中的next方法，执行语句case0-2其中一个return语句得到指向Circle Square Triangle 其中的一个引用，并将其以Shape类型发送出去。此例中当return语句指向Circle时 shp.draw() 即将shp元素变量替换为Circle对象，调用Shap类型 Circle对象draw 方法。 package javahaonan.shape; public class Shape { public void draw() {} public void erase() {} } package javahaonan.shape; import static javahaonan.Print.*; public class Circle extends Shape { public void draw() { print ("Circle。draw()");} public void erase() {print ("Cricle.erase()");} } package javahaonan.shape; import static javahaonan.Print.*; public class Square extends Shape { public void draw(){print("Square.draw()");} public void erase(){print("Square.erase()");} } package javahaonan.shape; import static javahaonan.Print.*; public class Triangle extends Shape { public void draw(){print("Trinagle.draw()");} public void erase(){print("Trinagle.erase()");} } package javahaonan.shape; import java.util.*; public class RandomShapeGenerator { private Random rand = new Random(); public Shape next(){ switch(rand.nextInt(3)){ default: case 0:return new Circle(); case 1:return new Square(); case 2:return new Triangle(); } } } package javahaonan.shape; public class Shapes { private static RandomShapeGenerator gen = new RandomShapeGenerator(); public static void main(String[] args) { Shape[] s=new Shape[9]; for (int i=0;i s[i]=gen.next(); for(Shape shp: s) shp.draw(); } }/output* Square.draw() Trinagle.draw() Square.draw() Circle。draw() Square.draw() Trinagle.draw() Trinagle.draw() Trinagle.draw() Trinagle.draw() */