package ming;import java.util.ArrayList;import java.util.Collection;import java.util.List;class Canvas {	// ? extends Shape,定义了子类，所以List
    
     可以用	public void drawAll(List
      shapes) {		for (Shape s : shapes) {			s.draw(this);		}	}}abstract class Shape {	public abstract void draw(Canvas c);}class Circle extends Shape {	public void draw(Canvas c) {		System.out.println("在画布上 " + c + "画一个圆");	}}class Rectangle extends Shape {	@Override	public void draw(Canvas c) {		System.out.println("在画布上 " + c + "画一个正方形");	}}public class Apple3Test {	public static void main(String[] args) {		// TODO Auto-generated method stub		Canvas ca = new Canvas();		//--------------------------------------------------		List
     
       list = new ArrayList
      
       ();		list.add(new Circle());		list.add(new Circle());		list.add(new Circle());		ca.drawAll(list);		//---------------不设置上面代码会报错------------------------		ArrayList
       
         li2 = new ArrayList
        
         ();		li2.add(new Circle());		li2.add(new Rectangle());		li2.add(new Circle());		ca.drawAll(li2);	}}