java_泛型，设置类型通配符的上限

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<Circle>可以用

	public void drawAll(List<? extends Shape> 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<Circle> list = new ArrayList<Circle>();

		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<Shape> li2 = new ArrayList<Shape>();

		li2.add(new Circle());

		li2.add(new Rectangle());

		li2.add(new Circle());

		ca.drawAll(li2);

	}



}