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);
	}

}