import java.util.*;

interface Shape{

public void draw();

}

class Circle implements Shape{

public void draw(){

System.out.println(1);

}

}

class Triangle implements Shape{

public void draw(){

System.out.println(2);

}}

public class Test0 {

public static void main(String[] args) {

// TODO Auto-generated method stub

Test0 a=new Test0();

List<Circle> b=new ArrayList<Circle>();

b.add(new Circle());

a.drawall(b);

List<Triangle> c=new ArrayList<Triangle>();

c.add(new Triangle());

a.drawall(c);

}

public void drawall(List<? extends Shape>c){

for(int i=0;i<c.size();i++){

c.get(i).draw();

}

}

}

Test0 a=new Test0();

List<Circle> b=new ArrayList<Circle>();

b.add(new Circle());

a.drawall(b);

上面这4行代码能解释一下么？？？？谢谢啦