public class C {

     protected A webDigester = new A(" first one ");

     public void test(){

         A a;

         a=webDigester;

         a.add(" second ");        

         a=null;

         webDigester.display();

         //猜猜会打印出什么呢？

     }

     public static void main(String[] args) {

         C c=new C();

         c.test();

     }

 }

 import java.util.ArrayList;

 public class A {

         public ArrayList<Object> al=new ArrayList<Object>();

         public void display()

         {

             System.out.println("-----start-------");

             for(Object o:al)

             {

                 System.out.println(" ["+o.toString()+"] ");

             }

             System.out.println("-----end-------");

         }

         public A(String msg){

             al.add(msg);

         }

         public void add(String msg){

             al.add(msg);

         }

         public void  distory()

         {

             al.clear();

         }

 }