java引用被设置为null的疑惑

a=null;

 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();
         }

 }