java引用被设置为null的疑惑

a=null;

 1 public class C {
 2
 3     protected A webDigester = new A(" first one ");
 4
 5     public void test(){
 6
 7         A a;
 8
 9         a=webDigester;
10
11         a.add(" second ");
12
13         a=null;
14
15         webDigester.display();
16         //猜猜会打印出什么呢？
17     }
18     public static void main(String[] args) {
19
20         C c=new C();
21
22         c.test();
23     }
24
25 }

 1 import java.util.ArrayList;
 2
 3
 4 public class A {
 5
 6         public ArrayList<Object> al=new ArrayList<Object>();
 7
 8         public void display()
 9         {
10             System.out.println("-----start-------");
11             for(Object o:al)
12             {
13                 System.out.println(" ["+o.toString()+"] ");
14             }
15             System.out.println("-----end-------");
16         }
17
18         public A(String msg){
19             al.add(msg);
20         }
21
22         public void add(String msg){
23             al.add(msg);
24         }
25
26         public void  distory()
27         {
28             al.clear();
29         }
30
31 }