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();
        }
        
}