JAVA中TreeSet的问题

添加一个对象到treeSet执行就正确
添加一个以上对象到treeSet执行就提示错误信息：java.lang.ClassCastException
    at java.util.TreeMap.compare(TreeMap.java:1085)
    at java.util.TreeMap.put(TreeMap.java:463)
    at java.util.TreeSet.add(TreeSet.java:209)
    at JiHe.main(JiHe.java:16)
Exception in thread "main" 
---------------------------------------------------------import java.util.*;public class Test{
    
    public static void main(String args[])
    {
        Address a1 = new Address("A1","beijing","baogong","100000");
        Address b1 = new Address("A2","beijing","huaxiang","100000");
        Address c1 = new Address("A3","beijing","wukesong","100000");  
        
        TreeSet treeSet = new TreeSet();
        treeSet.add(a1);
        treeSet.add(b1);//去掉正确
        treeSet.add(c1);//去掉正确        Iterator iterator = treeSet.iterator();
        while(iterator.hasNext())
        {
            Object t1=iterator.next();
            System.out.println(t1+"\n");
        }  
        
    }
}