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");
}

}
}