import java.rmi.*;

public class AddServerImpl extends UnicastRemoteObject implements AddServer
{
public AddServerImpl()
{
super();
}
public int AddNumbers(int firstnumber,int secondnumber) throws RemoteException
{
return firstnumber+secondnumber;
}
}
这段代码编译时，显示找不到符号类UnicastRemoteObject，怎么回事？

import java.rmi.server.UnicastRemoteObject;

 另AddServer是什么接口？！