我输入以下一段代码，显示找不到符号UnicastRemoteObject，请问怎么回事？代码如下：
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;
}
}