//define the interface
//建立和編譯服務接口的Java代碼。這個接口定義了所有的提供遠程服務的功能
import java.rmi.Remote;
public interface Calculator extends Remote
{
    public long add(long a, long b) 
        throws java.rmi.RemoteException;  
    public long sub(long a, long b) 
        throws java.rmi.RemoteException;  
    public long mul(long a, long b) 
        throws java.rmi.RemoteException;  
    public long div(long a, long b) 
        throws java.rmi.RemoteException; 
}