simple JDBC --> can't get started

I have a simple test.java which tries to connect to a database I have
created, my user name is "yangsu" which is created by "createuser yangsu"
My db_hba.conf has only the following lines:

local all trust
host all 127.0.0.1 255.255.255.255 trust
host all 129.186.68.160 255.255.255.255 trust

My machine name is inkle.cs.iastate.edu(ip 129.186.68.160), my execution
of "java test" yields:

Class.forName OK!
java.sql.SQLException: Connection failed: java.net.ConnectException: Connection refused

What is the error?(what causes the error?) Thanks for any suggestions.

yang
http://www.cs.iastate.edu/~yangsu/

Following is the test.java:

import java.io.*;
import java.net.*;
import java.sql.*;

public class test{

public static void main(String args[]){
try{
Class.forName("postgresql.Driver");
System.out.println("Class.forName OK!");

String url="jdbc:postgresql://inkle.cs.iastate.edu/test";
String usr="yangsu";
String pwd="";

Connection db = DriverManager.getConnection(url, usr, pwd);
if(db != null){
System.out.println("Connected to db");
}else{
System.out.println("Can't connect to db");
}

Statement st = db.createStatement();
ResultSet rs = st.executeQuery("select * from classmate");

while(rs.next()){
System.out.println(rs.getString("name"));
System.out.println(rs.getBoolean("sex"));
System.out.println(rs.getInt("age"));
}
}catch(Exception e){
System.out.println(e.toString());
}
}

}