Connecting postgreSQL using JDBC

I have just tried a sample Java program. It connects the PostgreSQL through
the JDBC. I ran it on the postgreSQL machine. It works fine. But, when I
tried to run it in another machine and connecting the database through the
network. The connection was refused. I have no idea what is wrong.

postgreSQL:
startup with 'postmaster -i'
java program:
...DriverManager.getConnection ("jdbc:postgresql:webtest", "nobody",
"");
where webtest is my testing database name.

-- Howard