> Could anyone tell what is wrong with the following error:

It cannot connect to the Firebird server running on the localhost. There are
several possible issues:

a) your Firebird server is not acepting network connections (for example it
is not started);
b) your Linux blocks socket connections (firewall);
c) your Linux does not know what "localhost" is (sometimes you have to add a
line into /etc/hosts);
d) your java.policy does not allow JayBird to open socket connections.

I would start with first, as the most likely situation. Please note that you
can check the Firebird installation with isql only if you use

CONNECT 'localhost:/opt/firebird/examples/employee.fdb' USER 'SYSDBA'
password 'masterkey';

but not

CONNECT '/opt/firebird/examples/employee.fdb' USER 'SYSDBA' password
'masterkey';

In the latter case isql will access the database file directly, it won't
tell you whether server is running or not.

Or just use

telnet localhost 3050

You will see whether it accepts connections or not.

Roman