Because Java can only use TCP/IP connections, the Postgres postmaster must be running with the -i flag.
Also, the pg_hba.conf file must be configured. It's located in the PGDATA directory. In a default installation, this file permits access only by UNIX domain sockets. For the JDBC driver to connect to the same localhost, you need to add something like:
host all 127.0.0.1 255.255.255.255 password
Here access to all databases are possible from the local machine with JDBC.
The JDBC Driver supports trust, ident, password and crypt authentication methods.