Changing hostname of a server without reboot

I need to change the hostname of my server. This is normally done by editing /etc/hosts. However, the new hostname will not be effective or placed in memory without rebooting. Fortunately, the command "hostname new_hostname" help me to solve this problem. The purpose is to set a overriding hostname in memory until next time the server reboot. This feature reallys help system administrator a lot.