192.168.* is a private address family intended for internal, private LAN usage. This is a defined feature of the TCP/IP addressing standards. What this means is that routers and gateways will not route or forward any packets for 192.168 address outside of the local network.
This has nothing to do with Java, it's a purely network thing. Like someone already suggested, you need to set up a NAT gateway. Most OTC LAN or broadband routers intended for home-usage have NAT capability so likely you have it already. All you need to do is create mappings to connect the address and ports of your webcams to ports that are then accessible from your public IP. I would consult the manual of your LAN router if you are unsure about how this works.
If your public IP is DHCP assigned (like most are on public ISP subscriptions), you might want to look into one of the dynamic DNS services out there (like dyndns.org) so you can access your home IP using a DNS name rather than the IP.
Routers will route anything that is outside the LAN. What is outside is determined by the subnet mask. You should use LAN addresses with 192.168.x.y and usually you use 255.255.255.0 as subnet mask for a home LAN. Then, anything ouside 192.168.x will be routed outside the LAN. This must be configured in the network settings, but no router will route or not route traffic outside the LAN just because of the IP-address range 192.168.x.y. – Martin – 2011-07-19T14:55:53.017