Portability
Program code should never ever contain "hard-coded", i.e. literal, values referring to environmental parameters, such as absolute file paths, file names, user names, host names, IP addresses, URLs, UDP/TCP ports. Otherwise the application will not run on a host that has a different design than anticipated. Such variables should be parametrized, and configured for the hosting environment outside of the application proper (e.g. property files, application server, or even a database).
As an extension, resources such as XML files should also contain variables rather than literal values, otherwise the application will not be portable to another environment without editing the XML files. For example with J2EE applications running in an application server, such environmental parameters can be defined in the scope of the JVM and the application should get the values from there.
Read more about this topic: Best Coding Practices